INFLUENCE OF DAY AND NIGHT TEMPERATURE ON SOYBEAN SEED YIELD

Growth temperature influences the seed yield of soybean [Glycine maw ( L.) Merr.]. Most studies have concentrated on the effects of concomita nt increases in day/night temperature during the entire life cycle of soybean. A better understanding of the influence of combinations of hi gh day and night temperature imposed during different reproductive gro wth phases on soybean seed yield would be beneficial in identifying yi eld limiting factors. Day/night temperatures of 30/20, 30/30, 35/20, a nd 35/30 degrees C were imposed during flowering and pod set (R1-R5), seed fill and maturation (R5-R8), and during the entire reproductive p eriod (R1-R8). increases in day temperature resulted in decreased seed formation when plants were exposed during dowering and pod set and de creased seed growth when exposed during flowering and pod set or seed fill. Seed growth reductions in plants exposed to the high day tempera ture were accompanied by decreased photosynthetic rates. The largest y ield reduction in this study was 27% and occurred when 35 degrees C oc curred for 10 h per day from flowering to maturity. No significant los ses in yield occurred at high night temperature at any reproductive gr owth phase. The only significant interaction between day and night tem perature for the yield components was for seed weight per plant during flowering and pod set. Night temperature stress did not occur at 30 d egrees C and a night temperature of 20 degrees C did not reduce the yi eld loss from daytime high temperature stress. This study suggests tha t soybean seed yield reductions from high temperatures are primarily a response to day temperature and moderate to high night temperatures h ave a small affect on soybean seed yield components.