Analysis of Water Supply of Plants Under Saline Soil Conditions and Conclusions for Research on Crop Salt Tolerance

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1 J. Agronomy & Crop Science (28) ISSN REVIEW ARTICLE (SALINITY STRESS) Analysis of Water Supply of Plants Under Saline Soil Conditions and Conclusions for Research on Crop Salt Tolerance U. Schleiff Independent Expert for Irrigation & Salinity Fertilizers & Crops Soils & Environment, Wolfenbuettel, Germany Keywords brackish irrigation; crop salt tolerance; rhizosphere; root morphology; soil salinity; water uptake Correspondence U. Schleiff Independent Expert for Irrigation & Salinity Fertilizers & Crops Soils & Environment, PO Box 1934, D Wolfenbuettel, Germany Tel.: +49() Fax: +49() Accepted November 28, 27 doi:1.1111/j x x Abstract In the dry areas of the world there is an increasing pressure to apply low quality brackish waters for plant irrigation (agriculture, horticulture, landscape greening). Consequently there is a demand to improve salt tolerance of conventional crops and to develop adequate irrigation techniques too. The efforts in the past decades to approach the understanding of salt stress mechanism by focusing on biochemical and physiological research were disappointing with respect to progress for crop growth and yields under saline soil conditions. However, it is generally agreed by all disciplines involved in research for crop salt tolerance that under saline soils conditions the reduced water supply of crops is the most critical growth factor. The paper presents some model calculations and field investigations that demonstrate the effect of root water uptake on the salinity of the root surrounding soil fraction (rhizospheric soil). It is shown that root hair length and rhizospheric soil volumes are factors most relevant for understanding crop salt tolerance, when growing in soils. It is postulated that short root hairs contribute to a lower salt tolerance (onions), whereas long root hairs enhance water uptake from saline soils and crop salt tolerance (rape). As interactions between roots and soil contribute to the salt tolerance of crops under field conditions, it is doubtful that selection for salt tolerant varieties and breeding for salt tolerance under conditions of water and flow culture experiments is very efficient. Breeding for more salt-tolerant crops and brackish irrigation techniques should consider root morphology and soil/ root contact zone. Introduction In the dry areas of the world there is an increasing pressure on agriculture, horticulture and landscape greening to replace good quality irrigation waters by waters of lower qualities such as brackish, drainage or treated wastewaters. The application of water of lower qualities often causes salinization of soils, which may affect growth of many crops depending on their salt tolerance (Rhoades et al. 1992). Consequently there is a general demand to improve crop salt tolerance in order to maintain yield levels and plant growth even at increasing levels of soil salinity. In the past decades the focus of research to elevate salt tolerance of plants mainly referred to biochemical and physiological aspects (Koyro and Huchzermeyer 1999). Genes responsible for salt tolerance of some crops (e.g. soya bean, tomatoes, grasses, rice) have been identified (Jaradat 1999). However, in spite of the large efforts put into the understanding of biochemical and physiological processes in plants grown under saline conditions, results are disappointing with respect to their relevance for crop yields under brackish or saline agriculture (Flowers 24, Yamaguchi and Blumwald 25, Jones 26). The presented concept for further progress of research on agronomic crop salt tolerance under irrigation Journal compilation ª 28 Blackwell Verlag, 194 (28) 1 8 1

2 Schleiff analyses the internationally applied standard procedure for rating of crop salt tolerance, which was developed by USDA and proposed by FAO (Ayers and Westcot 1985). This concept considers the upward and downward movements of water and salts in irrigated soils. No attention has been given to the fact that within irrigation cycles between two water applications the lateral movement of water and salts from the bulk soil to the root surface dominates. Based on the assumption that under saline field conditions usually plant water supply is the most critical growth factor, the conditions for water uptake by roots are given special attention. Present Concept for Rating of Crop Salt Tolerance in Agriculture In agronomy the term crop salt tolerance is defined as the relative crop yield (Y in %) at