Shade and temporal distribution of pod production and pod set in soybean

The temporal distribution of pod production and pod survival play an important role in determining pod and seed number in soybean (Glycine max L. Merrill). We investigated the effect of changing photosynthesis at growth stage RI (beginning flowering) on these temporal distributions in two greenhouse experiments. Plants ('Elgin 871) were exposed to two levels of shade (60 and 90%) from growth stage R1 to maturity. Other plants were removed from 90% shade or placed under 90% shade midway through flowering (transfer treatments). Temporal distributions of pod production and pod survival were determined by marking all unmarked pods >= 10 mm long on plants every three days with different colored paint. The color of paint on the mature pods identified when they started development. Continuous shade reduced mature pods by 27 (60% shade) and 82% (90% shade), but it shortened the pod-production period in only one of four comparisons. Pod production responded quickly to transfer treatments, and the mature pod load was always greater (nearly three fold) than the continuous 90% shade treatment and less (average of 53%) than the control. The mature pod load failed to recover from early shade because the increase in radiation did not lengthen the pod-production period and not enough pods were produced. Pod production was often more important than pod abortion in determining mature pod number. Adding the temporal distribution of pod production and survival to models predicting pod and seed number will improve their accuracy.