Seasonal variations in apparent photosynthesis among plant stands of different soybean cultivars.

The CO2- and H2O-exchange rates between soybean canopies and the atmosphere were measured in three mobile chambers (4 m(3)). Each chamber stopped at 8 or 9 plots (3.1-m(2) ground area) every 25 min. Diurnal and seasonal CO2-exchange rates (CER) of 13 soybean (Glycine max (L.) Merr.) cultivars are summarized here. The oldest two cultivars, released in 1927 and 1932, had the lowest CER values. The CER usually decreased in the afternoon (23.4 vs 27.8 μmol CO2 m(-2) s(-1) at 1.6 mmol photons m(-2) s(-1)), except shortly after rainfall. During a drought, these reductions occurred earlier in the day and were more pronounced. We present evidence for a nonstomatal component of the CO2 flux-reaction system causing CER reductions during a water stress. Daytime CER values were not correlated with temperature (24-34° C), but nighttime values were (15-25° C, r=0.85,* n=41).