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 crit ical period and the number of seeds produced. Response to photoperiod durin g post-flowering stages was evaluated in indeterminate soybean cultivars fr om maturity groups (MG) IV and V The study was conducted under field condit ions with two sowing dates (normal and late). Plants were grown under natur al photoperiod throughout the experiment or exposed, from the R3 stage (beg inning pod) onwards, to artificially extended regimes of 2 h longer than na tural daylength. Duration of the R3-R6 period increased in response to the extension of photoperiod, and cultivars of MG V exhibited a stronger sensit ivity to photoperiod than those of MG IV. Exposure to long photoperiods pro moted node production, mainly in branches, and increased node fertility. Wi thin each sowing date, the increased duration of R3-R6 under longer photope riods 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 p eriod was corrected for temperature differences between treatments. Seed nu mber was also related to the integral of solar radiation during R3-R.6. The possibility of using sensitivity to photoperiod after flowering as a crite rion for increasing yields through increasing seed number are discussed. (C ) 2001 Elsevier Science B.V. All rights reserved.