The Pennsylvania State University. The Graduate School. College of Agricultural Sciences THE UTILITY OF LATERAL BRANCHING OF THE PRIMARY ROOT FOR

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1 The Pennsylvania State University The Graduate School College of Agricultural Sciences THE UTILITY OF LATERAL BRANCHING OF THE PRIMARY ROOT FOR TOLERANCE TO PHOSPHORUS STRESS AND DROUGHT IN THE COMMON BEAN (Phaseolus vulgaris L.) A Thesis in Horticulture by Samuel A. Camilo 2014 Samuel A. Camilo Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science December 2014

2 The thesis of Samuel A. Camilo was reviewed and approved* by the following: Jonathan P. Lynch Professor of Plant Nutrition Thesis Adviser Kathleen M. Brown Professor of Plant Stress Biology Rick Bates Associate Professor of Ornamental Horticulture Rich Marini Professor of Horticulture Head of the Department of Horticulture *Signatures are on file in Graduate School. ii

3 ABSTRACT Common bean (Phaseolus vulgaris L.) is essential to the food security of millions people in developing nations. Inadequate precipitation and low soil fertility, especially low phosphorus availability, are the primary constraints in most rain-fed bean production regions. This study aimed to evaluate the utility of lateral branching of the primary root for tolerance of phosphorus stress and drought in the common bean (Phaseolus vulgaris L.). Recombinant inbred lines contrasting in taproot lateral branching were evaluated in pots with stratified water and phosphorus availability in a greenhouse with four treatments: 1) adequate water and phosphorus; 2) limited phosphorus and adequate water; 3) limited water and adequate phosphorus; and 4) combined drought and phosphorus stress. Another greenhouse study focused on drought stress in pots with stratified water availability in a greenhouse. In addition f i e l d studies were conducted in which drought stress was imposed by automated rainout shelters at Rock Springs; field experiment at URBC, Republic of South Africa and Sussundenga research station, Republic of Mozambique. Overall, drought and phosphorus treatments significantly reduced growth parameters (shoot biomass, leaf area and number of leaves). In the field study at Rock Springs drought stress caused a reduction of 50% in shoot biomass, 64% in leaf area, and 48% in number of leaves at flowering. Total taproot length in the bottom horizon of the stratified water greenhouse system (2013 greenhouse study ) was correlated with shoot biomass, although in these results as well as those from the field experiments (Rock Springs, URBC and Sussundenga), phenotype groups did not have significant effects on shoot biomass and most of the parameters measured. Results suggest the need for more research testing whether the differences in root length, branching, as well as the size of lateral branches (and their anatomical traits such as metaxylem vessels) has an influence in water uptake. How much water is taken up after imposition of drought would be related to the root length density and its size, since in our experiments we have observed that strong drought stress led to more fine roots production and allocation in the deeper horizons. Keywords: common bean, root architecture, taproot lateral branching, drought, phosphorus iii

4 TABLE OF CONTENTS LIST OF FIGURES... V LIST OF ABBREVIATIONS... IX ACKNOWLEDGMENTS... X 1. INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ANNEX: FIGURES AND TABLE REFERENCES APPENDIX: REGRESSION AND ANALYSIS OF VARIANCE iv

5 LIST OF FIGURES FIGURE 1: ROOT MORPHOLOGICAL COMPONENTS OF THE COMMON BEAN. A) ADVENTITIOUS ROOT; B) BASAL ROOT; C) TAPROOT; D) LATERAL ROOTS. ROOT CROWN WAS OBTAINED FROM FIELD EXPERIMENT AT SUSSUNDENGA RESEARCH STATION, REPUBLIC OF MOZAMBIQUE FIGURE 2: STRATIFIED WATER AND PHOSPHORUS GREENHOUSE SYSTEM. TOP 0-8 CM AND BOTTOM 8-36 CM HORIZONS ARE SEPARATED BY A 2MM SCREEN MESH IMPREGNATED WITH WAX AND COCONUT OIL, AND AN IRRIGATION RING BELOW THE SCREEN MESH WHICH PERMITTED TO IRRIGATE THE LAYERS INDEPENDENTLY. FIGURE WAS RECREATED FROM (HO ET AL. 2005), AND IS NOT DRAWN TO SCALE FIGURE 3: SOIL VOLUMETRIC WATER CONTENT AT 36 DAP IN THE DROUGHT AND CONTROL TREATMENTS OF THE TWO DEPTHS (0-8 AND 8-36 CM) IN THE STRATIFIED WATER GREENHOUSE SYSTEM AT UNIVERSITY PARK, PA. MOISTURE CONTENT WAS SIGNIFICANTLY DIFFERENT AT THE END OF THE EXPERIMENT FIGURE 4: SOIL VOLUMETRIC WATER CONTENT OF THE DROUGHT AND CONTROL TREATMENTS AT TWO DEPTHS (0-8 AND 8-36 CM) IN THE STRATIFIED WATER GREENHOUSE SYSTEM THROUGHOUT THE EXPERIMENT AT UNIVERSITY PARK, PA. IRRIGATION WAS STOPPED TWO WEEKS BEFORE HARVEST IN DROUGHT TREATMENTS TO IMPOSE STRONG STRESS, WHICH REDUCED THE TOTAL BIOMASS FIGURE 5: PLANT WATER STATUS MEASURED AT 35 DAP WITH PRESSURE BOMB UNDER DROUGHT AND WELL-WATERED TREATMENTS IN STRATIFIED WATER GREENHOUSE SYSTEM AT UNIVERSITY PARK, PA FIGURE 6: LEAF WATER POTENTIAL MEASURED AT 35 DAP WITH PRESSURE BOMB. PLANTS WERE GROWN UNDER DROUGHT TREATMENT IN STRATIFIED WATER GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 7: LEAF WATER POTENTIAL MEASURED AT 35 DAP WITH PRESSURE BOMB. PLANTS WERE GROWN UNDER WELL-WATERED TREATMENT IN STRATIFIED WATER GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 8: LEAF RELATIVE WATER CONTENT MEASURED THE DAY BEFORE HARVEST SHOWED DIFFERENCE BETWEEN THE DROUGHT AND WELL-WATERED PLANTS IN THE STRATIFIED WATER GREENHOUSE SYSTEM (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 9: RELATIONSHIP OF LEAF RELATIVE WATER CONTENT OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH AT THE BOTTOM HORIZON 35 DAP, SHOWED A STRONG CORRELATION UNDER DROUGHT PLANTS IN THE STRATIFIED WATER GREENHOUSE SYSTEM (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 10: RELATIONSHIP OF LEAF RELATIVE WATER CONTENT OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH AT THE BOTTOM HORIZON AT 35 DAP SHOWED A WEAK CORRELATION UNDER WELL-WATERED PLANTS IN THE STRATIFIED WATER GREENHOUSE SYSTEM (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 11: PHOTOSYNTHETIC ASSIMILATION MEASURED WITH LICOR DAP SHOWED PLANTS UNDER DROUGHT STRESS HAD LESS PHOTOSYNTHETIC ACTIVITY THAN THE WELL-WATERED PLANTS IN THE STRATIFIED WATER GREENHOUSE SYSTEM (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 12: THE RELATIONSHIP OF PHOTOSYNTHETIC ASSIMILATION OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER DROUGHT CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 13: THE RELATIONSHIP OF PHOTOSYNTHETIC ASSIMILATION OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER IRRIGATED CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 14: SHOOT BIOMASS AT 36 DAP SHOWED SIGNIFICANT DIFFERENCES BETWEEN THE DROUGHT v

