Photoperiod during stem elongation in wheat: is its impact on fertile floret and grain number determination similar to that of radiation?

Increasing duration of stem elongation by exposure to short photoperiod would result in higher spike dry weight at anthesis, which is positively associated with the number of fertile. florets and grains in wheat. However, it is not easy to determine whether photoperiod effects on fertile florets and grains are only mediated by assimilate supply to the growing spike when spikeweight variation is attained only with photoperiod treatments. The aim of this study was to determine whether photoperiod effects on number of fertile florets and grains may be direct, that is, not mediated by assimilate supply, by comparing the magnitude of photoperiod effects with those of shading the canopy. Spike dry weight at anthesis was changed through the factorial combination of different photoperiod ( natural and 6 h extended photoperiod) and shading (un-shaded and 67 +/- 3% shaded) treatments during stem elongation of Buck Manantial, a cultivar known for its photoperiod sensitivity in this phase. Both treatments modified spike dry weight at anthesis and the number of fertile florets and grains, independently. When duration of stem elongation was lengthened by exposure to natural photoperiod and when incident radiation was high, spike dry weight at anthesis increased by 33% (NP+0 v. NP+6) and 27% ( un-shaded v. shaded), respectively. The number of fertile florets increased similarly to spike dry weight (34% NP+0 v. NP+6 and 28% un-shaded v. shaded) resulting in higher number of grains. Most photoperiod effects on the number of fertile florets and, consequently, on the number of grains, were mediated by assimilate supply to the growing spike as the same relationship between the number of fertile florets and spike dry weight at anthesis was observed for photoperiod and shading treatments (R-2 = 0.99, P< 0.05).