a given soil salinity compared to the yield of a non-saline soil. Soil salinity refers to the average salinity of the rooted soil layer and is expressed as the electrical conductivity (EC) of the soil saturation extract, the ECe. For a specific crop the relative crop yield is calculated according to the following equation (Maas and Hoffmann 1977): Y ¼ 1 bðece aþ ð1þ where b is the yield loss in % per unit increase of soil salinity ECe (1 ds m )1 ) (ranging from <3 %/[ds m )1 ] for salt tolerant to >3 %/[ds m )1 ] for sensitive crops) and a is the soil salinity threshold value, ECe, where yield decrease starts (ranging from 1.5 ds m )1 for sensitive to 1 ds m )1 for salt-tolerant crops). Both values a and b are crop specific and were published by Maas and Hoffmann (1977) and Ayers and Westcot (1985). This approach is often helpful to compare the relative advantage of crops or cropping patterns. Based on Eqn (1) crops are classified into groups of different salt tolerance as presented in Fig. 1: sensitive (S; e.g. onion), moderately sensitive (MS), moderately tolerant (MT; e.g. rape), tolerant (T) and very tolerant (VT) halophytes. When the ECe exceeds a range of 4 45 ds m )1, soil salinity is even too high for halophytes. The principles of this soil salinity-based concept for rating the crop salt tolerance are presented on the lefthand side of Fig. 2. According to this concept, the relative root zone is divided into four layers of equal depth ( 25 %, 25 5 %, 5 75 %, 75 1 %). The different layers contribute different amounts of water to the water requirement of the crop (ET = evapotranspiration): the top layer ( 25 % soil depth) contributes 4 % (=.4 ET), the second layer (25 5 %) 3 % Y = relative crop yield in % HAL S MS MT T Sensitive Moderately Moderat. sensitive tolerant HAL Tolerant Very tolerant (halophytes) NO PLANT GROWTH Soil salinity as ECe in ds m 1 Fig. 1 Rating of relative crop salt tolerance after FAO (Ayers and Westcot 1985) completed for halophytes after Goodin et al. (199). (=.3 ET), the third layer (5 75 %) 2 % (=.2 ET) and the bottom layer (75 1 %) 1 % (=.1 ET) to the total ET of the crop. Consequently the amount of soil water available for leaching of salts (LF = leaching fraction) from the upper layers traveling downwards the soil profile into the non-rooted subsoil decreases with soil depth (LF = ECi/ECe: LF > LF1 > LF2 > LF3 > LF4). The decreasing amounts of leaching water result in a soil water salinity (ECsw) that continuously increases with soil depth (ECsw < ECsw 1 < ECsw 2 < ECsw 3 < ECsw 4 ). The rating of salt tolerance for all crops considers the average salinity (ECe) of the total soil layer. More details on the relevant calculation procedures are available elsewhere (Ayers and Westcot 1985, Schleiff 23). The right-hand side of Fig. 2 indicates an aspect that is neglected by this concept, but is assumed to be of great importance for our understanding of crop salt tolerance. The FAO concept does not take into consideration that the lateral movement of salts and water in soils that dominates between two succeeding water applications could play a prominent role for plant growth conditions in conjunction with morphological properties of roots (Schleiff 1982). Driven by transpiration of the shoot, saline soil solution moves from the bulk soil to the root surface, where water uptake occurs, but most ions are excluded from root uptake. As this aspect of ion dynamics in the root surrounding soil during irrigation cycles has been neglected in the past, we will return to the principles of these dynamic processes between roots and soil under brackish irrigation in a later section after summarizing recent research activities related to crop salt tolerance. 2 Journal compilation ª 28 Blackwell Verlag, 194 (28) 1 8

3 Research for Crop Salt Tolerance Evapotranspiration ET Transpiration SHOOT % Relative rootzone depth in % 5% 1%.4ET.3ET.2ET.1 Irrigation SOIL LF 4 ECsw 4 LF 3 ECsw 3 LF 2 ECsw 2 LF ECsw =ECi LF 1 * ECsw 1 ** root Drainage FAO - Concept for Plant Water Uptake and Soil Water Salinity under Brackish Irrigation * LF = leaching fraction ** ECsw = soil water salinity = directions of water and salt movement Transpiration causes a lateral water and salt movement leading to a significant salt gradient between bulk soil and soil at the soil/root-interface: Lack of research for crop salt tolerance! Fig. 2 Principles of the soil-based concept of FAO for rating of crop salt tolerance and development of this concept. Overview on Main Research Activities Related to Crop Salt Tolerance Research activities related to crop salt tolerance focus on an in-depth