6 STRESSED AND WELL-WATERED PLANTS IN THE STRATIFIED WATER GREENHOUSE SYSTEM (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 15: THE RELATIONSHIP OF DRY SHOOT BIOMASS OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER DROUGHT CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 16:THE RELATIONSHIP OF DRY SHOOT BIOMASS OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER WELL-WATERED CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 17: THE RELATIONSHIP OF PERCENTAGE REDUCTION OF DRY SHOOT BIOMASS OF THE GENOTYPES TO THE TOTAL ROOT LENGTH IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER DROUGHT CONDITIONS AT UNIVERSITY PARK, PA FIGURE 18: THE RELATIONSHIP OF TOTAL SHOOT BIOMASS TO THE TOTAL TAPROOT LENGTH OF THE MAIN AXIS IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER DROUGHT CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 19: THE RELATIONSHIP OF TOTAL SHOOT BIOMASS TO THE TOTAL TAPROOT LENGTH OF THE FIRST ORDER LATERAL ROOTS IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER DROUGHT CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 20: THE RELATIONSHIP OF TOTAL SHOOT BIOMASS TO THE TOTAL TAPROOT LENGTH OF THE SECOND ORDER LATERAL ROOTS IN THE BOTTOM HORIZON GROWN IN THE STRATIFIED WATER GREENHOUSE SYSTEM UNDER IRRIGATED CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 21: LEAF RELATIVE WATER CONTENT MEASURED THE DAY BEFORE HARVEST SHOWED NO DIFFERENCE BETWEEN DROUGHT STRESSED AND WELL-WATERED PLANTS IN THE STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 22: THE RELATIONSHIP OF LEAF RELATIVE WATER CONTENT OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH AT BOTTOM HORIZON UNDER DROUGHT AND HIGH PHOSPHORUS CONDITIONS MEASURED THE DAY BEFORE HARVEST, SHOWED POSITIVE CORRELATION IN THE STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 23: THE RELATIONSHIP OF LEAF RELATIVE WATER CONTENT OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH AT BOTTOM HORIZON UNDER DROUGHT AND LOW PHOSPHORUS CONDITIONS MEASURED THE DAY BEFORE HARVEST, SHOWED POSITIVE CORRELATION IN THE STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 24: LEAF PHOTOSYNTHETIC CO2 ASSIMILATION MEASURED UNDER DROUGHT AND LOW PHOSPHORUS TREATMENT IN THE STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 25: THE RELATIONSHIP OF PHOTOSYNTHETIC ASSIMILATION CO2 OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH IN THE TOP HORIZON GROWN IN THE STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM UNDER DROUGHT AND HIGH PHOSPHORUS CONDITIONS (2013 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 26: PHOSPHORUS CONCENTRATION (ΜM) FROM LEACHATE SAMPLES TAKEN 32 DAP SHOWED THE HIGHEST CONCENTRATION IN THE DROUGHT AND HIGH P TREATMENT AND LOWEST CONCENTRATION IN THE LOW P TREATMENT IN THE STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 27: PERCENTAGE OF PLANT P CONCENTRATION FROM PLANT TISSUES SHOWED THE HIGHEST CONTENT IN CONTROL & HIGH P TREATMENT AND LOWEST CONCENTRATION IN THE DROUGHT AND LOW P TREATMENT IN STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA vi

7 FIGURE 28: SHOOT BIOMASS SHOWED SIGNIFICANT DIFFERENCE BETWEEN THE DROUGHT AND WELL- WATERED PLANTS IN STRATIFIED WATER & PHOSPHORUS GREENHOUSE SYSTEM AT UNIVERSITY PARK, PA FIGURE 29: RELATIONSHIP OF SHOOT BIOMASS OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH AT TOP HORIZON GROWN IN STRATIFIED LOW WATER/HIGH PHOSPHORUS GREENHOUSE SYSTEM (GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 30: RELATIONSHIP OF DRY SHOOT BIOMASS OF THE GENOTYPES TO THE TOTAL TAPROOT LENGTH AT TOP HORIZON GROWN IN STRATIFIED LOW WATER/LOW PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 31: ROOT ALLOCATION FOR 4 TREATMENT COMBINATIONS SHOWED PLANTS UNDER HIGH PHOSPHORUS LOCALIZED RELATIVELY MORE ROOTS COMPARED TO THE PLANTS UNDER LOW PHOSPHORUS IN STRATIFIED WATER/PHOSPHORUS GREENHOUSE SYSTEM (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 32: THE RELATIONSHIP OF PERCENTAGE REDUCTION OF THE DRY SHOOT BIOMASS TO THE TOTAL TAPROOT LENGTH AT TOP HORIZON GROWN UNDER DROUGHT/HIGH P IN STRATIFIED GREENHOUSE SYSTEM CONDITIONS (2012 GREENHOUSE STUDY) AT UNIVERSITY PARK, PA FIGURE 33: SOIL VOLUMETRIC WATER CONTENT AT THE BEGINNING OF THE EXPERIMENT SHOWED NO DIFFERENCE IN DROUGHT PLOTS- RAINOUT SHELTER AND CONTROL PLOTS BETWEEN DEPTHS AT ROCK SPRINGS FIGURE 34: SOIL VOLUMETRIC WATER CONTENT THROUGHOUT THE EXPERIMENT WAS MUCH LOWER IN THE TOP HORIZON (0-15CM) IN THE RAINOUT SHELTER, AT ROCK SPRINGS FIGURE 35: SHOOT BIOMASS (A), NUMBER OF LEAVES (B), PER PLANT OF THE GENOTYPES GROWN IN THE RAINOUT SHELTER UNDER DROUGHT AND IRRIGATED REGIMES AT ROCK SPRINGS FIGURE 36: TOTAL ROOT LENGTH (CM) PER CORE UNDER DROUGHT AND IRRIGATED REGIMES. SOIL CORES WERE TAKEN 5CM FROM THE STEM IN THE RAINOUT SHELTERS AT ROCK SPRINGS FARM.35 FIGURE 37: SOIL VOLUMETRIC WATER CONTENT MEASURED WITH TDR 100 THROUGHOUT THE EXPERIMENT WAS MUCH LOWER AT THE TOP HORIZON (0-10CM) AT 18 DAP, AT SUSSUNDENGA RESEARCH FARM FIELD. INTERMITTENT RAINFALL AVOIDED STRONG DROUGHT FIGURE 38: SHOOT BIOMASS PER PLANT GROWN UNDER COMBINED WATER/PHOSPHORUS TREATMENT AT SUSSUNDENGA RESEARCH STATION, REPUBLIC OF MOZAMBIQUE. INTERMITTENT RAINFALL AVOIDED STRONG DROUGHT FIGURE 39: NUMBER OF LEAVES PER PLANT GROWN UNDER COMBINED WATER/PHOSPHORUS TREATMENT AT SUSSUNDENGA RESEARCH STATION, REPUBLIC OF MOZAMBIQUE. INTERMITTENT RAINFALL AVOIDED STRONG DROUGHT FIGURE 40: PHOSPHORUS CONTENT IN SOIL OF FOUR DEPTHS MEASURED FROM SOIL CORES TAKEN AT 42 DAP AT SUSSUNDENGA RESEARCH STATION, REPUBLIC OF MOZAMBIQUE. SOIL CORES WERE DIVIDED IN SECTIONS OF 10CM FOR EACH DEPTH FIGURE 41: DISTRIBUTION OF ROOT WITHIN THE SOIL PROFILE GROWN UNDER COMBINED WATER/PHOSPHORUS TREATMENTS AT SUSSUNDENGA RESEARCH STATION. ROOT CORE SAMPLES WERE TAKEN 5CM FROM THE PLANT STEM AT 42 DAP FIGURE 42: DRY SEED WEIGHT (A), DRY NUMBER OF PODS (B) AND DRY POD WEIGHT (C) PER PLANT GROWN UNDER COMBINED WATER AND PHOSPHORUS TREATMENTS AT SUSSUNDENGA RESEARCH STATION, REPUBLIC OF MOZAMBIQUE. INTERMITTENT RAINFALL AVOIDED STRONG DROUGHT. 39 FIGURE 43: SHOOT BIOMASS (A) AND NUMBER OF LEAVES (B) GROWN UNDER COMBINED WATER AND PHOSPHORUS TREATMENTS (HIGH AND LOW P & DROUGHT AND IRRIGATED REGIMES AT URBC, SA. INTERMITTENT RAINFALL PREVENTED STRONG DROUGHT FIGURE 44: PHOSPHORUS CONTENT IN SOIL OF FOUR DEPTHS MEASURED FROM SOIL CORES TAKEN AT 62 DAP AT URBC, REPUBLIC OF SA. SOIL CORES WERE DIVIDED IN SECTIONS OF 10CM FOR EACH DEPTH FIGURE 45: PERCENTAGE OF TOTAL PLANT P CONTENT (A) AND MILLIGRAMS OF TOTAL PLANT P CONTENT (B) UNDER DROUGHT AND IRRIGATED AT URBC, REPUBLIC OF SA vii

8 FIGURE 46: DISTRIBUTION OF ROOT WITHIN THE SOIL PROFILE GROWN UNDER DROUGHT/LOW AND HIGH PHOSPHORUS TREATMENTS (A) AND WELL WATERED & LOW AND HIGH PHOSPHORUS (B) AT URBC, REPUBLIC OF SA. ROOT CORE WERE SAMPLED 5CM FROM THE PLANT STEM AT 62 DAP.42 viii

9 List of Abbreviations RIL recombinant inbred line DAP Days after planting VWC Volumetric water content RWC Relative water content HP- High phosphorus LP Low phosphorus WW well watered/irrigated WS Water stressed/drought URBC- Ukulima Root Biology Center ix

