Biomass, Growth and Matter Partitioning in Soybean Plants under Long-term Moisture Deficit

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1 Jpn. J. Trop. Agr. 44(1) : 20-29, 2000 Biomass, Growth and Matter Partitioning in Soybean Plants under Long-term Moisture Deficit Ashok K. GHOSH, K. ISHIJIKI, M. TOYOTA, A. KUSUTANI and K. ASANUMA Faculty of Agriculture, Kagawa University Ikenobe 2393, Miki-cho, Kagawa-ken , Japan Abstract Information on growth, matter partitioning and yield response of soybean plants [Glycine max (L.) Merr.] grown at different levels of moisture stress imposed from the early stage until harvest seems to be insufficient. In this experiment, a determinate cultivar e Akiyoshi f was grown in a greenhouse at optimal (80% of field capacity) and three sub-optimal soil moisture levels (65%, 50% and 40% of field capacity). Moisture stress significantly decreased root length, plant height, leaf area and the number of branches, flowers, pods and seeds. Moisture deficiency resulted in smaller leaves and a slower rate of leaf expansion but higher weight per unit area. Increasing moisture deficit significantly decreased the dry matter weight of all the plant organs and also the shoot:root ratio at all stages of growth. With decreasing soil moisture levels, plants accumulated a greater proportion of dry matter in the main stem leaves and roots but a lower proportion in their branches and the whole shoot. Nonsignificant differences in assimilate partitioning to the pods suggest that the seed yield depends on the dry weight of the whole plant. Moisture deficiency decreased the weight per seed and also seed yield per plant. Continuous moisture deficiency reduced the number of branches per plant and the seed yield per branch. Hence, the significant yield decrease per plant resulted from the reduced dry matter production of plant and low productivity of branches in conjunction with the smaller decrease of main stem yield. Key words Dry matter, Moisture deficit, Soybean, Yield Introduction Water limitation exerts numerous effects on the physiology, growth and yield of soybean plants. Generally, soybean is cultivated under non-irrigated conditions. Therefore, the naturally available soil moisture is very important for growth and yield. There are wide fluctuations in the amount of plant available water in soybean growing areas of the world. Sometimes the soil moisture contents remain below the optimum level throughout the whole growing season. This kind of soil moisture status is especially observed in the tropical and sub-tropical regions A decrease in the amount of plant available water at the earlier stage of growth resulted in the reduction of the leaf expansion rate and dry, matter accumulation16). Vegetative and reproductive growth was almost equally suppressed by the leaf moisture stress applied at the R3 stage of growth, and even after recovery, the yield resulting in poor growth and low yield of potential was restricted by the decrease in the soybean crops. Moreover, precipitation varies from number area of to pods area or and seed17). also Plants from season grown to in Drought season. occurs very often. Ther th deficient soil moisture until flowering can Received Feb. 12, 1999 Accepted Aug. 27, 1999 produce higher dry matter and grain yield if soil

2 GHOSH et al.: Soybean growth and yield at moisture stress 21 moisture supply was adequate after flowering stage compared to the plants receiving sufficient moisture until flowering but face moisture deficiency after that timely. Reduced turgor during inflorescence development reduces the number of flowers, which may lead to the abortion at pod development and smaller seed. Significant yield differences were observed between irrigated and non-irrigated soybean18). Moisture deficit decreases the pod number and reduces the seed size3,4,7,16) In all these experiments, moisture stress was imposed on soybean plants only at certain stages of growth. To our knowledge, published data do not enable to explain the patterns of growth and yield of soybean plants grown under continuously deficient soil moisture from the early growth stage until harvest. Within the same ecotype, growth responses may differ in different cultivars depending on the soil moisture status10) On