Evaluation of the effects of limitations of source and sink on traits of soybean genotypes planted at different dates in Mazandaran

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1 International Journal of Agriculture and Crop Sciences. Available online at IJACS/2013/5-15/ ISSN X 2013 IJACS Journal Evaluation of the effects of limitations of source and sink on traits of soybean genotypes planted at different dates in Mazandaran Fatemeh Marzban 1 and Esmaeil Yasari *2 1. Instructor, Department of Agricultural Sciences, Payame Noor University, I.R of Iran 2. Assistant Prof, Department of Agricultural Sciences, Payame Noor University, I.R of Iran *Corresponding author e_yassari@yahoo.com ABSTRACT: An experiment was conducted at the Agricultural Research Station of Baye Kola in 2010 in the form of split- split using the randomized complete block design in order to evaluate the effects of imposing source sink limitations on some traits of soybean genotypes as planted at different dates. The factors studied in this experiment included the planting date as the main factor, four soybean genotypes as the sub-factor, and five levels of source sink limitations as the sub-sub factor. Results showed that the different soybean genotypes significantly differed in all the traits studied, except in the harvest index and the number of pods on the main stem, the effects of planting date were significant only on the length of the pods, the number of unfilled pods on the main and lateral, and the harvest index. The effects of source sink limitations were significant on the distance of the first pod from the soil surface, the seed yield, the number of unfilled pods on the lateral, and the number of pods on the main stem and on the lateral. According to the results we obtained, Line 033 had the highest seed yield, the largest number of nodes, the tallest plants, the greatest distance of the first pod from the soil surface, and the maximum pod length. Key words: Soybean, Cultural traits, Source and sink, Planting date INTRODUCTION Because of the importance of soybean in connection with the production of oil, soybean meal, and various food products, the need to identify and employ appropriate methods for expanding the acreage under soybean cultivation and for increasing the yield per unit area of this crop is increasingly felt in our country. The higher yields of new soybean cultivars, as compared to the old ones, result from increasing the genetic potential through plant breeding, and from employing the advances of cultural technology in agricultural production systems. Agricultural technology and its great capability in increasing the production of food products have been promising developments for the people of the world in the recent decades, hence the optimal use of the production resources in various regions of the world seems to be a necessity. One of the methods considered in this respect has been to maximize the utilization of solar energy in the production of a greater quantity of dry and usable matter (Naseri, 1991). Factors influencing this system have been divided into two categories: I. the environmental factors (sunlight, carbon dioxide, temperature, etc.), and II. factors related to the plants (photorespiration, age of the leaves, hormones, control on the photosynthesis etc. (Ainsworth, 2004). The performance of a plant is the end result of the production of assimilates by the leaves (the source) and the transfer of these substances to the growing seeds (the sink, where they are used for synthesizing starches, fats, and proteins). In crop plants, the movement of photosynthesis from the source to the sink depends on the potential of producing photosynthesis, on the one hand, and, on the other hand, on the capacity of storing photosynthesis; and if there is an imbalance between these two, the yield will decline. Potts and Gardiner (1980) reported that the economic performance of a crop plant is influenced by the limitations of the source and of the sink. Limitations of the source mean that the plant is not able to produce the photosynthesis necessary for satisfying the intended goals. In contrast, limitations of the sink refer to the fact that the plant is able to produce sufficient photosynthesis but the economic goals of the plant do not have the capability of receiving and storing all the photosynthesis produced at the source (Sassanpour, 2008).