10 Acknowledgments Thank you to Dr. Jonathan Lynch and Dr. Kathleen Brown for their mentorship and for giving me the opportunity to be a Penn State student and work in their Lab with a variety of skilled and experienced people. Many thanks to Bob Snyder who gave tireless assistance helping me with all Lab procedures. I want to thank all my Lab mates for helpful guidance while working in the field, Lab, and through data analysis. Special thanks to Jimmy Burridge for his readiness because this work would not have been possible without his help at every stage, from setting up experiments to the end of writing my thesis; and to Anica Massas who gave me unconditional help, setting up experiments, harvesting and processing data. x

11 THE UTILITY OF LATERAL BRANCHING OF THE PRIMARY ROOT FOR TOLERANCE OF PHOSPHORUS STRESS AND DROUGHT IN THE COMMON BEAN (Phaseolus vulgaris L.) 1. Introduction Inadequate precipitation and suboptimal nutrient availability are the principal constraints of staple crop production for smallholder farmers in most rain-fed lands. In most agricultural ecosystems, low availability of water and phosphorus decreases yields (Lynch & Brown, 2008); this factor leads to high prices of agriculture commodities as well as food insecurity. Irrigation and fertilization are obvious solutions to alleviate these constraints, however intensive irrigation and fertilization are not feasible options for many smallholder farmers and create negative environmental impacts in the USA and other developed countries (Vitousek et al., 2009). Furthermore, these two essential plant resources elements (water and phosphorus) have particular characteristics that can make them unavailable to plant growth; while water is more mobile and mainly found in deep soil horizons under drought conditions, phosphorus is immobile and is found primarily in the topsoil. The availability of phosphorus is also determined by a set of chemical and biological reactions among several elements in the soil (Adams et al., 2002) causing only a small amount of phosphorus in the soil to be present as orthophosphate. Other reasons for not using conventional P fertilization in many bean production systems in (mainly tropical) developing countries such as lack of knowledge of agricultural practices among farmers, soil P fixation, and the dubious sustainability of intensive fertilization (Lynch & Brown, 2008). In addition, high costs and scarcity of capital prevent many farmers from obtaining commercially available P fertilizer. All of these factors highlight the need to develop crop cultivars with greater tolerance to low P and water availability in order to increase food security for millions of smallholder farmers around the tropics. Phosphorus (P) deficiency is a major limiting factor in crop production and productivity, mainly in the tropics and subtropics (Lynch & Brown, 2008; Ramaekers et al., 2010). Low P availability is associated with several factors such as chemical and physical soil properties. Compared to other macronutrients, P is the least mobile in the soil and therefore the least available to plants (Hinsinger, 2001). The reactivity of orthophosphate (Pi) with iron and alumina oxide in soil result in compounds that are not available to plants (Lynch & Brown, 2008). Physical soil properties are determinants in P availability. Sandy soils have low total P content and have limited retention of fertilizer P (Deckers et al., 2001). Another limitation for P fertilization is its economic sustainability (Lynch & Brown, 2008) since periodic applications of fertilizers are needed in order to maintain high yields. An alternative approach to increase productivity in a soil where P is limited is to enhance crop adaptation to low P availability (Fageria et al., 2008). Mechanisms for improving P efficiency are associated with soil exploration and phosphate mobilization from unavailable P sources in the rhizosphere (Lynch & Brown, 2012). Traits that enhance topsoil foraging also improve P acquisition, such as shallower root growth angle (Lynch & Brown, 2012). Studies of the common bean have shown that genotypes with shallower root growth angle (RGA) performed better than genotypes with deeper root growth angle under P stress (Ho et al., 2005). Root growth angle also improves P acquisition due to decreased competition among roots in the same plant (Ge et al., 2000); this is an important trait for topsoil foraging and P acquisition in annual crops (Lynch & Brown, 2012). Basal root whorl number is another trait which affects phosphorus acquisition from low P soils. An increase in basal root whorl number is beneficial for soil exploration because it increases the vertical range of root deployment (Lynch & Brown, 2012). Basal roots are effective in P acquisition if seeds are planted at the proper depth (Walk et al., 2006); if seeds are planted below the topsoil layer with greatest P bioavailability, plants will allocate biomass to adventitious roots to compensate for inadequate acquisition of P by the basal root (Walk et al., 1

12 2006). A greater number of adventitious roots i s another trait that increases P uptake (Miller et al., 2003). Adventitious roots have shallow growth angles, large specific root length, and low tissue construction cost (Ho et al., 2005), which all improve the metabolic efficiency of P acquisition. Although adventitious roots are beneficial for P uptake, Walk et al. (2006) have suggested that increasing production of adventitious roots leads to high overall root competition in the rooting zone. Root hair formation plays an important role in phosphorus uptake since the greater number and length of root hairs that a plant has, the greater will be the volume of rhizosphere subject to P depletion (Lynch & Brown, 2012). Several studies have demonstrated that root hairs are more advantageous for phosphorus acquisition in nonmycorrhizal plants since in the mycorrhizal association the functioning of root hairs is replaced by hyphae (Lynch, 1995). Further studies also indicate that root hairs may influence the exudation of P mobilizing compounds such as organic acids which released into rhizosphere in exudates leads to metal chelation and subsequent desorption of Pi thereby increasing P availability (Ramaekers et al., 2010; Dick, 2012). Soil exploration at minimal metabolic cost is another important plant adaptation strategy for phosphorus acquisition efficiency (Lynch & Brown, 2008). One mechanism of reducing metabolic cost is the allocation of relatively more biomass to root classes such as adventitious that are metabolically efficient in P uptake and reduce competition within and among plants (Lynch & Brown, 2012). Another mechanism of reducing metabolic cost is the formation of root cortical aerenchyma, which replaces living cortical cells with air spaces, thereby reducing root respiration (Postma & Lynch, 2011), and the P from dead tissue is used for root elongation. Besides increased acquisition of soil P, aerenchyma formation is an important mechanism of recycling internal pools of acquired P for later use (Vance et al., 2003). Other strategies associated with P acquisition efficiency that are not directly influenced by improvement of root architecture, and are of interest in this review include phenology and mycorrhizal symbioses. Delayed phenology could be considered a strategy for P acquisition since roots would be able to explore the existing P for a relatively long period of time and thus increase the P efficiency (Nord et al., 2011). Association of plants with mycorrhizal fungi mediates the availability of phosphorus by improving the soil volume that is exploited beyond the depletion zone of the roots (Ramaekers et al., 2010). The common bean (Phaseolus vulgaris) is a legume that plays an important role in human diets. Beans are rich in proteins, carbohydrates, vitamins and minerals, which are necessary for human growth and development, and are generally affordable and easy to obtain. However the common bean is mostly grown on marginal lands with low fertilizer and water inputs. It is estimated that of bean production areas in developing countries 73% of the area in Latin America and 40% in Africa are prone to drought (Broughton et al., 2003); this situation causes severe reduction in yields (Wortmann & Kaizzi, 1998). Bean yields are also strongly affected by drought and low phosphorus availability and it is estimated that 60% of bean production in Latin America and approximately 44% in Africa are grown in soil under phosphorus and drought stress (CIAT, 1987; Wortmann & Kaizzi, 1998). The production of the common bean in developing countries has not achieved potential yields due to limitations in the production system (Hong & Long, 2000). Besides its importance in food security, common bean is also a cash crop for many famers in Africa. 2

13 Rationale and significance of the study Drought and low soil phosphorus availability are major constraints in common bean production and productivity in developing countries. Most agricultural systems rely on P fertilizers derived from rock phosphate to overcome soil P deficiency for food production and therefore food security (Bouwman et al., 2009). Since rock phosphate reserves are not renewable resources, the use of P fertilizers in sustainable ways in order to improve its efficiency is a major concern (Bouwman et al., 2009). Moreover the sustainability of using P fertilizer is questionable in the long-term due the depletion of natural resources. In developing countries, the high cost, inaccessibility, and lack of knowledge of how to manage fertilizers make them impracticable in terms of increasing agriculture yields. Therefore, the development of common bean varieties, which are less dependent on phosphorus fertilizers, is an important impetus of a new sustainable green revolution. This is especially true in developing countries where agriculture production is characterized by low inputs and very low yield. Root architectural traits that improve water and phosphorus acquisition efficiency are key tools in developing crop plants that can achieve higher yields in stressful soils. As previously noted, the negative consequence of increasing plant acquisition efficiency is a decrease in the total content of soil resources in the long-term. Still, the use of plants adapted to drought tolerance and low P availability is beneficial in most rain-fed lands of developing countries since these plants can maximize use of the total content of soil resources even though it is low and generally unavailable. In the particular case of the common bean, numerous advantages for the soil exist, such as the addition of organic matter derived from increased root biomass which thereby improves the overall soil fertility and reduces soil erosion (Henry et al., 2010a). The use of biotechnology, specifically genetically modified organisms, would make the agricultural production in developing countries even more impractical and uneconomical due to reliance on multi-national seed companies. As a local agricultural practice, most farmers in developing countries rely on their own seeds for subsequent crop seasons. The use of transgenic seeds would imply the need to buy seeds for every crop season. Another limitation of using GMO s is the environmental risk assessment, since there is no clear evidence of how they interact with non GMO s plants nor of their impact on the biodiversity of the area where they are planted. All of these aspects suggest that the hope for increasing productivity in most developing nations lies in crop adaption to low soil resources under classical breeding by improving root traits that increase nutrient and water acquisition efficiency. The identification of traits that improve the utilization of environmental resources would be beneficial for breeders in developing genotypes of the common bean that can achieve greater yields with the use of minimum resources. This particular study aimed to identify a particular trait (phene) that would improve crop growth with limited water and phosphorus availability. Studies have been conducted in order to determine the importance of root architectural tradeoff for water and phosphorus acquisition. Results from previous studies (Ho et al. 2005) have shown, that root traits that are beneficial in single stress scenarios maybe detrimental to accessing several limiting soil resources. Other results demonstrated that root foraging strategies which improve the acquisition of soil resources, such as P and water, have high yields when the resources are limited in the soil and thus fewer externally applied resources are required for plant growth (Beebe et al., 2006). Combining root architectural traits in multiline has potential for yield improvement under multiple edaphic stress environments (Henry et al. 2010b). 3