the basis of the literature, it was assumed that at different levels of moisture stress, soybean plants display a different pattern of growth in terms of both morphology and dry matter content. These differences in growth may be reflected in the yield per plant. Therefore, monitoring of the soil moisture level in relation to the climatic conditions of a particular area might be important for soybean production. A relationship between the available moisture and the seed yield may be observed. Based on the soil moisture status, prediction of yield may be possible provided other factors are identical. In this report, studies on soybean growth at optimum and sub-optimal soil moisture levels from the early growth stage until harvest were carried out. Materials and Methods Plants were grown in clay pots filled with coarse sandy loam soil equivalent to 9.0 kg of dry weight. The field capacity (FC) of the soil used in the pot was 29.8% of dry weight and was determined by the gravimetric method. The soil moisture content at 80% of FC level was considered to be optimum for standard growth of the soybean plants. Other three sub-optimal levels of soil moisture treatments in this experiment included 65%, 50% and 40% of FC. According to the soil analysis recommendation, 5.0g calcium superphosphate and 2.0g potassium sulphate were mixed into the soil of each pot prior to seeding. On the 7th day after emergence, 2.0g ammonium sulphate was applied per pot. A second application of nitrogen was made after 40 days of emergence (2.0g ammonium sulphate per pot). A soybean cultivar eakiyoshi f was used in this experiment. Akiyoshi is a late maturing and determinate type cultivar which requires about 135 days from emergence to harvest in an optimum growing season. The seeds were not inoculated with Rhizobium sp. Five seeds were sown in each pot on 26th June, Two plants were finally grown per pot. The whole experiment was conducted in a naturally lit glasshouse, with four replications. The pots were arranged in a randomized block design where replications are placed as blocks and treatments as plots. One pot with two plants was considered as one replication. Hence, the data represented the mean of a total of eight plants. The changes in the atmospheric temperature throughout the growing season are shown in Fig.1. Soil moisture treatments were started on the 10th day after emergence (DAE). Every day, water was applied twice (at 7:00 am and 2:00 pm) to maintain the soil moisture levels. The amount of water added to each pot was determined by weighing the pot and adding a sufficient amount of water to maintain the desired weight. A similar water management technique in pot experiment was reported by DE SOUZA et al.5) Fig. 1. Changes in ambient temperature during the experimental period in Maximum and minimum temperatures within ten-day periods are indicated on the X-axis.

3 22 Jpn. J. Trop. Agr. 44 (1) 2000 and XIA 20) to impose water stress on soybean plants. After every 15 days, the mean fresh weight of whole plants per pot was measured. The adjustment of the weight of the whole pot under the respective treatments was carried out adding the increases in fresh plant weights. Plants were sampled six times at the V3, V11, R1, R3, R5 and R8 stages of growth8). Morphological data included plant height, node number, petiole length, leaf area, root length, number and length of branches, number of flowers and pods. Data of leaves from the main stem and branches were separately recorded. Leaf area was measured using an automatic area meter (Hayashi Denko Co. Ltd., Japan). Leaf expansion rates and patterns under the imposed soil moisture levels were monitored at the growth stage R3. Length, width and area of all the leaflets of the trifoliates on the 11th node of the main stem of five plants were monitored. The area of individual leaflets was measured by tracing on black sheets of paper and subsequently cutting the papers in exact shapes and passing through the area meter. These observations of leaf growth began on day 2 of unfolding and continued until day 32. The number of flowers that had recently bloomed was recorded every day throughout the flowering period of about 30 days to obtain the number of flowers per plant. Soil masses including roots