Effect of Defoliation and Depodding on 14CO2 Assimilation and Photosynthate Distribution in Y‐Shaped Soybean Plants1

Abstract

To determine the importance of long distance translocation to the yield of soybeans (Glycine max L. Merrill), field‐grown plants were modified by selective pruning to create an imbalance in the number of leaves and the number of pods on opposing branches of a Y‐shaped plant. The Y‐shaped plants were created early in vegetative growth by pinching off the growing apex when the first trifoliolate leaf appeared. After flowering, one group of these Y‐shaped plants was pruned by removing one‐third of the leaflets and one‐third of the pods from both branches (the even treatment), another group was pruned by removing two‐thirds of the leaflets from one branch and two‐thirds of the pods from the opposite branch (the uneven treatment). Plants given the even and uneven treatment were labelled with 14CO2 on three occasions corresponding to days early, middle, and late in pod‐filling. After 24 h, labelled plants were harvested, dried, and the specific radioactivity in each part and the distribution of radioactivity in the various parts of the plant were determined. Some of the labelled plants were allowed to mature before harvest and analysis.The changed leaf area of the treatments remained to the end of the season, but the changed pod distribution did not. Increased abortion of pods and seeds on the deleafed branch and decreased abortion on the depodded branch of the uneven treatment equalized the reproductive load on all the pruned branches during most of the pod‐filling period. The pruning had no effect on photosynthesis per unit leaf area, as measured by assimilation of 14CO2, or on the rate of disappearance of radioactivity from leaves in the first few hours after labelling. Moreover, the movement of the radioactivity out of the leaf or into the seed was unaffected. The uneven treatment did induce a change in the amount of photosynthate retained in the stem of the two different branches; the depodded stem retained twice the radioactivity of the deleafed stem after 24 h. The difference in radioactivity in the stems of different treatments noted 24 h after labelling was also found at the end of the season, though the level of radioactivity was reduced by a factor of 2.The resistance to translocation is sufficient to alter the distribution of concurrent photosynthate to the stem, petiole, and podwall parts but it is not sufficient to change the uptake of photosynthate into the seeds of Y‐shaped plants with an uneven distribution of sources and sinks.