Yield Potential in a Dwarf Spring Wheat and the Effect of Shading1

A dwarf spring wheat (Triticum aestivum ‘Yecora 70’) was grown under irrigation and optimal agronomy in northwest Mexico during four winter seasons. Single shading treatments of approximately four (1971, 1972), three (1973), and two (1974) weeks duration and various intensities were used to elucidate the importance for grain yield of radiation and crop photosynthesis at different stages of development. Shading always reduced crop growth, approximately in direct proportion to the reduction in radiation. Grain yield reduction was however curvilinearly related to radiation such that small reductions had little effect on yield at any developmental stage. Yield reductions with heavier shading only occurred with shading after about 60 days after seeding and were greater before anthesis (90 days) than after anthesis, especially in 1973. The shorter shading treatments indicated that the period of rapid spike growth, approximately 35 to 5 days before anthesis, was that showing greatest sensitivity to shading. Shading then led to fewer spikes/m2 (lower tiller survival) and fewer grains/splkelet (fewer florets produced), hence reduced grains/m2 Responses to post‐anthesis shading suggest that the degree of yield limitation by post‐anthesis photosynthesis is not constant for a given genotype, varying substantially from year to year. Thus in 1973 immediate pre‐anthesis conditions were unfavorable due to cloudiness, grains/m2 was low, and yield was entirely limited by this factor. In other years, yield was more sensitive to post‐anthesis shading. However it is concluded that grains/m2, hence crop photosynthesis in the immediate pre‐anthesis period, is the key to higher yield in this cultivar.