understanding of plant and soil factors that may be relevant for the limitations of plant growth under saline conditions. Based on results from this research strategies can be devised to elevate yields of crops growing on salt-affected fields (Pasternak et al. 1989, Pasternak and de Malach 1995). As plant salt tolerance under field conditions is recognized as an extremely complex affair, many disciplines are involved in salinity research and contribute many interesting details. Table 1 lists some scientific topics that have contributed in the past decades to achieve a better understanding of salt tolerance. The overview is subdivided into research activities focusing on the shoot, the root and the bulk soil. Research into shoots has focused at the level of the whole plant, the organelles and at the molecular level (Ashraf 24, Hu and Schmidhalter 24). Many interesting aspects were investigated, only some are mentioned here. Interesting results have been obtained on the contribution of roots, the reduction in nutrient and water uptake, the exclusion of toxic ions and many other details. Research into soils has been mostly orientated to farmers needs such as the relationship between soil solution salinity and crop growth, ion-specific effects on soils and plants, long term effects of water qualities on soil fertility parameters and other aspects. In this context it should be stressed that there is no need for a further debate on the antiquated concept of Wadleigh and Ayers (1946) concerning the additive effect of the soil matric and osmotic water potentials on water supply of plants. Experiments with various crops and soils have clearly demonstrated that the additivity of the water potentials on the plant water supply and plant growth cannot be considered as a general rule, but there are specific combinations of soils and plants, where the matric and osmotic water potential may affect plant growth additive (Schleiff 1983; Schleiff and Schaffer 1984, Schleiff 1986, 1987a,b, 25). The approach generally applied by agronomists, irrigation engineers or plant breeders to calculate the effective soil solution salinity roots are exposed to at decreasing soil water contents between water applications is based on the average soil salinity and water contents of a specific soil layer. This approach does not consider aspects concerning interactions between root and soil at the soil/root Journal compilation ª 28 Blackwell Verlag, 194 (28) 1 8 3

4 Schleiff interface. Riley and Barber (197) were the first who observed in greenhouse experiments with soya beans that after a period of water depletion salinity of a rooted soil layer is far from being a relatively homogeneous affair. They found that a steep salinity gradient had developed between the bulk soil (far from roots) and the soil close to the roots (rhizospheric soil). The soil fraction close to the root surface was up to 15 times more saline than the Table 1 Research topics of various disciplines related to soil salinity and crop salt tolerance Shoot Root Soil root interface (the forgotten link ) Bulk soil salinity ESP, exchangeable sodium percentage. Research at whole plant, organelle and molecular level Water deficit, ion toxicities, nutrient deficiencies, ionic imbalances Salt distribution among leaves, stage of plant, transpiration, photosynthesis, respiration, smaller plant, reduced final production Altered leaf/stem cell ultrastructure, anatomy, cell wall hardness, membranes, salt exclusion, salt inclusion, excretion and leaf drop Cell expansion, cell division, cytoplasm, vacuole, gene expression Biochemical processes: accumulation of metabolites (proline, glycinebetaine, other amino acids, polyamides, sucrose, polyols, oligosaccharides) Phytohormones: ABA, auxin, cytokinin, gibberellin, etc. Enzymes (e.g. H + -ATPases, V-ATPase, PEPC, NADP-MDH) Reduction in water and nutrient uptake, root mortality Selectivity of ion uptake, barriers Phytohormones: root to shoot signals, ABA Transpiration as driving force causing mass flow of salts from the bulk soil directed to the root surface Ions excluded from uptake by roots accumulate in the root surrounding soil resp. soil solution at the soil root interface Interactions between root properties, water potentials of the root surrounding and bulk soil with respect to their relevance for the water supply of plants and crop salt tolerance Salinity of soil saturation extract, soil solution salinity, specific ions Irrigation water quality, ESP, soil physical effects, ph Matric, osmotic and total water potential of the rooted soil layer General soil fertility parameter including fertilization