SOYBEAN SEED QUALITY REDUCTIONS BY HIGH DAY AND NIGHT TEMPERATURE

Environmental stress during reproductive growth of soybean [Glycine ma r (L.) Merr.] may reduce seed germination and vigor. Most studies of e nvironmental influence on soybean seed germination and vigor have conc entrated on the effects of drought, maximum day temperature, or averag e daily temperature during seed fill. This study was conducted to dete rmine the effects of different combinations of day temperatures and ni ght temperatures during reproductive growth on soybean seed quality. D ay/night temperatures of 30/20, 30/30, 35/20, and 35/30 degrees C were imposed at flowering and pod set (R1-R5), seed fill and maturation (R 5-R8), and during the entire reproductive growth period (R1-R8). High day and night temperature reduced soybean seed germination and seedlin g vigor. The extent of reductions in germination and vigor by high tem perature was influenced by duration of temperature exposure and phase of reproductive growth. Temperatures during flowering and pod set exer ted little influence on soybean seed quality. A combination of 35 degr ees C day temperature and 30 degrees C night temperature during seed f ill and maturation or 35 degrees C day temperature from flowering to m aturity decreased germination. Seedling vigor was reduced by 35/20, 30 /30, and 35/30 degrees C during seed fill and maturation and the entir e reproductive period. Increase in night temperature from 20 to 30 deg rees C decreased seedling vigor at both 30 and 35 degrees C day temper atures. These results suggest that locational and seasonal differences in soybean seed quality may be induced by high temperatures during re productive growth and indicate that night temperatures along with day temperatures are important in determining soybean seed quality.