Effect of Photoperiod at Different Growth Stages on the Initiation of Spikelet Primordia in Wheat

Leaf and spikelet initiation of wheat (Triticum aestivum L.) was studied on plants grown in cabinets under a range of constant photoperiod regimes and on plants transferred, at different growth stages, between these regimes. The existence of genotypically specific limits to the effects of short photoperiods on the delay in floral initiation and on the increase in the number of spikelets was demonstrated. A long photoperiod applied after the onset of apex elongation, but not earlier, induced the differentiation of spikelets in the axils of already existing primordia, which under a short photoperiod would have developed into leaves. The initiation of the terminal spikelet was induced by a long photoperiod applied not later than the double ridge stage. The final number of spikelets had been determined by the time of swelling of the spikelet primordia in the central portion of the spike.