Effect of and Allelic Variants on Grain Number and Thousand Kernel Weight of Durum Wheat and Their Impact on Final Grain Yield.

The main yield components in durum wheat are grain number per unit area (GN) and thousand kernel weight (TKW), both of which are affected by environmental conditions. The most critical developmental stage for their determination is flowering time, which partly depends on photoperiod sensitivity genes at loci. Fifteen field experiments, involving 23 spring durum wheat genotypes containing all known allelic variants at the PHOTOPERIOD RESPONSE LOCUS ( and ) were carried out at three sites at latitudes ranging from 41° to 27° N (Spain, Mexico-north, and Mexico-south, the latter in spring planting). Allele at , which causes photoperiod insensitivity and results in early-flowering genotypes, tended to increase TKW and yield, albeit not substantially. Allele , also causing photoperiod insensitivity, did not affect flowering time or grain yield. Genotypes carrying the allele conferring photoperiod sensitivity had consistently higher GN, which did not translate into higher yield due to under-compensation in TKW. This increased GN was due to a greater number of grains spike as a result of a higher number of spikelets spike. Daylength from double ridge to terminal spikelet stage was strongly and positively associated with the number of spikelets spike in Spain. This association was not found in the Mexico sites, thereby indicating that had an intrinsic effect on spikelets spike independently of environmental cues. Our results suggest that, in environments where yield is limited by the incapacity to produce a high GN, selecting for may be advisable.