2 Loss inflicted by pests and diseases (through the damage they inflict on leaves and pods), and also the selection of the best cultivar with respect to its capability in transferring assimilates and stored photosynthesis after the treatments of eliminating some of the leaves or some of the flowers, are some of the reasons why it is necessary to understand the source sink relationships. MATERIALS AND METHODS This experiment was conducted at the Baye Kola Agronomy Research Station situated in the west of the city of Neka in 2010 to evaluate the effects of source sink limitations on some of the cultural traits of different soybean genotypes. The geographical features of the experimental site are as follows: latitude: North, longitude: East, altitude: 4 meters above sea level, average yearly rainfall: 524 mm, soil texture: Silty clay, and soil ph: 8.2. The experiment was carried out in the form of twice split plots (split split plots) using the randomized complete block design in three replications. The planting date was the main factor (with two levels: June 6 and June 27); while the cultivar with four levels (Line 033, Line 032 or the cultivar Nekador, the cultivar JK or Sari, and the cultivar BP or Telar), and the source sink limitations imposed (with the five levels of cutting one third of the leaves on the lower, the middle, and the upper parts of the stems, removing one third of the flowers, and the control treatment) were considered as the minor and minor minor factors, respectively. In order to impose the source sink limitations on the intended cultivars, in all of the experimental units the limitations were imposed at the start of flowering: from the seven lines planted in each plot, lines 1 and 7 were left out to eliminate the border effect, the plants situated at the distance of half a meter from the two ends of each line were discarded, leaves on the lower one third of the plants in the first half of line 2 and leaves on the middle one third of the plants in the second half of the next line (line 3) were eliminated, line four was selected as the control, and lines 5 and 6 were treated similarly to lines 2 and 3 (for imposing the limitation of eliminating leaves on the upper one third of the plants and eliminating one third of the flowers, respectively). Colored threads were tied to the plants included in the treatments to distinguish them from the other plants present in the plots. Necessary data, including the height of the plants, the distance from the first pod to the soil surface, the number of lateral, the length of the pods, the number of nodes on the main stem, the seed yield of each plant, the number of pods on the main and lateral, the number of unfilled pods on the main stem and on the lateral, and the harvest index, was recorded from the plots (after eliminating the border effect). This data was analyzed using the statistical software MSTATC; and the spreadsheet software Excel was employed to draw the Figures. RESULTS AND DISCUSSION Plant height and the distance from the first pod to the soil surface Plant heights differed significantly among soybean cultivars at the 1% probability level (Table 1), with the tallest plants observed in Line 033 (Table 3). Line 033, because of having long stems (which is a genetic trait), has a higher level of photosynthetic activity than other cultivars, and this causes the transfer of a greater quantity of photosynthetic (and hence, an increase in seed yield). Numerous reports have been submitted concerning the decline in plant height due to delays in planting date (Thomas and Raper, 2009), but the results of our experiment showed that planting date had no significant effect on plant height (and these results conform to those obtained by Potts and Gardiner in 1980). With an increase in plant height, the distance from the lowest pod to the soil surface is increased, which is a desirable trait for mechanized harvesting with combines (Walch, 1992). Imposing source sink limitations was significant for the trait of the distance from the first pod to the soil surface at the 1% probability level, so that the first pod in the control treatment had the greatest, and that in the treatment of eliminating leaves from the upper one third of the plants the shortest distance from the soil surface (Table 3). The number of nodes on the main stem Results of analysis of variance indicated there was a significant difference concerning the factor of cultivar at the 1% probability level (Table 1). Comparison of the means for the factor of cultivar showed that Line 033 and Line 032 with and nodes, respectively, had the largest number of nodes on the main stem (Table 3). It seems that Line 033, because of having longer stems, had produced more nodes. The number of lateral The number of lateral differed significantly among the cultivars at the 1% probability level (Table 1). Line 032 had the maximum number of lateral (5.77) among the cultivars (Table 3). This difference among