were soaked in water for 12 hours and then washed on a net under a gentle flow of water. The clean roots were immediately stored in a household refrigerator and the length was measured within 2-3 days using a Comair Rootlength Scanner (Commonwealth Aircraft Corporation Limited, Australia). Plant parts were dried for 6 days at 65 Ž prior to recording the dry mass of individual organs. In addition to the dry matter accumulation, several plant growth indicators were computed to compare the seasonal patterns of growth components affected by the soil moisture treatments, namely specific leaf area, shoot:root ratio, absolute growth rate and biomass duration (BMD). BMD is the value for biomass persistence with time. According to HUNT13), BMD was calculated by the formula, D=(W1+W2) (t2-t1) /2, where, W1 and W2 are the initial and final dry weight, and t1 and t2 are the time interval (in days) after emergence. Dry matter partitioning to different organs was calculated as the percentage of whole plant weight. The plants were harvested after the color of the leaves of almost all the branches turned brown. The plants were cut at the soil level and left for air drying on a concrete floor for about two weeks. Pods grown on the main stem and branches were collected separately. The data on seed yield components per plant were recorded. The weight of 100 seeds was also calculated. Analysis of variance was used to evaluate the effect of the treatments on selected traits. Mean separation was achieved through the calculation of the least significant differences at the 5% level. Results and Discussion Morphological development Different levels of moisture deficit from the early growth stage resulted in significant differences in the morphological development of soybean plants. Plants grown at optimum soil moisture levels had an average root length of 928m per plant at the R3 stage (Fig. 2a). Root length decreased successively at 65%, 50% and 40% FC levels (P<0.05). The lowest soil moisture level led to the lowest root length (346m per plant). Therefore, a reduction of 50% soil moisture from the optimum level resulted in a 63% reduction of root length. KONO et al.14) reported the reduction of root length in 38-dayold soybean plants grown in narrow root boxes under moisture-deficient conditions. BROWN et al.1) reported the reduction of root growth in field-grown soybean for a moisture deficit imposed at the R2 and R4 stages. But HULK et al.12) observed a greater root length in fieldgrown soybean plants under water stress. With the gradual decrease of the moisture level in field soil, roots may have the tendency to grow deeper in search of moisture. In our case, the reduction of root growth by water stress may be due to rooting volume limitation in the pot experiments. An increase in water deficit always led to a smaller root length throughout the six stages of growth when the data were taken (all the data are not shown). These data suggest that the soybean roots cannot develop well in moisture deficient soils. Plant height was shorter with a reduction in the soil moisture levels (Fig. 2b). The differences were significant for all the treatment levels except between 40% and 50% FC, indicating that the rate of decrease is high up to a soil moisture content of 50% FC and that beyond that level

4 GHOSH et al.: Soybean growth and yield at moisture stress 23 Fig. 2. Morphological growth parameters of soybean cultivar Akiyoshi at four soil moisture levels. The data of the figures a, e, f and g were determined at the R3 stage; while for b, c and d, at the R5 stage; in h, the data were recorded from the R1 to R3 stages; and in i, at the R8 stage of growth. FC denotes the field capacity. Vertical bars represent the standard errors of the mean. plant height continues to be shorter but with a slower rate. Plant height was reduced by 44% when the soil moisture level decreased from 80% to 40% of the FC values. Doss and THURLOW7), and MOMEN et al.15) also observed shorter plants when grown under moisture deficit conditions. In these experiments, water stress was applied at a specific stage of growth and for a short duration. However, the data from this experiment clearly showed the extent of changes in the height of the plants cultivated at different levels of soil moisture throughout the growing season. The number of branches per plant decreased from 15.8 at 80% FC to 8.5 at 40% FC with significant changes (P<0.05) at every step of the treatment levels (Fig. 2c). This value was equivalent to approximately a 46% reduction when the highest and lowest available water levels were compared. Length of the branches per plant (Fig. 2d) showed a highly significant reduction (P<0.01). The length of the branches per plant at 80% FC was 247 cm, while only 33