14 Since there is evidence that root architecture plays an important role for P and water acquisition and therefore, less work has been done in determining the utility of specific traits such as the utility of taproot laterals branching, this study offers a substantial contribution for manipulating root physiological traits for minimal resources use. The overall objective of this study was to assess the value of taproot branching under low phosphorus availability and water stress in two common bean phenotypes: one phenotype with many taproot lateral branches and another phenotype with few taproot lateral branches. The specific objectives were to: (1) compare performance of contrasting root phenotypes under water and phosphorus stress, (2) evaluate the response of lateral branching of the 2 phenotypes under suboptimal water supply, and (3) assess differences in root architecture which have an impact on water and phosphorus acquisition (by determining angle, basal root number, adventitious root number, diameter of taproot). We hypothesized that: (1) phenotypes with few taproot lateral branches will perform better in environments where water availability is concentrated in topsoil; (2) phenotypes with deep roots and many taproot lateral branches will perform better in drought environments; and (3) phenotypes that are adapted to both environments will have dimorphic characteristics. 2. Materials and Methods 2.1 Research approach The research consisted of evaluating common bean phenotypes (phaseolus vulgaris L.) with contrasting branching of the primary root in three different environments. Experiments were conducted at four locations: a) controlled experiments were conducted in 20 liters containers filled with solid media in a stratified water and phosphorus system in the greenhouse at University Park, PA, and b) a drought study was conducted at the field site in the rainout shelter at the Agricultural Research farm facilities in Rock Springs, PA (40º 43 N 77º 56 W); c) High and low phosphorus field sites at Ukulima Root Biology Center (URBC) 24º 31 E, 28º 7 S in the Republic of South Africa (RSA), and at Agriculture Research Institute of Mozambique (IIAM)-Sussundenga Research Station, Manica province in the Republic of Mozambique. 2.2 Greenhouse experiment University Park, PA Growth conditions Four greenhouse experiments were conducted; in the fall o f 2011, the spring of 2012, the summer of 2012 and the summer of In the first three trials, seeds of six genotypes comprising two phenotypes that differ in primary root branching were grown in a system of stratified water and phosphorus (Ho et al., 2005) as shown in Fig. 1. Seeds were sterilized in 10% NaOCl for 3 min, rinsed twice with distilled water, pre- germinated in rolls of brown germination paper soaked in 0.5 mm CaSO4, and placed in a dark growth chamber for 48h at 28ºC. The seedlings then were planted at a depth of 4 cm in pots of 30 cm diameter x 36 cm height which were filled with a mixture of (1:2:2 v/v) of sand, coarse grade A vermiculite, and coarse grade A perlite. The containers were separated into two layers, 0 to 8 cm and 8 to 36 cm depths by a round 2 mm nylon screen mesh impregnated with wax and coconut oil. A ratio of 30g wax/70g coconut oil was mixed and heated on a hot plate, until the wax melted completely, and then the mixture was embedded in each screen mesh, air-dried for 3 minutes, and then kept in the cold room. All trials were a randomized complete block design with four replicates, four treatments combining water and phosphorus levels with time of planting as the block: (i) control: high water and high phosphorus in both layers; (ii) stratified low phosphorus: high phosphorus in the top 0-8 cm, low phosphorus in the bottom 8-36 cm, and adequate water 4

15 in both layers; (iii) stratified low water: low water in the top 0-8 cm, adequate water in the bottom 8-36 cm, and high phosphorus in both layers and (iv) stratified low water and phosphorus: high phosphorus and low water in the top 0-8 cm and low phosphorus and adequate water in the bottom 8-36 cm layers. The fertilization in each pot was maintained using solid fertilizer that was mixed thoroughly with the media using a mixer and which contained (in grams per pot): 3 urea, 20 KNO3, and 8 micromax granular micronutrients (6.0% Ca, 3.0% Mg, 12.0% S; 0.10% B 1.0% Cu, 17.0% Fe, 2.5% Mn, 0.05% Mo and 1.0% Zn), obtained from Everris NA Inc. P.O.Box 3310, Dublin, OH Phosphorus levels were maintained by using solid-buffered Al-P according to the treatments of low and high P (Lynch et al., 1990), which was a concentration of less than 5µm considered as low P fertilization, and above 10µm as adequate phosphorus supply. The water supply in the system was maintained by placing two irrigation rings in each pot, one below the impregnated screen mesh and another on the top of the pot, which permitted separate irrigation of the two layers. The irrigation frequency depended on the pot water content in the pots that was monitored by TDR probes. In the bottom layers, the moisture content was maintained at a constant level in all treatments, while in the water stress treatment irrigation was withheld 7 days after planting (DAP). To avoid waterlogging, the top layer was irrigated first followed by watering the bottom layer to the saturation point. The drip ring irrigation used in the experiments (36 cm diameter) supplied ml of water in 3 minutes of irrigation, an amount sufficient to maintain adequate growth of the plants. The last greenhouse experiment was mainly focused on drought stress and included 14 genotypes that contrast in primary root branching, planted in the same growth conditions as the previous experiments, but differing in treatments that were imposed (i) adequate water in both layers: 0-14 cm and cm, and (ii) stratified low water: low water in the top 0-14 cm and adequate water in the bottom cm. 2.3 Field experiment at rainout shelter The rainout shelter at the Russell E. Larson Agricultural Research farm in Rock Springs, PA (40º43 N, 77º56 W) was used to impose drought stress of six genotypes of the two classes of phenotypes that contrast in primary root branching. The experiment was planted in the summer months of 2012 in two precipitation-sensing shelters covered with clear plastic film (0.184 mm) obtained from Griffin Greenhouse & Nursery Supply Company, Morgantown, PA, U.S.A. These precipitation sensing shelters prevented rainfall in the plots by moving over the drought treatments plots when precipitation was detected and withdrawing from the plot at the end of rainfall to expose the plots to the normal environment, permitting a controlled environment of drought stress. Two adjacent unsheltered plots served as control plots. The plots with shelters were irrigated by drip irrigation and received water regularly before imposing drought at 21 DAP, while the control plots were watered by drip irrigation and also received precipitation until the end of season. At the Rock Springs site each phenotype group was represented by three genotypes grown at each of the two treatments and arranged as a split-plot design for water treatments with water regime as the main plot, genotypes as a sub-plot, and time of planting as block. Each genotype was randomly distributed throughout the control and drought stress sub-plots planted in a three 3m row; one inside row served for data collection and the two outside rows as borders. The spacing between plants in row was 10 cm and the spacing between rows was 60 cm. The first block was planted on June 9, 2011 and the second block on June 11 and 12, Plants were harvested by using shovelomics methods at the flowering period (Kaeppler et al., 2011). 5