Vertical salinity gradients, gypsum, leaching, groundwater, irrigation management bulk soil. This gradient was even more expressed under conditions of higher ET demand of the atmosphere. It is hard to explain, why during the past three decades the international scientific community involved in research concepts for brackish irrigation or breeding of more salt-tolerant crops did not appreciate this basic observation and overlooked the potential of this significant innovation. The following results obtained in Hofuf, Saudi Arabia, from a field experiment under brackish irrigation prove that dynamic processes around roots can be even found under field conditions (Fig. 3; Schleiff 1979). In field experiments onions were irrigated with waters of meq l )1 chloride (corresponding to ds m )1 ). The salinity of the bulk soil increased from about.5 meq 1 g )1 soil Cl ) under the lowest to about meq l )1 Cl ) under the highest water salinity level. The Cl contents of the rhizospheric soil (soil fraction adhering to the roots) was determined at two occasions: immediately (4 h) after a water application and before the following (4 days later) one. Figure 3 shows that even after the water application of 5 mm the rhizospheric soil salinity was three to five times higher than in the bulk soil reaching values between 2.5 and 5 meq 1 g )1 soil Cl ). A water application of 5 mm did not leach the rhizospheric soil salinity to the level of the bulk soil. It is also shown that after a water depletion period of 4 days the rhizospheric soil salinity increases the Cl-contents in the range of 8 12 meq 1 g )1 soil. Consequently, the osmotic water potentials (Yo) of the soil solutions contacting the root surface are also expected to be significantly lower than in the soil solution of the bulk soil. As the Yo of soil or water solutions strongly affects plant water supply and crop growth, processes happening Cl-contents of soils in mmol 1 g Rhizospheric soil Bulk soil 4 days after a water application 4 h after a water application Cl-concentration of irrigation water in mmol l 1 Fig. 3 Chloride contents of the bulk and rhizospheric soil under brackish irrigation of onions (Hofuf, Saudi Arabia). 4 Journal compilation ª 28 Blackwell Verlag, 194 (28) 1 8

5 Research for Crop Salt Tolerance between bulk and rhizospheric soil solutions are most relevant for further progress in research of crop salt tolerance and brackish irrigation. Principles on the Water Uptake by Roots under Saline Soils Conditions An overview of the principal processes contributing to the plant water supply under brackish irrigation or under saline soils conditions is presented in Fig. 4. After a water application soil water content is highest in the bulk soil as well as in the rhizospheric soil, which is penetrated by root hairs. Conditions for high water uptake rates by roots are excellent, as there is much water in close contact with roots and root hairs (Schmidhalter 1997). The depletion of soil water from the root hair zone causes a gradient of soil water potentials between bulk and rhizospheric soil, which principally initiates a flow of soil solution from the bulk soil directed to the root surface or the root hair zone (mass flow). A further contribution to the plant water supply comes from the soil volume that is gained by the growing root system expanding into not yet explored bulk soil (root interception). The focus of the next section is on the fact that plants are equipped with roots of different morphology. As presented in Fig. 6 there are large differences in the volumes of the root hair cylinders of various crops. The roots of onions form a very small rhizocylinder, which is only less than 5 mm 3 cm )1 root length. Maize and tomatoes form a medium rhizocylinder volume in the range of 2 4 mm 3 cm )1 root length. The largest rhizocylinder soil volume was reported for rape with >6 mm 3 cm )1 root length. Furthermore, Fig. 6 shows that there is a good relationship of the K-uptake rate by roots to the volumes of their root hair cylinders. The K-uptake rate of rape roots was 2 to 3 times higher than that of onions. Similar investigations were also reported for other nutrients such as phosphate and nitrate (Jungk and Claassen 1997, Kovar and Claassen 25). Doubtless, root morphology has to be considered as an important factor for nutrient supply of crops. Based on these fundamental observations it is the intention of the following model calculation to transfer the principles of this concept to the process of water Brackish Irrigation Modelling Soil Solution Salinity around Roots of Different Morphology Figure 5 shows that a