When crops of wheat (Triticum aestivum L.) are stressed, grain set in potentially fertile florets is reduced. Cold temperatures and boron (B) deficiency during reproductive development cause grain set failure in wheat. Patterns of grain set in cold-stressed and B-deficient wheat ears were studied under field conditions in Nepal and in controlled environments in the UK. In both B-deficient and cold-stressed circumstances, ear fertility was reduced by up to 98% but the pattern of grain set within an ear was similar. Under cold-stressed conditions, florets in the uppermost one-third of the ear were 41 to 53% less fertile than those located in the middle and basal regions. Even in the unstressed crops, the top one-third of the ear was less fertile than below by as much as 8–13%. Similarly, within a spikelet, the distal florets always had fewer grains than the proximal ones. Decreased grain set following stress markedly reduced yield per ear. We conclude that fertility should be assessed on the entire ear. The determination of competent florets should be based on the presence of well-developed ovaries, feathery stigmas and the structures of anthers (which can still be seen in the sterile florets at maturity) rather than on the length of the lemma or on judgements based on visual appearance or other subjective criteria.
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