3 the cultivars could have a genetic origin, because the first component of the yield to be controlled by genetic and environmental factors is the number of pod bearing in the plants (Emami and Niknejad, 1994). The length of the pods According to the results obtained from the table of analysis of variance, the length of the pods at the various planting dates and in the different cultivars tested, differed significantly at the 5% probability level. Among the different treatments of imposing source sink limitations, the length of the pods did not differ significantly (Table 1). According to Board et al. (1999), the shorter pod length at the second planting date could be due to the short growing period which deprives the plants from the chance of better completing the pods (because the amount of photo-assimilates received from the leaves, stored in the sink, and transferred to the pods is limited), and hence fewer seeds are formed in the pods, which leads to the production of shorter pods. Among the cultivars, Line 033 had the longest pods. This could be a genetic trait (which would agree with the observations of Campbell and Kondra (1978) who stated that the length of the pods is determined by the genetic structure of the plants); or it may be that Line 033, because of its greater height and due to the fact that more sunlight penetrates into its community of plants, is more efficient in transferring photo-assimilates to the pods, and hence more seeds are formed in the pods, which also leads to an increase in the length of the pods (Table 3). The number of pods on the main stem Analysis of variance reveals a significant difference at the 1% probability level for the factor of imposing source sink limitations (Table 2). Results of imposing source sink limitations showed that the largest number of pods on the main stem belonged to the control treatment; while in the treatment of eliminating the leaves from the upper part of the plants, the smallest number of pods was produced on the main stem (Table 4). It seems that with the elimination of the leaves from the upper part of the plants were confronted with the condition of limited source capability in transferring photosynthetic to reproductive organs at the flowering stage, and hence the number of pods on the main stem declined. In this connection, Abbaspour et al. (2005) reported similar results in sunflower plants. The number of pods on lateral Analysis of variance indicated significant differences at the 1% probability level for the factor of cultivar, for the Interaction effects of planting date and cultivar, and also for the factor of imposing source sink limitations (Table 2). Among the cultivars, Line 033 had the largest number of pods on the lateral, and the smallest number was that of cultivar BP. In the treatment of imposing source sink limitations, in which the smallest number of pods on the lateral was obtained in the treatment of eliminating the leaves of the middle part of the plants and the largest in the control treatment and in the treatment of eliminating the lower leaves of the plants (Table 4), the leaves in the middle part of the plants can also play the role of supplying the needed nutrients of the lower pods and, in case the leaves in the upper part of the plants are eliminated, the nutrients needed by the upper nodes; and with the elimination of the leaves in the middle part of the plants results similar to the elimination of the leaves of the upper part of the plants were observed. The results we obtained are compatible with those observed by Yassari et al. (2009). Comparison of the Interaction effects of cultivar and planting date showed that the largest numbers of pods on lateral were observed in Line 033 and line 032 at the first planting date, and the smallest in the cultivars BP and JK, respectively, planted at the second planting date. Donald (1998) reported that at earlier planting dates more flowers and pods are produced and kept on lateral, compared to later planting dates; and the results of our experiment conform to those in his report (Figure 1). The number of unfilled pods on lateral Results obtained suggested that for the treatments of imposing source sink limitations, and for the treatment of planting date, there were significant differences at the 5% probability level, that significant differences existed for the factor of cultivar at the 1% probability level (Table 2), and that the smallest number of unfilled pods on lateral (0.5) was observed at the first, and the largest (1) at the second planting date. Mobasser et al (2007) studied the effects of planning date and concluded that with delays in the planting date the number of unfilled pods on lateral increases. Their observations are in agreement with the results we obtained. Comparison of the means for the factor of cultivar indicated that the maximum numbers of unfilled pods on lateral (0.95) were those of Line 032 and cultivar JK (1.08), and the minimum (0.27) that of Line 033. In the comparison of the means for the factor of imposing source sink limitations, the largest number of unfilled pods on lateral (1.078) was that of the treatment of eliminating the leaves on the upper part of the plants (since

