Kernel Number Per Spike in Wheat ( <i>Triticum aestivum</i> L.): Responses to Preanthesis Shading

Several cultivars of wheat, grown in pots in a controlled environment, were subject to single shading periods of 8-11 days within the interval from 36 days before anthesis until anthesis. The aim was to confirm sensitivity of kernel number to reduced irradiance as observed in the field and to investigate its causes. Shading periods centred 10-13 days before anthesis had biggest effects on kernel number per spike through effects on kernels per spikelet. This was associated with greatest reductions in the spike dry weight at anthesis and with reductions in the number of competent florets (florets with full anther development) per unit spike dry weight; floret fertility (kernels per competent floret) was unaffected. Approximately 30% of the total dry weight increment of the plant was being allocated to the spike at the stage of greatest sensitivity to shading, but shading did not alter this partitioning. Maximum water soluble carbohydrate (WSC) content of the spike (30-40% on a dry basis in unshaded plants) occurred at 15 days before anthesis. Shading at the most sensitive period reduced temporarily WSC concentration in the spike, reduced stainable starch in carpels of florets of central spikelets, and reduced floret survival by increasing degeneration of distal florets. Shading before or after this period reduced kernel number less, reducing spike dry weight but not competent florets per unit spike dry weight. Reduced assimilate supply is probably the cause of these responses in the spike.