5 24 Jpn. J. Trop. Agr. 44 (1) 2000 cm of the total branch length (13% of the optimal condition) was obtained in the plants grown at 40% FC. Branch node number was significantly lower in the plants grown under deficient moisture conditions (data not shown). Thus branching was severely affected by the water deficit. In general, branches produce the major portion of pods in most of the soybean cultivars grown in the warmer regions. Moisture deficit during the vegetative phase leads to poor branching and branch development which become irreversible even if the soil moisture status is improved in the later part of the growing season. Therefore, the growth and development of the important yield related organs were impaired due to increased water limitation. The length of the petiole (Fig. 2e) and leaf area (Fig. 2f) per plant were reduced at deficient soil moisture levels (P<0.05). A soil moisture deficit of 50% from the Optimum level resulted in a 73% reduction of petiole length and 66% reduction of leaf area per plant. Increasing moisture deficit led to a slower growth of leaves (Fig.2.g). Apparently, the completion of leaf expansion occurs slightly earlier at higher soil moisture levels. BUNCE2) reported that soil water stress reduced the leaf expansion rate in the early vegetative growth. In this experiment, we measured the leaf expansion rate at the R3 stage. These results showed that water deficit reduced the leaf expansion rate at the vegetative as well as reproductive stages of growth. The number of flowers per plant (Fig. 2h) decreased sharply from 280 to 96 (65% reduction) with significant differences among the treatments (P<0.05) as the soil water decreased from 80% FC to 40% FC. The number of pods per plant consistently decreased (P<0.05) for every decrease in the soil moisture content (Fig. 2i). An overall reduction of 50% of soil moisture from the optimum resulted in a 59% reduction in pod number per plant. Considering the total number of flowers that bloomed and the final number of pods per plant, the numbers of aborted flowers and undeveloped pods were significantly low (P<0.01) in the plants under increasing moisture deficiency. The percentage of flowers that dropped was Fig. 3. Dynamics of dry matter gain per plant at four high in the plants grown under higher soil soil moisture levels. FC denotes the field moisture conditions. These differences ranged capacity. Vertical bars indicate the standard between 70 to 77% and were not statistically significant. error of Therefore, the mean. under increasing mois-ture deficiency, soybean plants produced fewer and bor fef

6 GHOSH et al.: Soybean growth and yield at moisture stress 25 not shown). The foremost effect of moisture deficiency begins with a slower expansion of leaves and ends up with smaller leaves depending on the degree of deficiency, followed by slower overall growth of the plants which is reflected in the dry matter weight. Dry weight per plant was correlated with the dry matter weight of roots (r=0.994**), main stem (r=0.971*), branches (r=0.997**), leaves (r=0.995**) and pods (r=0.999**). Pod dry weight per plant also showed a close correlation with the dry weight of leaves per plant (r=0.998**). These relationships suggest that under increasing water shortage, soybean plants displayed a balanced reduction of dry matter production and accumulation in all the plant organs, including pods. Dry matter partitioning patterns differed among the plant organs (Fig. 4). The proportion of assimilates partitioned to the main stem did not show any definite pattern. Although the number of pods Fig. 4. Patterns of dry matter partitioning in soybean plants grown at four soil moisture levels. MS is for main stem, Br is for branches and FC is for field capacity.