plant principally can be equipped with roots that develop relatively short root hairs (e.g. <.5 mm; left-hand side), and also with roots that develop relatively long root hairs (e.g. >2 mm; right-hand side). Consequently, the soil volume in immediate contact with the root and the root hairs (soil root interface) is significantly larger in the case of longer root hairs (Jungk 22). Small volume of soil/root interface Root hair zone Root Soil surface Large volume of soil/root interface Fig. 5 Plant available soil water as affected by root hairs and the volume of the soil root interface..6 Brackish Irrigation Mass flow Root Root hair zone Soil surface Bulk soil Root interception K-uptake rate in µmol (cm*sec) soil type: silt loam with 21% clay Onion Maize Tomato Lolium Rape Volume of the root hair cylinder (mm³ cm 1 ) Fig. 4 Basic components contributing to the water supply of plants from saline soils. Fig. 6 Rate of potassium uptake by roots of increasing root hair cylinder (Jungk 22). Journal compilation ª 28 Blackwell Verlag, 194 (28) 1 8 5

6 Schleiff uptake by roots from the rhizospheric soil solution, when their morphology differs. The model calculation will show the effect of water depletion by roots from the rhizospheric soil, when the volumes of the rhizocylinders differ due to the length of the root hairs. The calculation procedure considers the following steps: l Calculation of the rhizospheric soil volume (VOLrh in mm 3 ) and the volume of soil water (VOLw-rh in mm 3 ) affected by the water depletion, based on the soil water content at field capacity: VOLs-rhðmm 3 Þ¼p r 2 1 ð2aþ where r is the radius of rhizospheric soil volume in mm and l is the length of root in mm. VOLw-rhðmm 3 Þ¼VOLs-rh h ð2bþ where h is the water content in ml ml )1 soil (= vol.%/ 1) at field capacity. l Calculation of the effect of water depletion by roots (VOLdepl) from the rhizospheric soil solution (VOLwrh) on the salinity of rhizospheric soil solution ECrh-fin in ds/m): ECrh-fin ¼ ECin ½VOLw-rh=ðVOLw-rh-VOLdeplÞŠ ð3þ where ECin is the initial EC (ds m )1 ) at the beginning of the water depletion period. In a first step the calculation procedure determines the soil fraction considered as rhizospheric soil volume (VOLs-rh, Eqn 2a). The amount of rhizospheric soil solution at field capacity (h) is calculated in a second step (Eqn 2b). The effect of the water depletion by roots on the development of the rhizospheric soil solution salinity can be calculated from the initial salinity (ECin) at the beginning of a water depletion period and the amount of soil water depleted by roots (VOLdepl) from the rhizospheric soil volume according to Eqn (3). This calculation does not consider the ion uptake by roots, as this effect is negligibly low. Based on the presented formulas Fig. 7 shows the effect of water uptake by roots, when the water is taken up by roots of different root hair cylinders. In all three cases A, B and C, the initial rhizospheric soil solution salinity at field capacity is 1 ds m )1. In case A, where the radius R of the root hair cylinder is.5 mm only and the affected soil volume 78.5 mm 3 (VOLs-rh), a water depletion of 15 mm 3 1 cm )1 root length causes an increase in EC to nearly 28 ds m )1. In case B, where R is 1. mm and the root hair affected soil volume 314 mm 3, the EC increases only from 1 to 12 ds m )1 and in case C with R of 2. mm (VOLs-rh = 1256 mm 3 ) the EC increase can be nearly neglected achieving 1.4 ds m )1 only. It is concluded from these model calculations that root morphology must be of great relevance to extend our understanding of crop salt tolerance und water supply of crops under saline soil conditions. Water Uptake Rates from Saline Soil Solutions by Roots of Different Morphology Some first experimental results can be taken as a confirmation that root morphology is not only a very important factor for nutrient absorption by roots, but also for water uptake by roots from saline soils. Figure 8 shows results of a pot experiment with onions and rape, where the water uptake rates by roots from a densely rooted salinized sandy loam soil were determined from the transpiration rates (pot water losses) during a 4-day period. Details of the experimental set-up and vegetation technique are reported elsewhere (Schleiff 1987a). The EC (ds m 1 ) of soil solution A B Root hair cylinder Sandy loam: R mm mm cm 3 Vol. % at field cap. A B C C 5 % field capacity 1 field capacity ECin Water depleted by roots in mm 3 1 cm 1 root length Fig. 7 Calculated effect of water depletion from the rhizospheric soil solution by roots with different volumes of rhizocylinders. Root water uptake [ml (g DM * h) 1 ] day 1.day ONION 4.day 2.day RAPE 3.day 4.day Soil osmotic potential [-MPa] Fig. 8 Water uptake rates by roots of onions and rape (based on root dry matter) from saline soil solutions following a water application with 5 mmol l )1 NaCl (Schleiff 25). 6 Journal compilation ª 28 Blackwell Verlag, 194 (28) 1 8

