Fruit development and reproductive survival in soybean: Position and age effects

Abstract

Flowering and pod set in soybean ( Glycine max L. Merrill) is a dynamic system in which pod survival may depend on when a pod is initiated and where it is located. Greenhouse (cvs. Lambert and Elgin 87) and field (cvs. Elgin 87 and Essex) experiments were conducted to evaluate, first, the effect of pod size and the timing of pod appearance on pod survival and, secondly, inter-nodal interactions between assimilate supply and reproductive failure. Plants were grown in 3.0 L pots (1 per pot) in a greenhouse where the temperature was maintained between 20 and 30 °C. Large (maximum length) and small (∼15 mm long) pods were marked several times during flowering and then plants were placed under shade (60, 80 or 90%) for 4, 8, 12, and 16 days or until maturity. Large pods almost always survived until maturity—significant reproductive failure (22%) occurred only when pods marked late in the flowering period were exposed to 90% shade. Survival of small pods was reduced by shade (lowest survival was 0%) and the greatest reduction occurred when pods were marked late in flowering when there was a large reproductive sink. Reaching maximum length was the key event in pod survival and the chances of reaching maximum length depended upon when the pod was initiated and the assimilate supply. Leaf and flower removal from individual nodes in greenhouse (cv. Lambert) and field experiments were used to evaluate inter-nodal interactions between assimilate supply and reproductive failure. Removing the leaf from the target node on the main stem when flowering started had no effect on mature pod number at that node; but removing leaves from three adjacent nodes reduced pod number by up to 50% on Elgin 87, but not Essex. Continuously removing flowers from target nodes had no effect on pod number at adjacent nodes. Apparently pod survival at individual nodes is relatively independent of the photosynthesis of the subtending leaf, but the interaction among early and late pods is strictly an intra-nodal phenomena. Survival of a late developing pod at the top of the plant is apparently not affected by large rapidly growing pods at lower nodes. These descriptions of timing, pod size and location effects on pod survival are needed to build realistic dynamic models of pod set in soybean.