16 2.4 Field experiments at low and high phosphorus sites (URBC and Sussundenga) These two sites were used for field experiments in high and low phosphorus conditions as well as with and without irrigation. The trial at URBC was planted in the middle of the rainy season December 2012 and extended into a typical dry season The study consisted of 12 common bean genotypes, three known to have dense primary root branching, three as with sparse primary root branching and the additional six genotype were picked from the same population BAT477 X DOR 364 to increase the sampling size for comparison. The trial at URBC was planted in a split-split plot design, where phosphorus and water level were each the main factors. The soil type on site was a sandy, well-drained deep Clovelly soil (RSA classification), or Ustipsamment Entisol (USDA classification), with a low CEC (1.34). The following 4 treatments were imposed: 1 Low phosphorous with less irrigation (LP-LW); 2 Low phosphorus with irrigation (LP-WW); 3 High phosphorus and less irrigated (HP-LW) and; 4 High phosphorus and well irrigated (HP-WW). Each of the four main plots was blocked according to slope and time of planting in four replicates. The 12 genotypes were randomly planted in a plot of four 3.0 m rows spaced 0.60 m and the within-row plant spacing was 0.10 m. The two middle rows were for data collection and the outer rows served as border rows. There were buffer rows surrounding the entire field as well as 3.0 m buffer zone separating the high and low phosphorus plots. Block 1 was planted on 15 December 2012 and Block 2 on 16 December After planting there was regular occurrence of rainfall that guaranteed irrigation and seed germination. Later, after 3 weeks the entire field was drip irrigated. Drought was imposed 28 DAP, however the occurrence of frequent abnormal rainfall prevented a strong drought stress treatment. All plots were harvested at flowering starting with Block 1 from February A field site at IIAM- Sussundenga Research Station in the Republic of Mozambique, was used to impose phosphorous and drought stress treatments during the dry season of This site has particular characteristics of low soil P availability as well as less occurrence of rainfall during the dry season, therefore is considered a potential site for phosphorus and drought studies. Twelve common bean genotypes that differ in primary root branching (dense and sparse taproot branching) were grown in high and low phosphorus environment with and without irrigation. In this study a split-split plot design was used, where phosphorus and water levels were each main factors. The plots were blocked into 4 replicates and the twelve genotypes were randomly assigned to a plot within each block. Each genotype was planted in a three 3.0 m row spaced by 0.60 m and the within-row plant spacing was 0.10 m. The first block was planted on 01 March 2013 and the second block on 02 March There were b u f f e r plots surrounding the entire field as well as 6.0 m buffer zone separating the high and low phosphorus and 4.0m buffer zone separating the irrigated and no-irrigated plots. Three days after planting the field received 28 mm of precipitation, which was enough to ensure uniform germination. Then entire field was sprinkler irrigated every 7-12 days. Irrigation to the drought treatments was withheld at 23 DAP and the irrigated treatment plots continued to receive sprinkler irrigation every 7-12 day for the entire duration of the growing season, baring rainfall events. Although drought was imposed at 23 DAP, the season was characterized as abnormal because of intermittent rainfall and this factor unfortunately prevented a strong drought until the period of root harvest. Part of the plants in the plots was harvested at flowering period for root measurements, and the rest of plants were harvested at the end of the growing season for grain yield. 6

17 2.5 Plant material (common bean RILs used) All common bean (Phaseolus vulgaris L.) seeds used in these studies were propagated from seeds originally obtained from the International Center for Tropical Agriculture (CIAT, Cali, Colombia). Initially 18 genotypes from the RIL (recombinant inbred line) population DOR364 x BAT477 were selected for these studies (See table 1 for the RIL numbers used and their primary root branching density). DOR364 is a Mesoamerican genotype with an indeterminate erect bush growth habit with deep root architecture and is characterized as phosphorus inefficient (Liao et al., 2001). BAT477 is also Mesoamerican genotype with an indeterminate erect bush growth habit and is tolerant to drought due to high rooting density at the deep horizons (Liao et al., 2001). The criteria of categorical grouping of the DOR364 x BAT477 genotypes were based on phenotypic characterization of taproot lateral branching density, from a previous experiment planted at Rock Springs (experiment of James Burridge) during the summer season of In addition these six genotypes were tested in a pilot experiment, and their categorical groups were confirmed before the trials. Due to limited space in the greenhouse at the University Park, PA and the rainout shelter at the Rock Springs only six genotypes were used in the trials, three characterized as with dense lateral branches and other three as with sparse lateral branches. For the field experiments at URBC and IIAM- Sussundenga Research Station twelve genotypes were used and in the final greenhouse trial 14 genotypes were used to impose only drought stress, and were categorized into two groups (dense and sparse primary root branching density). 2.6 Plant measurements Growth and plant biomass measurements For the greenhouse studies growth measurements were taken during the growing season. Leaf appearance rate and leaf size were taken after 14, 21, and 35 DAP to determine differences in length, width, and number of leaves in control and drought & phosphorus treatments pots. The day before harvest, leaves were collected and measured for leaf area using a leaf area meter - LI-3000C portable area meter (Licor Bioscience, Inc., 4421 Superior Street Lincoln, Nebraska, USA). At the Rock Springs site, leaf count was taken the day of harvest at 44 DAP, and leaves were collected for leaf area measurements using a portable leaf area meter (LI-3000C). At the URBC site leaf appearance rate and leaf size were taken weekly after 21 DAP, while at IIAM Sussundenga Research station only leaf appearance rate was taken at 14 and 35 DAP. In both sites a subsample of t h e entire plant from each plot were dried and preserved for phosphorus analysis. Shoot and root biomass were harvested at DAP for the greenhouse trials. Shoot tissue was dried at 60ºC until constant mass and weighed. Dry tissue was ground and ashed at 490ºC for 10-12h in a muffle furnace and analyzed for phosphorus content using spectrophotometry (Murphy & Riley, 1962). Roots were harvested by root type and horizon. The entire primary root from both the top and bottom layers was sampled for primary root lateral branching analysis, while a subsample of a single basal root from the both layers was collected for root length and lateral branching analysis. All root samples were preserved in ethanol (25% v/v) immediately after harvest for subsequent root scanning. After scanning, root images were analyzed with WinRhizo Pro V 7

18 2009b (Regent Instruments Canada Inc., 2672 Chemin Ste-Foy Quebec QC G1V 1V4 Canada) for total root length (cm), total root surface area (cm 2 ), average root diameter (mm) and root length by diameter class. The remaining root biomass from each horizon was stored separately and dried at 60ºC until constant mass and then weighed for root biomass allocation assessment in each layer and treatment. At IIAM-Sussundenga research Station, Rock Springs and URBC sites, biomass samples were harvested at 38, 44, and 66 DAP respectively. Three plants of each plot were harvested and dried at 60ºC and weighed. Dried plants were ground, ashed then analyzed for phosphorus content as described above. The root crowns of three subsamples in each plot were evaluated. Root crown examination included basal root growth angle (BRGA), basal root whorl number (BRWN), basal root number (BRN), number of representative adventitious roots (greater than 1.5mm diameter), stem diameter, and taproot diameter. The primary root of each subsample was preserved in ethanol (25% v/v) for later scanning. After scanning, root images were analyzed with the WinRhizo Pro V 2009b as described above for apparent total root length and root surface area as well as apparent average root diameter and root length by root diameter class. 2.7 Plant water status In the greenhouse at University Park, plants were grown in summer as well as in the winter season. Several measurements were taken in both control and the drought treatment pots to determine the plant water status. Leaf gas-exchange measurements were determined at 21, 28, and 36 DAP with a LiCor 6400 portable photosynthesis gas exchange system (Licor Bioscience, Inc., 4421 Superior Street Lincoln, Nebraska, USA). The day before the harvest, which ranged between DAP, pre-dawn leaf water potential was measured with a Scholander leaf pressure chamber (Wescor Scientific Inc. M 615 Scholander chamber 0-40 bar) in active leaves in all pots. On the same day after pre-dawn measurements, relative water content (RWC) was determined in both control and drought treatments. First, six 2.54 cm diameter leaf discs were collected randomly from each plant and immediately sealed in a plastic bag for fresh weight (FW) determination. Then the discs were immersed in water 18-20h for turgid weight (TW) measurement and finally dried at 60ºC until constant mass for dry weight (DW). Leaf relative water content was determined by ((FW-DW)/(TW-DW))*100 ( Turner, 1981). In all field sites except Rock Springs where only leaf gas-exchange was taken, other measurements related to the plant water status were not determined due the intermittent rainfall that prohibited strong drought stress. 2.8 Root distribution in the soil At the Rock Springs, URBC and IIAM-Sussundenga sites, root distribution in the soil profile was determined from soil cores taken 2-3 days before root harvest (at flowering). At Rock Springs all 6 genotypes were sampled by taking 3 subsamples of soil cores in each treatment area for the control and drought plots. At URBC, 3 subsamples of cores of three representative genotypes of each phenotypic class (dense and sparse lateral branches) were taken, while at IIAM- Sussundenga 2 subsamples of cores in each plot and 3 representative genotypes of each phenotypic category were chosen to be sampled. Cores were taken in-row between two neighboring plants at a distance of 5cm between cores. The cores were divided in depth increments of 10 cm, and depending on the depth of profile reached, 3-6 sections were sampled, washed, scanned, and analyzed for total root length and root length by diameter classes using the WinRhizo Pro V 2009b. Samples from IIAM-Sussundenga site were preserved in alcohol (25% 8

