Photoperiod sensitivity after flowering and seed number determination in indeterminate soybean cultivars

Seed and pod numbers are the main yield components in soybean (Glycine max (L.) Merrill). They are related to canopy photosynthesis during a critical period, occurring between the RI (be-inning of flowering) and R6 (full seed) stages. We investigated the relationship between the duration of the critical period and the number of seeds produced. Response to photoperiod during post-flowering stages was evaluated in indeterminate soybean cultivars from maturity groups (MG) IV and V The study was conducted under field conditions with two sowing dates (normal and late). Plants were grown under natural photoperiod throughout the experiment or exposed, from the R3 stage (beginning pod) onwards, to artificially extended regimes of 2 h longer than natural daylength. Duration of the R3-R6 period increased in response to the extension of photoperiod, and cultivars of MG V exhibited a stronger sensitivity to photoperiod than those of MG IV. Exposure to long photoperiods promoted node production, mainly in branches, and increased node fertility. Within each sowing date, the increased duration of R3-R6 under longer photoperiods was corresponded with increments in pod and seed number. Seed number was related to the duration of R3-R6, particularly when the length of the period was corrected for temperature differences between treatments. Seed number was also related to the integral of solar radiation during R3-R.6. The possibility of using sensitivity to photoperiod after flowering as a criterion for increasing yields through increasing seed number are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.