4 photosynthetic organs had been eliminated, and hence there were not enough nutrients to fill all the seeds, and therefore the number of unfilled pods increased) (Table 4). The number of unfilled pods on the main stem The table of analysis of variance revealed that, at the 1% probability level, the effects of the cultivar and the planting date on the number of unfilled pods on the main stem were significant (Table 2). Comparison of the means of the planting dates showed that at the first planting date the number of unfilled pods on the main stem (0.66) was smaller than the number of unfilled pods on the main stem at the second planting date (2.28). Mobasser et al (2007), similar to what we observed in our research, in their study of the effects of planting date concluded that delays in planting date increases the number of unfilled pods on the main stem. Comparison of the means of the factor cultivar revealed that the maximum number of unfilled pods on the main stem (1.90) was that of the cultivar BP, and the minimum (1.72 and 0.88) belonged to Line 032 and Line o33, respectively (Table 4). Seed Yield Results of our experiment indicated that there were significant differences at the 1% probability level among cultivars and among the treatments of imposition of source sink limitations, and also that there were significant differences at the 5% probability level for the Interaction effects of the planting date and the cultivar (Table 2). Comparison of the seed yield for the different treatments showed that the highest yield was achieved in Line 033. Results obtained indicated that the higher seed yields of Line 033, compared to the other cultivars, were accompanied by the maximum 1000 seed weight, the largest number of pods on the main stem and on lateral, a relatively large number of seeds per pod, and the smallest number of unfilled pods on the main stem and on lateral, and also that the low seed yield of Line BP was accompanied by the smallest number of pods on lateral, a relatively low seed weight, and fewer pods on the main stem. Results of the effects of the treatments of the imposition of source sink limitations on seed yield showed that the highest seed yield was achieved in the control treatment (Table 4). When the leaves of the upper part of the plants were eliminated, it was observed that the seed yield declined sharply, so that the minimum seed yield in this treatment was 35% less than that of the control. It seems that the treatment of eliminating leaves has a lower economic yield due to the source - sink limitations, the decline in photosynthesis and in assimilates, and the hormonal disorder. Yield is a somewhat complex trait that is controlled by a large number of genes, and that is strongly influenced by the environment. Yield is the outcome resulting from many traits that individually or collectively influence it (Zahabi, 2003). In Figure (2), which shows the Interaction effects of cultivar and planting date, it can be seen that the highest yield of Line 033 was achieved at the first planting date, and the lowest belonged to the cultivar BP planted at the second planting date. These results are compatible with the findings of Abdoli et al (2004) concerning the differences in the responses of cultivars to various planting dates. The Figure on the Interaction effects of cultivar and imposition of source sink limitations also indicates that there are significant differences between the seed yields of the control treatment and the treatment of eliminating leaves from the lower one third of the plants and those of the other treatments of imposing limitations. The reason for the increase in seed yield in the treatment of eliminating leaves from the lower one third of the plants can be attributed to the lack of appropriate efficiency of leaves in the lower parts of the plants: the respiration rate of these leaves is probably much higher than the level of photosynthesis taking place in them, the level of the production of photosynthesis in them is low, and the photosynthesis produced are not available for increasing seed yield. The control treatment had the maximum seed yield because the photosynthetic organs were not eliminated. Moreover, the decline in seed yield in the treatment of eliminating the leaves from the upper one third of the plants suggested that light absorption, and carbon fixation to carbohydrate forms, exhibit their greatest effects in the photosynthesis taking place in the leaves of the upper part of the plants so that, with the elimination of these leaves in the early stages of seed formation (the flowering and the milk stages), seed yield sharply decreases. These results conform to those obtained by Mali (1999). The harvest index Results of analysis of variance for the factor of planting date were significant at the 1% probability level (Table 2). These results show that the harvest index at the first planting date (56%) was higher than that of the second planting date (51%). These results contradict the report submitted by Wilcox and Frankenberg (1987). They stated that the harvest index does not change much at different planting dates, except when plants are killed by cold weather; i.e., they came to the conclusion that the harvest index is insensitive to the planting date; however, Hashemi Jazi (2001) reported that with delays in planting date the harvest index of soybean decreased; and the results they obtained were confirmed in our experiment (Table 4).