19 v/v) immediately after washing for later scanning at URBC and for analysis with WinRhizo at University Park, PA. 2.9 Soil Moisture measurements For the greenhouse studies at University Park, volumetric water content was monitored in both layers (as shown in figure 1) with a portable TDR-100 time-domain reflectometry system (Campbell Scientific Inc., Logan, UT, USA). Forty TDR probes were used in all experiments; two were placed in each pot at the top and bottom horizon in control and drought treatments. Measurements of each TDR probe were taken twice per week, and supplementary irrigation was made if the VWC in control pots was below 15% and 30% at the top and at the bottom horizons, respectively. Irrigation in drought treatments was withheld 7 DAP in the top layer, and in the bottom, water content was kept between 10-15% since the preliminary experiments showed that this amount was enough to induce drought stress. At the URBC site field, soil VWC was collected continuously (every 30min) using a CR100 data-logger and TDR system (Campbell Scientific Inc., Logan, UT, USA) in control and drought treatments of Block 1. In Block 2 a portable TDR100 was used to manually monitor VWC, because of cable length limitation of the data-logger. At Rock Springs and IIAM- Sussundenga a portable TDR100 was used to monitor the VWC. TDR probes were placed at depths of 15 and 30cm and each treatment plot had 3-4 locations per block. Samples measured using the soil gravimetric water content method was compared to the TDR100 measurements. Soil cores for soil water content were taken to determine the gravimetric water content and later converted to volumetric water content using bulk density of the soil of 1.28, 1.5 and 1.6g /cm 3 at the URBC, IIAM-Sussundenga and Rock Springs sites respectively, estimated from website ( My Agriculture Information Bank) Phosphorus content in the soil Phosphorous content of media in green house studies was determined by collecting leachate samples at 28 DAP from each pot and analyzed for its concentration (Murphy & Riley, 1962). At URBC and IIAM-Sussundenga research station soil cores were taken in each treatment plot (low and high P) and divided into cm segments. Phosphorus concentration was measured by depth-bray No I extract method using spectrophotometer (Cross & Schlesinger, 1995) Statistical Analysis The statistical software Minitab Ver. 16 (Minitab Inc. State College, PA, USA) was used for all data analyses. Data from the greenhouse studies were analyzed as a randomized completeblock design, and a fixed model analysis of variance (ANOVA) was used for leaf area, shoot biomass, root biomass, relative root biomass in each horizon, where phenotype, phosphorus and water treatments, and replicates were the independent variables. ANOVA was also performed for total root biomass, total root length and relative tap root lateral branching by classes, where phenotype, phosphorus and water treatment, pot depth and replicate were independent variables. Regression analysis was performed for the total taproot length in the top and bottom horizons. Field studies at Rock Springs, URBC and IIAM were analyzed as a split-split-plot design and ANOVA was used for number of leaves and shoot biomass, where phenotype, phosphorus and water treatments are fixed variables and the block is a random variable. Regression was used for total apparent root length by class and other traits as mentioned in the results. In all cases significance level was set at 0.05 and all data were tested for normality 9

20 before ANOVA. 3. Results Greenhouse study in controlled environment stratified water treatment at University Park, PA. 3.1 Soil moisture availability Although drought was imposed 7 DAP, soil moisture content at both depths (0-8 and 8-32 cm) at the beginning of the measurements (10 DAP) did not differ between treatments and between blocks. All pots were watered to the saturation point at 12 DAP due to the necessity of marathon application (a systemic insecticide effective against trips). This factor increased the moisture content in the pots; it began to decline at 17 DAP. However, at the start of measurements, there was a difference between the moisture content in the top and bottom layers in the stratified water system. At harvest there was a s i g n i f i c an t difference between well-watered and drought treatments (p<0.000, F= 55) as well as a difference in the depths in the two sections of the stratified water system (p<0.000, F=21); See Figures 3 e 4 for soil VWC of the drought and control treatments at the end and throughout the experiment. The moisture content of the bottom horizon (8-36 cm) in the drought treatment differed significantly from the moisture content in the well-watered treatment because irrigation in the drought pots at the bottom horizons was completely stopped two weeks before harvest in order to impose a strong drought stress. 3.2 Plant water status Plant water status measured at 35 DAP with pressure bomb showed that plants grown under drought were strongly stressed compared with the well-watered in stratified water greenhouse system at University Park, PA (Figure 5). An increased taproot length in the lower horizon was also correlated with leaf water potential. Plants grown under drought with greater taproot length had better plant water status than those with less total taproot length under the same conditions. There was a significant effect of total taproot length (p= ) and water treatment (p<0.0000) on leaf water potential which may have implication on better water status and an increased taproot lateral formation (Figures 6 and 7). Leaf water relative content and gas exchange measurements revealed that there was strong drought stress imposed on the plants at the harvest period (Figures 8 and 11). An increased taproot length in the lower horizon was positively correlated with leaf water relative content as well as with photosynthetic assimilation. Plants grown under drought with greater total taproot length showed better water status. There was significant effect of water treatment (p = ) which may influenced an increase of total root length resulting in better plant water status (leaf water relative content), as well as an effect of total taproot length (p = ) on photosynthesis activity which may have a beneficial effect on water acquisition (Figures 9 and 12). 3.3 Root localization differs between drought and control treatments There was a significant effect on water treatment on total root allocation in the lower horizon (p<0.000, F=33.31). Plants grown under drought localized more roots at lower horizon compared to the well-watered plants. However the total root weight in the system (top and bottom horizon) was greater in the plants grown under irrigated than the plants under drought conditions. 10

21 3.4 Taproot lateral formation may influence growth when water is limited Shoot biomass assessed at the harvest showed greater reduction of total shoot biomass in drought treatment compared to well-watered treatment in the stratified water greenhouse system (Figure 14). In addition, an increased taproot length in the lower horizon was strongly correlated to shoot biomass production at 36 DAP. Plants grown under drought with greater total taproot length had better performance in total shoot biomass (Figure15). There was significant effect of water treatment (p< ) on shoot biomass reduction and total taproot length (p= ) on total shoot biomass production, which may have increased plant water efficiency. Shoot biomass was also correlated with the total taproot length in the bottom horizon when partitioned into root classes. An increased taproot length in the bottom horizon of the main axis, first order and second order of plants grown under drought, had a positive effect on increasing shoot biomass (Figures 18,19 and 20), possible an indication of resources acquisition efficiency. Greenhouse study in controlled environment stratified water/phosphorus system at University Park, PA. 3.5 Plant water and phosphorus status Leaf water relative content measured the day before harvest showed no significant difference between plants grown under drought phosphorus and well-watered phosphorus conditions. Overall plants grown under high phosphorus in the both environments (drought and irrigated) performed better compared to low phosphorus (Figure 21). An increased taproot length in the top horizon was correlated with leaf water relative content (p= ). Plants grown in combined drought/high phosphorus treatment with greater total taproot length had better water status compared to those with less total taproot length under the same conditions (Figure 22). Gas exchange measurements also revealed that plants grown in drought conditions in both high and low phosphorus treatments had low performance in photosynthetic activity compared to those grown in well irrigated under the same treatments (Figure 24). Greater total taproot length at top horizon was correlated to high rate of photosynthesis assimilation (p= ). Plants with dense taproot lateral grown in combined drought and phosphorus treatment had better performance compared to those with sparse taproot lateral under the same treatments. There was significant effect of water treatment (p= ) and phosphorus (p= ) treatments on photosynthesis activity, which may have a beneficial effect on water acquisition (Figure 25). Phosphorus analysis from leachate samples taken at 32 DAP showed high P concentration in combined drought/high P treatments than in the well-watered/high P treatments pots in stratified water/phosphorus system (p<0.000, Figure 26). The higher P concentration in the combined drought/high P treatment may attributed to the less frequency of irrigation in the pots, which avoided the leaching process. Phosphorus analysis from plant tissue in the stratified water/phosphorus system revealed significant difference of percentage of total plant P content between high and low P treatments (p<0.000) under drought and irrigated conditions. Phosphorus content was much lower in combined drought and low phosphorus treatments (Figure 27). 11

