SEPARATING PHOTOPERIOD AND TEMPERATURE EFFECTS ON THE DEGREE DAY REQUIREMENT FOR FLORAL EVENTS IN SOYBEAN

A date-of-planting study of time from emergence (VE) to floral bud initiation (R0, detection of floral bud primordium) and first flower (R1) was carried out at Urbana IL USA for soybean genotypes representing maturity groups (MG) 00 through VIII. Growing degree days (GDD), base 10℃ with a cutoff at 30℃, were calculated for VE-R0, R0-R1 and VE-R1 and were plotted against the averge photoperiod between events. We found a temperature relationship among GDD values between events for MG's 00-I; subtracting these GDD values from those of MG II or greater and plotting the residual or adjusted GDD (AGDD) values (or 1/AGDD values) vs. average photoperiod improved the linear or curvilinear relationship involved. Photoperiod effects on R0-R1 were significant for later -maturing genotypes (MG IV+or greater) as reported earlier. These phenology-temperature-photoperiod relationships greatly improve our ablility to predict phenology with computers for real world situations.