5 Figure1. Interaction effects of number pods on lateral shoot and planting date Figure 2. Interaction effects of seed yield and planting date

6 Table 1. Analysis of variance of soybean traits as influenced by planting date, cultivar, and imposition of source sink limitations Degree of Distance of Pod Source of variance Freedom first pod to soil length Plant height Replications 2 surface No. of lateral (cm) 0.025** No. of nodes on the main stem Planting date (A) n.s n.s 0.057** n.s n.s Error (a) Cultivar (B) * ** * ** ** A x B n,s 0.326n.s 0.003n.s 0.515n.s n.s Error(b) Source sink Limitations C AC BC ABC Error ** 0.172n.s 0.168n.s 0.120n.s n.s 0.288n.s 0.183n.s 0.111n.s n.s 0.007n.s 0.003n.s 0.006n.s n.s 0.276n.s 1.295n.s 0.862n.s n.s n.s n.s n.s Coefficients of Variance The symbols n.s, *, and ** stand for not significant, significant at 1% probability level, and significant at 5% probability level, respectively. Table 2. Analysis of variance of soybean traits as influenced by planting date, cultivar, and source sink limitations Source of variance Degree of freedom No. of pods on the main stem No. of pods On lateral No. of unfilled pods on lateral No. of unfilled pods on the main stem Harvest Index (%) Replications Seed yield Per Plant Planting date (A) n.s n.s 2.008* 0.879** 0.065** n.s Error (a) Cultivar (B) n.s ** 1.682** 0.965** 0.005n.s ** A x B Error (b) n.s 5.640** 0.082n.s 0.419n.s 0.010n.s * Source sink Limitations ** 3.279** 0.319* ** AC n.s 1.038n.s 0.122n,s 0.162n.s 0.003n.s 0.400n.s BC n.s 0.251n.s 0.154n.s 0.065n.s 0.003n.s 7.578** ABC * 0.836n.s 0.140n.s 0.166n.s 0.007n.s 4.309n.s Error Coefficients of Variance The symbols n.s, *, ** stand for not significant, significant at 1% probability level, and significant at 5% probability level, respectively.

7 Table 3. Comparison of the means of soybean traits as influenced by planting date, cultivar, and source sink limitations Treatments\ Traits Distance from the first pod to soil surface (cm) Pod length (cm) Plant height (cm) No. of pods on the main stem No. of lateral Planting date A 1: June 6 A 2: July 27 Cultivar B 1: Line 032 B 2: Line 033 B 3: Cultivar JK B 4: Cultivar BP Source sink limitations C1: Eliminating leaves of upper 1/3 C2: Eliminating leaves of middle 1/3 C3: Eliminating leaves of lower 1/3 C4: Eliminating 1/3 of Flowers C5: Control a a b a c bc c bc bc b a 4.232a 4.056b 4.026b 4.461a 3.978b 4.110b 3.995b 4.163ab 4.169ab 4.163ab 4.229a a a b a c d b a ab ab a a a a a b b b ab 13.84ab a a 4.340a 5.778a 4.164b 2.880c 2.618c 3.529a 3.241a 3.904a 4.200a 4.426a Similar letters in each column represent no significant differences on the basis of Duncans test at the 5% probability level Table 4. Table of comparison of the means of cultural traits of soybean as influenced by planting date, cultivar, and source sink limitations Treatments\Traits Seed Yield Per plant (g/m 2 ) No. of pods on the main stem No. of pods on the lateral No. of unfilled pods on the lateral No. of unfilled pods on the main stem Harvest index HI (%) Planting date A 1: June 6 A 2: July 27 Cultivar B 1: line 032 B 2: Line 033 B 3: Cultivar JK B 4: Cultivar BP Source sink limitations C1: Eliminatin Leaves of the Upper 1/3 C2: Eliminating Leaves of the Middle 1/3 C3: Eliminating Leaves of the Lower 1/3 C4: Eliminating 1/3 of flowers C5: Control a a ab a b c b b a b a a a a a a a c b ab ab a a a a a b c c a ab a-c a 0.509a 1b 0.954a 0.275b 1.085a 0.703ab 1.078a 0.760b 0.713b 0.560b 0.660ab 0.665a 2.283b 1.384ab 0.888b 1.720b 1.903a 2.497a 1.142a 1.097a 1.302a 1.329a Similar letters in each column represent no significant differences on the basis of Duncans test at the 5% probability level 56a 51b 54a 54a 54a 52a 53a 53a 55a 53a 54a CONCLUSIONS Results obtained in this experiment indicated that Line 033 is the best cultivar regarding the seed yield and other cultural traits. Although Line 032 has a low seed yield, its harvest index is higher; that is, it can send a greater percentage of the assimilates produced to the seeds, and hence has a higher efficiency in distributing photosynthesis. If we were to choose one of these two cultivars, we would select Line 033, because it has a higher seed yield and produces more dry matter. However, Line 032 which, despite its low seed yield, has a

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