Reproductive development and yield components in indeterminate soybean as affected by post-flowering photoperiod

Pod and seed number are the most important yield components in soybean ( Glycine max (L.) Merrill) crops. Crop growth rate during post-flowering and the duration of the period when pod and seeds are formed explain much of the variation of soybean yields across genotypes and environments. Exposing post-flowering stages to long photoperiod has been found to extend the period R3–R6 and to increase seed number in soybeans grown under field conditions. In this paper, post-flowering development and yield components responses to photoperiod were quantitatively analysed and the degree of coupling between both responses was investigated. Indeterminate soybean cultivars, A-5409 (maturity group V) and Dekalb CX-458 (maturity group IV), were grown under field conditions and exposed to natural photoperiod from sowing to the beginning pod stage (R3). From then onwards, they were either kept under natural daylength or exposed to four photoperiod regimes that were artificially extended in relation to natural daylength by 1.5, 3.0, 4.5 or 6.0 h. All the extended photoperiod regimes increased the duration of R3–R6 period. Both cultivars showed a quantitative type of response through the whole range of explored photoperiods, though A-5409 exhibited a stronger sensitivity. Development responses during the R6–R8 phase were less noticeable and more variable. Exposing plants to extended photoperiod increased the number of nodes per plant and improved node fertility, thus increasing the number of pods and seeds produced per unit area. Average seed weight tended to be reduced in plants exposed to extended photoperiod and the magnitude of these effects depended both on cultivars and treatments. However, seed size was reduced in ca. 20% while seed number was increased by more than 75% due to the treatments. These results strengthen the hypothesis that manipulating photoperiod sensitivity during post-flowering in indeterminate soybean may actually be an avenue to increase seed number and yield.