7 26 Jpn. J. Trop. Agr. 44 (1) 2000 from the main stem (refer to the following chapter) decreased in the plants grown under increasing moisture deficiency, it was not related to the percentage of dry matter partitioning. Percentage of assimilates partitioned to the pod per plant remained unchanged irrespective of the stages of pod development and soil moisture levels. Since the percentage of dry matter did not change, the dry matter weight of the whole plant determined the pod weight per plant. Percentage of dry matter allocated to the main stem leaves and roots increased with increasing moisture deficit at all the stages of growth. In contrast, the amount of dry matter as well as the percentage of dry matter partitioned to the branches, branch leaves and the whole shoot decreased with increasing soil moisture deficit. A high correlation (r=0.998**) was observed between the proportion of dry matter partitioned to the branches and the number of pods produced on branches. This finding indicates that the amount and proportion of dry matter in branches are a determining factor for the productivity of branches. HULK et al. 12) observed a significant reduction of shoot dry weight when water stress was imposed at different stages of growth. Therefore, it is concluded that regardless of whether the moisture stress occurs from the early growth stage and continues until harvest or occurs suddenly at any other,stages of the plants, the result is a lower dry matter weight of the shoot. The specific leaf area (SLA) significantly increased by the increase of the moisture supply throughout the growth of plants (Fig. 5), indicating that the amount of dry matter per unit leaf area are high in plants grown under moisturedeficient conditions. SLA was highest at the V3 stage of growth, showed the lowest values at the V11 stage and then increased. The shoot to root ratio of dry weight also, increased (P<0.01) by the increase in the soil moisture supply, which resulted in a larger dry matter weight in the above ground parts. Pod and seed dry matter weight per plant showed a close relationship with the shoot:root ratio, indicating that a higher shoot weight corresponds to a higher pod weight per plant. Absolute growth rate per plant decreased significantly (P<0.05) with increasing moisture deficit (Table 1), mainly due to the low increase of the dry matter weight in moisture-deficient plants at all the stages of growth. Plants receiving a higher soil moisture also showed a significantly higher biomass duration throughout the stages of growth (P<0.05) which reflected the larger accumulation of dry matter per plant with time. Thus moisture deficiency markedly affected the growth of the soybean plants. Fig. 5. Specific leaf area and root: shoot ratio of dry weight in soybean plants grown at four soil moisture levels.

8 GHOSO et al.: Soybean growth and yield at moisture stress 27 Yield and yield components The number of pods and seeds on the main stem per plant was reduced (P<0.05) by increasing moisture deficiency (Table 2). A highly significant reduction in seed yield from branches per plant was observed due to water deficiency. These differences in branch yield also influenced the total seed yield per plant. As described previously, increasing water deficit significantly reduced the number of branches per plant. Number of pods per branch was also reduced at the same time, and the seed yield per branch significantly decreased (P<0.05). Number of productive branches per plant and seed yield per branch are the two major yield related factors in soybean. A 50% decrease in soil moisture from the optimum value resulted in a reduction of seed yield by 48% in the main stem and 65% in the branches. Therefore, the productivity of the branches was impaired more than that of the main stem. It was observed that at every step of treatment level, the seed weight per plant significantly changed. Individual seed weight was smaller in the moisture-deficient plants. Reduced yield per plant was also frequently reported by many authors when Table 1. Changes in absolute growth rate and biomass duration of soybean plants grown at four soil moisture levels. FC denotes the field capacity Table 2. Yield and yield components of soybean plants (cv. Akiyoshi) grown at four soil moisture levels. MS, main stem; Br, branch; pl, plant; no., number; wt, weight. Data followed by the same letter(s) in rows do not significantly differ at 5% level.