22 3.6 Biomass accumulation Plant performance assessed by recording shoot biomass production showed significant effect of water treatment (p<0.000) and P effect (p=0.013) on total shoot biomass in the stratified water and phosphorus greenhouse system (Figure 28). Furthermore, an increased taproot length in the top horizon was positively correlated to shoot biomass production in combined drought and high phosphorous treatment (Figure 29). Greater taproot length in the top horizon was also highly correlated to the shoot biomass in plants grown under drought/low phosphorus (r 2 = 0.72; Figure30). Root allocation assessed in both top and bottom horizons showed significant effect of phosphorus (p<0.000) on total root allocation in the bottom horizon. However, plants grown under wellirrigated/high phosphorus allocated more in the bottom horizon (Figure 31). Field experiment at Agriculture Research Farm in Rock Springs, PA summer Soil water availability Soil volumetric water content started to decline at 29 DAP after initiating the drought under the rainout shelters compared to the neighboring control plots. Soil VWC measured continuously by portable TDR in the top horizon (15cm), showed constantly a decrease in water content throughout the experiment (Figure 34). The VWC from TDR confirmed no difference before initiating drought in the rainout shelter (Figure 33). 3.8 Plant performance Overall plant performance was assessed by growth measurements taken at harvest. Shoot biomass (p<0.000) and number of leaves (p = 0.000) were statistically different between drought and wellirrigated treatments (Figure 35). There was a growth reduction caused by imposition of drought of 50% of shoot biomass and 47.6% of number of leaves, suggesting that the strong drought stress imposed in this experiment had sufficient effect on growth reduction. Root distribution from soil cores Total root length from soil cores showed greater total root length at harvest in drought plots compared to well-watered plots (Figure 36). At harvest root length within the profile showed significant difference between drought and well-watered treatment (p<0.000), and it was greater in the top 10 cm horizon of the drought plots. Overall there was greater total root length in the first 10 cm of the drought plots than in the 10 cm divisions below in drought plots. Contrary to the well-watered plots, greater total root length was observed in 10cm divisions bellow the top 10 cm horizon. Field Experiment at Sussundenga Research station, Republic of Mozambique. 3.9 Soil water availability Water availability at Sussundenga research station field remained almost at a constant level throughout the experiment. Heavy rain fell 2 DAP and continued intermittently throughout the experiment, preventing the development of drought stress (Figure 37). 12

23 3.10 Phosphorus content in plant tissue and soil Phosphorus analysis from plant tissue grown under combined water/phosphorus treatment revealed significant difference in P content between plants grown in drought/high P and wellwatered/high P plots (p<0.000; F=47.30). Soil phosphorus content measured from soil cores showed greater phosphorus content in the combined well watered/high P and combined drought/high P plots. Overall there was greater phosphorus content in the topmost 10 cm soil horizon in the combined drought/high P plots (Figure 40) Plant performance Plant performance was assessed by recording the total number of leaves and shoot biomass at 42 DAP, and number of pods, dry pod weight, and seed weight per plant at maturity. General linear model analysis revealed that there was no significant effect of drought total shoot biomass (p=0.584), number of leaves (p=0.93), number of pods (p=0.78, F=0.08), pod weight (p=0.15, F=2.14), or seed weight (p=0.071) Figures, 38,39,42. However, there was a significant effect of phosphorus stress which reduced the total shoot biomass (p<0.000), number of leaves (p<0.0000), number of pods (p<0.000), dry pod weight (p<0.000), and dry seed weight (p<0.000), Figure Root distribution from soil cores Similar to results from the drought study at Rock Springs, total root length from soil cores in the Sussundenga field study showed greater total root length in high P plots compared to low P plots. In general, total root length was greater in the topmost 10 cm, and phenotypes with dense taproot lateral had greater total root length in deeper horizons (40 cm) in high P plots, compared to the phenotype with sparse taproot lateral branches (Figure 41). Field Experiment at URBC, Republic of South Africa 3.13 Phosphorus content in soil and plant tissue Soil core samples were taken to determine P content with depth. Contrary to results from the Sussundenga field experiment, which showed greater total soil phosphorus content in the well watered/high P plots, the water/phosphorus study at URBC, had the greatest total P content in combined drought/high P plots. Phosphorus content was greater in the first 10 cm segments; followed to the next 10 cm division bellow in the drought/ high P plots. At deeper horizons, the scenario was different, since the greater phosphorus content was observed in well watered/high P plots at depths of 30 cm and 40 cm (Figure 44). Analysis of phosphorus content from plant tissues showed greater percentage of P concentration in plants grown under high P (in either drought or well-watered plots) compared to the low phosphorus treatment (Figure 45) Plant performance Plant performance assessed by shoot biomass and number of leaves under combined water/phosphorus treatment showed a decrease (p<0.000) for shoot biomass and (p=0.017) for number of leaves (Figures 43). Furthermore, phenotype categories were not different in shoot biomass (p=0.389, F=0.59), but did differ for the number of leaves which less in the drought/low P treatment (p=0.043, F=4.15). Shoot biomass and leaf number were slightly greater in the well 13

24 watered/high phosphorus treatments compared to under the drought/low phosphorus conditions Root distribution from soil cores Soil cores show similar total root length for all phenotypes at harvest. Overall the phenotype with dense taproot lateral branches had slightly more total root length in deeper horizons. Plants under drought/high P as well as irrigated/high P treatments, greater total root length was observed in the topmost 10 cm and last 10 cm division, which corresponded to 50 cm depth. In general, plants grown under drought showed greater total root length in all segments of cores compared to wellwatered plants (Figure 46). 14

25 4. Discussion The main objective of this study was to test the hypothesis that phenotypic variation in the formation of taproot lateral branches influences the acquisition of soil resources (water and nutrients), particularly when these resources are limited. Theoretical analysis from modeling has shown that increasing the root depth of maize to the rooting depth of sorghum would contribute to yield increase in harsher drought environments (Sinclair & Muchow, 2001). Unfortunately, similar work using the same approach is limited in legumes, although rooting depth is one of the three traits that potentially contribute to drought tolerance among the several traits (Ludlow & Muchow, 1990). We hypothesized that phenotypes with dense lateral branching from the taproot would adapt better to drought and that phenotype with sparse lateral branching from the taproot would adapt better to low phosphorus environments. To test our hypothesis plants were grown in greenhouse and field experiments using recombinant inbred lines of common bean that were previously selected and categorized as having either dense and sparse taproot lateral branching. Increasing root depth often leads to increased drought tolerance (Kashiwagi et al., 2006; Vadez et al., 2008). Our results from t h e greenhouse study 2013 in stratified water system support this assumption. In fact the relationship of the total taproot length in the bottom horizon grown under drought agree with our data from the stratified water greenhouse system where we run regression with shoot biomass. This strong correlation reveals that plants were able to perform under limited water supply thereby increasing the water uptake, which resulted in a relative increase in shoot biomass. Greater shoot biomass provides an advantage for survival in drought condition as it stated by (Shishkova, et al., 2013). Furthermore, our results supported that plants under water stressed increased total taproot length at deeper profiles; as some studies have demonstrate, that the basic difference between shallow and deeper rooted genotypes is expressed in respective of stress conditions imposed (Vadez et al., 2008). In addition, some of the reported studies clearly showed that the presumed relation between rooting traits and drought tolerance is in some way overlapped by escape mechanisms related to the phenology of the genotypes (Kashiwagi et al., 2006). Therefore, the contribution or differences in root phenotype to drought tolerance can only be truly assessed once genetic variation in root is found sharing a similar phenology (Vadez et al., 2008). As with the results from the stratified water greenhouse study, we found good correlation between deeper-rooted phenotype and depth of rooting at the Rock Springs site. Plants with dense taproot lateral under water deficit localized relatively greater root length below 40 cm compared to the plants with sparse taproot lateral branching. This is in agreement with what some studies have shown supporting that a small amount of roots in deeper layers where water is plentiful would be enough to fully supply water to the plants when the topsoil is dry (Sharp & Davies, 1985), thereby improving water uptake. Our interpretation is that the stronger drought the plant faces the more roots tend to allocate at deeper profiles as a result of survival mechanism of foraging water. In summary, the effect of drought on growth and photosynthesis depended somewhat on the level of taproot lateral branching. Since we imposed adequate drought in the stratified water greenhouse system (2013 greenhouse study) and in the field experiment at Rock Springs, we assumed the responses we observed mostly reflect the capability of plants with dense taproot branching to acquire water in deeper horizons under terminal drought. In the stratified water greenhouse system we were able to show that an increasing root length in the bottom horizon under drought stress was related to increase shoot biomass. Clearly from our results we would suggest more research testing whether the differences in root 15

26 length, branching level as well as the size of lateral branches (and related traits such as metaxylem vessels) has an influence in water uptake. How much water is taken up after imposition of drought would evidently related to the root length density and its size, since in our experiments we have observed that the more a plant is imposed a strong drought the more fine roots it grow. Whether dense taproot lateral branching has a benefit in increasing shoot biomass as intermittent drought occurs is still unknown and needs further investigation since there is little published documentation regarding the matter. Such work would be extremely important in the case of most legumes especially common bean which is usually grown in the marginal rain-fed lands mostly in Africa where most smallholder farmers face episodes of drought throughout the cropping cycle. 16

