DIURNAL CHANGE OF PHOTOSYNTHESIS AND ITS RELATION TO YIELD IN SOYBEAN CULTIVARS

Apparent photosynthetic rates (AP) of plant leaves often decline in midday even when solar radiation is adequate. The objective of this study was to determine whether this phenomenon is responsible for the cultivar differences in growth and yield of soybean (Glycine max (L.) Merr.). In a 3 year experiment using nine (1990) or ten (1991, 1992) cultivars, the association of the following parameters with growth and yield of cultivars was examined: APam (AP measured about 10h), APpm (AP measured about 14h), daily mean AP ([APam + APpm]/2) or APdec (APpm/APam, %) during flowering to early pod-filling. Cultivar differences in these parameters were observed and the ranking among cultivars tended to be consistent, although the significance varied with the day or year. None of the parameters were consistently correlated with growth rates or yields across cultivars. For each cultivar, however, yearly variation in yield closely cor-responded to variation in APdec. The results suggest that less decline of midday photosynthesis contributes to yield stability in soybean cultivars.