9 28 Jpn. J. Trop. Agr. 44 (1) 2000 water stress was imposed only during the reproductive phase of growth1,6,9,19) The results of this experiment revealed that the magnitude of the changes in yield varied when plants were exposed to different levels of stress throughout their growth. Seed yield per plant showed a highly significant positive correlation with the dry weight (at R5 stage) of leaf (r=0.901**), stem (r=0.936**) and the whole plant (r=0.917**). These are clear indications about the dependency of seed yield on the dry matter components in the plants grown at different levels of soil moisture. Moreover, dry matter accumulation depended on the soil moisture level. Therefore, a relationship between the seed yield per plant and the prevailing moisture level in the soil could be revealed. The seed dry weight data per plant were plotted against the corresponding soil moisture level expressed in terms of field capacity (Fig. 6). The fitted curve followed the regression equation: Seed weight= ln (soil moisture) R2= 0.999*** This simple equation could help to predict the yield of soybean plants grown in similar soil types and under similar weather conditions. For other conditions, detailed experimentation is essential to standardize an equation suitable for the purpose. The results discussed above can be summarized in the way that an inadequate supply of moisture restricted the growth of plants both in terms of morphology and dry matter production. This effect was eventually reflected in the size and number of seeds per plant and the seed weight as well. The soil moisture levels throughout the growing season had a greater impact on the development and productivity of branches compared with the main stem. That is, soil moisture deficit inhibits branching, and leads to a decrease of node number, leaf expansion, flower number, pod number and seed number. However, based on the percentage of yield shared by the branches and main stem, branches produced the major portion of seed per plant. Therefore, the growth factors which limit branch development can directly inhibit seed yield per plant. In the warmer regions including tropical and subtropical areas, branches are more important for soybean yield. Hence, it is essential for the farmers to monitor the soil moisture status of their field. By monitoring the moisture level in soil and the amount of irrigation water (if necessary) which is practically affordable, a farmer can forecast the quality and production of his crop in combination with other related factors. The relationships derived from the data of this experiment could be more useful for soybean growers and agronomists in the tropical and sub-tropical countries. References 1. BROWN, E. A., C. E. CAVINESS and D. A. BROWN 1985 Response of selected soybean cultivars to soil moisture deficit. Agron. J. 77: BUNCE, J. A Effects of water stress on leaf expansion, net photosynthesis, and vegetative growth of soybeans and cotton. Can. J. Bot. 56: CONSTABLE, G. A. and A. B. HEARN 1978 Agronomic and physiological responses of soybean and sorghum crops to water deficits. 1. Growth, development and yield. Aust. J. Plant Physiol. 5: COX, W. J. and G. D. JOLLIFF 1986 Growth and yield of sunflower and soybean under soil water deficits. Agron. J. 78: DE SOUZA, P. I., D. B. EGLI and W. P. BRUENING 1997 Water stress during seed filling and leaf senescence in soybean. ibid. 89: DOSS, B. D., R. W. PEARSON and H. T. ROGERS 1974 Effect of soil water stress at various growth stages on soybean yield. ibid. 66: \, and D. L. THURLOW 1974 Irrigation, row Fig. 6. Relationship between the soil moisture level and seed dry weight per plant in soybean cv. Akiyoshi. width, and plant population in relation to growth characteristics of two soybean varieties. ibid. 66: FEHR, W. R., C. E. CAVINESS, D. T. BURMOOD and J. S.

10 GHOSH et al.: Soybean growth and yield at moisture stress 29 PENNINGTON 1971 Stage of development descriptions for soybeans, Glycine max (L.) Merrill. Crop Sci. 11: HEATHERLY, L. G Response of soybean cultivars to irrigation of a clay soil. Agron. J. 75: HIDA, Y., T. HIRASAWA and K. ISHIHARA 1995 Differences in dry matter production and root system development between soybean cultivars under deficient soil moisture conditions. Jpn. J. Crop Sci. 64: HIRASAWA, T, K. TANAKA, D. MIYAMOTO, M. TAKEI and K. ISHIHARA 1994 Effects of pre-flowering soil moisture deficits on dry matter production and ecophysiological characteristics in soybean plants under drought conditions during grain filling. ibid. 63: HUCK, M. G., C. M. PETERSON, G. HOOGENBOOM and C. D. BUSCH 1986 Distribution of dry matter between shoots and roots of irrigated and non-irrigated determinate soybeans. Agron. J. 78: HUNT, R Integral durations. In: Basic growth analysis. Unwin Hyman Ltd. (London, UK) pp KONG, Y., K. TOMIDA, J. TATSUMI, T. NONOYAMA, A. YAMAUCHI and J. KITANO 1987 Effects of soil moisture conditions on the development of root systems of soybean John plants NILSEN, Wiley E. and T. (Glycine Sons, ARJMAND and 1979 Moisture-stress Inc. D. max M. (New Merr.). ORCUTT effects York) Jpn. pp. on 1996 J the Crop PATTERSON, R. Water yield P., In: Sci. of C Physiology D. two MOMEN, RAPER limitation. components soybean of 56: N. JR. cultivars. plants N., and Agron. R. J. under H. E. 71: D. stress. CARL GROS R. Groan H. SH