Alterations in Plant Growth and Dry Matter Distribution in Soybean

The partitioning of assimilate between vegetative and reproductive plant parts of soybean [Glycine mar (L.) Merrill] during flowering and fruit set could be related to fruit and seed number, and yield. Experiments were conducted in the field (cv. Cumberland) for 2 yr to investigate the effect of alterations in plant growth on the distribution of dry matter during flowering and fruit set. Alterations in plant growth were created by reducing the insolation (63%) during flowering and fruit set by using commercial shade cloth and by low density plantings (spaced plants, approximate equidistant spacing with 0.9 m between plants). The control and shade treatments were grown in 0.76-m rows (0.71-m in 1984), and conventional cultural practices were used. Partitioning ratios (reproductive mass/total biomass) were calculated from four samples collected between R1 and R6. Leaves (uppermost fully expanded leaf and the leaf four nodes below it) were sampled during flowering and fruit set for carbohydrate and N analysis. Shade reduced crop growth rate (Rl-R5), vegetative mass at R5, seed number per square meter, and yield. The spaced plants were much larger and had nearly 80% of their reproductive mass on the branches at R6 compared with 5% for the control plants. Leaf starch and soluble sugar concentrations tended to be reduced by the shade treatment and increased in the spaced plants. There were no treatment effects on leaf N concentration. The treatments had a minimal effect on the partitioning ratios, indicating that distribution of dry matter between vegetative and reproductive plant parts was relatively constant across large differences in plant size and growth rate. This suggests that the fruit and seed numbers may be more closely related to crop growth rate than to the partitioning of assimilate. Contribution from, the Dep. of Agronomy, ,Keritucky Agnc. Exp. Stn., Lexington. This paper (87-3-17) i s published with the approval of the director of the Kentucky Agnc. Exp. Stn. This research was supported, in part, br USDA-CSRS grant no. 59-221 1-0-2-01 9.