7 Research for Crop Salt Tolerance osmotic water potential of the soil solutions (Yo) shown in the figure was calculated from the EC (ds m )1 ) determined for the soil solution (ECss) and applying the classical relationship published by Richards (1954): W o ðmpaþ ¼ECss ðds m 1 Þ:36 ð4þ It is clearly shown that there is a significant difference in water uptake rates of onion and rape roots. During the first 2 days, when soil water contents were highest due to the previous water application up to field capacity, the water uptake rate by onion roots was only 5 6 ml g )1 root DM h )1 at a Yo between ).3 and ).4 MPa, but five to six times higher (26 28 ml g )1 root DM h )1 ) by rape roots at the same Yo and soil water content. Until the end of the 4-day water depletion period rape roots were exposed to a solution of )1.2 MPa at significantly lower soil water contents. The Yo and soil water contents were much less affected due to lower water uptake rates by onion roots. It is concluded that differences in root morphology certainly play an essential role for root water uptake from saline soils and agronomic crop salt tolerance as shown for rape and onion, even when other factors (leaf properties) that have not been investigated here were also involved. It is recommended that further research should also take relevant shoot investigations and soil physical properties (texture) into consideration. Selection of salt tolerant varieties and breeding for salt tolerance is expected to be more effective, when salt dynamics at the soil root interface are also reflected. It will be a challenge to develop adequate experimental set-ups. References Ashraf, M., 24: Some important physiological selection criteria for salt tolerance in plants. Flora 199, Ayers, R. S., and D. W. Westcot, 1985: Water Quality for Agriculture. Irrigation and Drainage Paper 29. FAO, Rome. Flowers, T. J., 24: Improving crop salt tolerance. J. Exp. Bot. 55, Goodin, J. R., E. Epstein, C. M. 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K., and N. Claassen, 25: Soil root interactions and phosphorus nutrition of plants. In: Phosphorus: Agriculture and the Environment, Agronomy Monograph No 46, pp Am. Soc. of Agron, Crop Sci. Soc. Am., Soil Sci. Am., Madison, WI. Koyro, H. W., and B. Huchzermeyer, 1999: Plant and crop stress: salt and drought stress effects on metabolic regulation in maize. In: Advanced Short Course on Saline Irrigation: Halophyte Production and Utilization. CIHEAM-IAMB/ UNESCO, Agadir, Morocco. Maas, E. V., and G. J. Hoffmann, 1977: Crop salt tolerance. In: K. K. Tanji, ed. Agricultural Salinity Assessment and Management Manual, pp ASCE, New York. Pasternak, D., and Y. de Malach, 1995: Irrigation with brackish water under desert conditions (X). Irrigation management of tomatoes (Lycopersicon esculentum) on desert sand dunes. Agric. Water Manage. 26, Pasternak, D., Y. de Malach, S. Mendlinger, and I. Borovic, 1989: Irrigation with brackish water under desert conditions (VIII). 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8 Schleiff Schleiff, U., 23: Handbook Salinity & Soil Fertility Kit A Portable Field Laboratory for Soil, Water and Plant Analysis. self-published, Wolfenbuettel, Germany. (ISBN ) Schleiff, U., 25: Research aspects for crop salt tolerance under irrigation with special reference to root environment. In: S. Haneklaus, R.M. Rietz, J. Rogasik and S. Schroetter, eds. Research Accents in Agricultural Chemistry, pp Special Issue/Sonderheft FAL Agricultural Research, Braunschweig, Germany. Schleiff, U., and G. Schaffer, 1984: The effect of decreasing soil osmotic and soil matric water potential in the rhizosphere of a loamy and a sandy soil on the water uptake rate of wheat roots. J. Agron. Crop Sci. 153, Schmidhalter, U., 1997: The gradient between pre-dawn rhizoplane and bulk soil matric potentials, and its relation to the pre-dawn root and leaf water potentials of four species. Plant Cell Environ. 2, Wadleigh, C. H., and R. S. Ayers, 1946: Growth and biochemical composition of bean plants as conditioned by soil moisture tension and salt concentration. Plant Physiol. 2, Yamaguchi, T., and E. Blumwald, 25: Developing salt-tolerant crop plants: challenges and opportunities. Trends Plant Sci. 1, Journal compilation ª 28 Blackwell Verlag, 194 (28) 1 8