27 Annex: Figures and table DOR 365 x BAT 477 RIL number Taproot branching characterization/clasification 5 Sparse 11 Dense 13 Dense 14 Sparrse 16 Dense 18 Sparse 19 Dense 32 Dense 35 Dense 40 Sparse 44 Sparse 48 Sparse 54 Sparse 881 Dense Table 1: The phenotype grouping of the recombinant inbred lines (RILs) from DOR 364 x BAT 477 population used in the study. Sparse denotes phenotype with sparse taproot lateral branching and dense phenotype with dense taproot lateral branches. A B D C Figure 1: Root morphological components of the common bean. A) Adventitious root; B) Basal root; C) Taproot; D) Lateral roots. Root crown was obtained from field experiment at Sussundenga research station, Republic of Mozambique. 17

28 irrigation ring d = 30 cm Greenhouse treatments Control Stratified Stratified Stratified P H2O P & H2O (HP-WW) (LP-WW) (HP-WS) (LP-WS) 0-8cm irrigation ring TDR wax layer Top horizon H 2O content % 15-20% 5-11% 5-11% P level HP HP HP HP Bottom horizon H 2O content >26% >26% 15-20% 15-20% P level HP LP HP LP 8-28cm TDR Figure 2: Stratified water and phosphorus greenhouse system. top 0-8 cm and bottom 8-36 cm horizons are separated by a 2mm screen mesh impregnated with wax and coconut oil, and an irrigation ring below the screen mesh which permitted to irrigate the layers independently. Figure was recreated from (Ho et al. 2005), and is not drawn to scale. 18

29 b Figure 3: Soil volumetric water content at 36 DAP in the drought and control treatments of the two depths (0-8 and 8-36 cm) in the stratified water greenhouse system at University Park, PA. Moisture content was significantly different at the end of the experiment. Figure 4: Soil volumetric water content of the drought and control treatments at two depths (0-8 and 8-36 cm) in the stratified water greenhouse system throughout the experiment at University Park, PA. Irrigation was stopped two weeks before harvest in drought treatments to impose strong stress, which reduced the total biomass. 19

30 Figure 5: Plant water status measured at 35 DAP with pressure bomb under drought and wellwatered treatments in stratified water greenhouse system at University Park, PA Leaf H2O potential (MPa)/plant taproot length bottom (8-36cm) Figure 6: Leaf water potential measured at 35 DAP with pressure bomb. Plants were grown under drought treatment in stratified water greenhouse system (2012 greenhouse study) at University Park, PA. 20

31 leaf H2O potential (MPa)/plant taproot length bottom(cm) Figure 7: Leaf water potential measured at 35 DAP with pressure bomb. Plants were grown under well-watered treatment in stratified water greenhouse system (2012 greenhouse study) at University Park, PA. Figure 8: Leaf relative water content measured the day before harvest showed difference between the drought and well-watered plants in the stratified water greenhouse system (2013 greenhouse study) at University Park, PA. 21

32 Figure 9: Relationship of leaf relative water content of the genotypes to the total taproot length at the bottom horizon 35 DAP, showed a strong correlation under drought plants in the stratified water greenhouse system (2013 greenhouse study) at University Park, PA. Figure 10: Relationship of leaf relative water content of the genotypes to the total taproot length at the bottom horizon at 35 DAP showed a weak correlation under well-watered plants in the stratified water greenhouse system (2013 greenhouse study) at University Park, PA. 22

33 Figure 11: Photosynthetic assimilation measured with Licor DAP showed plants under drought stress had less photosynthetic activity than the well-watered plants in the stratified water greenhouse system (2013 greenhouse study) at University Park, PA. Figure 12: The relationship of photosynthetic assimilation of the genotypes to the total taproot length in the bottom horizon grown in the stratified water greenhouse system under drought conditions (2013 greenhouse study) at University Park, PA. 23

34 Figure 13: The relationship of photosynthetic assimilation of the genotypes to the total taproot length in the bottom horizon grown in the stratified water greenhouse system under irrigated conditions (2013 greenhouse study) at University Park, PA. Figure 14: Shoot biomass at 36 DAP showed significant differences between the drought stressed and well-watered plants in the stratified water greenhouse system (2013 greenhouse study) at University Park, PA. 24

35 Figure 15: The relationship of dry shoot biomass of the genotypes to the total taproot length in the bottom horizon grown in the stratified water greenhouse system under drought conditions (2013 greenhouse study) at University Park, PA. Figure 16:The relationship of dry shoot biomass of the genotypes to the total taproot length in the bottom horizon grown in the stratified water greenhouse system under well-watered conditions (2013 greenhouse study) at University Park, PA. 25

36 Figure 17: The relationship of percentage reduction of dry shoot biomass of the genotypes to the total root length in the bottom horizon grown in the stratified water greenhouse system under drought conditions at University Park, PA. Figure 18: The relationship of total shoot biomass to the total taproot length of the main axis in the bottom horizon grown in the stratified water greenhouse system under drought conditions (2013 greenhouse study) at University Park, PA. 26

37 Figure 19: The relationship of total shoot biomass to the total taproot length of the first order lateral roots in the bottom horizon grown in the stratified water greenhouse system under drought conditions (2013 greenhouse study) at University Park, PA. Figure 20: The relationship of total shoot biomass to the total taproot length of the second order lateral roots in the bottom horizon grown in the stratified water greenhouse system under irrigated conditions (2013 greenhouse study) at University Park, PA. 27

38 Figure 21: Leaf relative water content measured the day before harvest showed no difference between drought stressed and well-watered plants in the stratified water/phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. Figure 22: The relationship of leaf relative water content of the genotypes to the total taproot length at bottom horizon under drought and high phosphorus conditions measured the day before harvest, showed positive correlation in the stratified water/phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. 28

39 Figure 23: The relationship of leaf relative water content of the genotypes to the total taproot length at bottom horizon under drought and low phosphorus conditions measured the day before harvest, showed positive correlation in the stratified water/phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. Figure 24: Leaf photosynthetic CO2 assimilation measured under drought and low phosphorus treatment in the stratified water/phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. 29

40 Figure 25: The relationship of photosynthetic assimilation CO2 of the genotypes to the total taproot length in the top horizon grown in the stratified water/phosphorus greenhouse system under drought and high phosphorus conditions (2013 greenhouse study) at University Park, PA. Figure 26: Phosphorus concentration (μm) from leachate samples taken 32 DAP showed the highest concentration in the drought and high P treatment and lowest concentration in the low P treatment in the stratified water/phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. 30

41 a Figure 27: Percentage of plant P concentration from plant tissues showed the highest content in control & high P treatment and lowest concentration in the drought and low P treatment in stratified water/phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. Figure 28: Shoot biomass showed significant difference between the drought and well-watered plants in stratified water & phosphorus greenhouse system at University Park, PA. 31

42 Figure 29: Relationship of shoot biomass of the genotypes to the total taproot length at top horizon grown in stratified low water/high phosphorus greenhouse system (greenhouse study) at University Park, PA. Figure 30: Relationship of dry shoot biomass of the genotypes to the total taproot length at top horizon grown in stratified low water/low phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. 32

43 Figure 31: Root allocation for 4 treatment combinations showed plants under high phosphorus localized relatively more roots compared to the plants under low phosphorus in stratified water/phosphorus greenhouse system (2012 greenhouse study) at University Park, PA. Figure 32: The relationship of percentage reduction of the dry shoot biomass to the total taproot length at top horizon grown under drought/high P in stratified greenhouse system conditions (2012 greenhouse study) at University Park, PA. 33

44 Figure 33: Soil volumetric water content at the beginning of the experiment showed no difference in drought plots- rainout shelter and control plots between depths at Rock Springs. Figure 34: Soil volumetric water content throughout the experiment was much lower in the top horizon (0-15cm) in the rainout shelter, at Rock Springs. 34

45 Figure 35: Shoot biomass (a), number of leaves (b), per plant of the genotypes grown in the rainout shelter under drought and irrigated regimes at Rock Springs. Figure 36: Total root length (cm) per core under drought and irrigated regimes. Soil cores were taken 5cm from the stem in the rainout shelters at Rock Springs farm. 35

46 Figure 37: Soil volumetric water content measured with TDR 100 throughout the experiment was much lower at the top horizon (0-10cm) at 18 DAP, at Sussundenga research farm field. Intermittent rainfall avoided strong drought. Figure 38: Shoot biomass per plant grown under combined water/phosphorus treatment at Sussundenga research station, Republic of Mozambique. Intermittent rainfall avoided strong drought. 36

47 Figure 39: Number of leaves per plant grown under combined water/phosphorus treatment at Sussundenga research station, Republic of Mozambique. Intermittent rainfall avoided strong drought. Figure 40: Phosphorus content in soil of four depths measured from soil cores taken at 42 DAP at Sussundenga research station, Republic of Mozambique. Soil cores were divided in sections of 10cm for each depth. 37

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