SOYPOD: A Model of Fruit Set in Soybean

The soybean [ Glycine max (L.) Merr.] plant matches its reproductive potential to environmental conditions by adjusting the number of fruits and seeds that it produces. This adjustment is an important, but not well understood, part of the yield production process. Developing a model of fruit set will contribute greatly to a better understanding of the determination of fruit number and yield. The model (SOYPOD) is based on the well documented principle that fruit number is related to assimilate availability during flowering. Inputs, including daily photosynthesis, flower production, and potential pod and seed growth rates, were taken from field and greenhouse experiments. Fruit survival at an isolated node was determined by comparing the daily assimilate supply with the assimilate requirement of the developing fruits, which are sensitive to supply only until the pod wall reaches its maximum size. Abortion occurs when the fruit does not receive assimilate for ABRTM consecutive days during the sensitive period (SEN). The most mature fruits receive assimilate first when assimilate is limiting. It was possible to find realistic combinations of ABRTM and SEN that mimicked the curvilinear response of fruit number to increasing assimilate supply observed in planta. Increasing the individual seed growth rate decreased fruit number when the assimilate supply was held constant, as it does in planta. SOYPOD accurately mimics the fruit set process at isolated nodes in soybean suggesting that it can be used as the basic building block of a whole plant model.