Development and seed number in indeterminate soybean as affected by timing and duration of exposure to long photoperiods after flowering

Background and Aims Long photoperiods from flowering to maturity have been found to delay reproductive development in soybean (Glycine max) and to increase the number of seeds per unit land area. This study was aimed to evaluate whether sensitivity to photoperiod after flowering (a) is quantitatively related to the length of exposure to long days and (b) persists throughout the whole pod-setting period. It was also evaluated whether seed number was related to changes in the duration of post-flowering phenophases. Methods Two field experiments were conducted with an indeterminate cultivar of soybean of maturity group V. In expt 1, photoperiods 2 h longer than natural daylength were applied during different numbers of days from the beginning pod stage (R3) onwards, while in expt 2 these photoperiod extensions were imposed during 9 consecutive days starting at different times between R3 and R6 (full seed) stages. Key Results There was a quantitative response of development to the number of cycles with a long photoperiod. The exposure to long photoperiods from R3 to R5 (beginning of seed growth) increased the duration of R3-R6 regardless of the timing of exposure. The stages of development comprised in the R3-R6 phase were delayed by current as well as by previous exposure to long days. A positive relationship was found between seed number and the duration of R3-R6, irrespective of the timing and length of exposure to the long photoperiod. Conclusions Sensitivity to photoperiod remained high during the reproductive period and was highly and positively coupled with the processes of generation of yield.