Effect of photoperiod and chlormequat on apical development and growth in a spring wheat (Triticum aestivum) cultivar

Spring wheat, cv. Timmo, was grown under three photoperiod regimes (16, 13 and 11 h) with and without treatment with the plant growth regulator chlormequat (applied at the glume primordium stage of apical development) and the relationships between apical development, primordium initiation and growth stage examined. The effects of photoperiod were generally similar to those reported from other studies; shorter photoperiod slowed the rate of apical development, increased the duration of the primordium initiation phases and reduced the rate of primordium initiation. The final number of spikelets was increased, but there was no effect on number of floret primordia per spikelet The number of tillers produced was also higher in the shorter photoperiods. Chlormequat treatment had a similar effect to imposing short-days: flowering was delayed and tiller production increased. There were strong correlations between certain development events and the phasing of primordium initiation and growth stages and these were not affected by photoperiod or chlormequat treatments. For example, the end of spikelet primordium initiation, i.e. terminal spikelet (TS) formation, coincided with the floret-stamen primordium stage (of the most advanced spikelet) and the end of floret primordium initiation with the stigma tic branches and hairs on ovary wall elongating stage. Similarly, rapid stem extension growth always started at TS formation while spike extension and spike growth commenced at TS formation and the stigmatic branches stage, respectively. Tiller production also ceased at TS formation, when rapid stem growth started. Although the timing of the phases of primordium initiation and certain growth events were linked to apical development, the rate of apical development did not determine either the rate of spikelet primordium initiation or the rates of stem and ear growth. However, there was a strong relationship between rate of development and rate of floret primordium initiation. There was also a strong relationship between spike length and apical development stage.