Photosynthesis and Productivity during High‐Temperature Stress of Wheat Genotypes from Major World Regions

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Abstract
Photosynthetic rates and thylakoid activities are adversely affected by high temperatures in wheat (Triticum aestivum L.), but their role in determining productivity during stress is uncertain. Our objectives were to ascertain relationships and genotypic variability among single‐leaf photosynthesis, thylakoid activity, and productivity during high‐temperature stress at vegetative and reproductive stages. Ten genotypes from major world wheat‐producing regions were grown under moderate (22/17 °C day/night) and high (32/ 27 °C day/night) temperatures for 2 wk as seedlings or from anthesis to maturity. Net photosynthesis, chlorophyll variable fluorescence (Fv), and productivity were measured at both stages. High temperature decreased mean photosynthetic rates 32 and 11%, mean Fv 42 and 11%, and mean total biomass 32 and 15% in seedling and maturing plants, respectively. The regime also reduced mean kernel weight 20% and mean grain yield 23% relative to the moderate temperature. Decreased photosynthetic rates and diminished productivity of plants were highly significantly correlated at both growth stages. Relative grain yields were strongly influenced by decreased duration of photosynthetic activity, which ranged widely in the 10 genotypes. Thylakoid membranes from seedlings of the 10 genotypes were heated at 20 to 35 °C, and uncoupled electron transport and Fv were determined. Thylakoid stability to heating varied similarly to whole‐plant responses to high temperature by seedlings, but not by maturing plants. Genotypes that were most tolerant of high temperature had stable rates and/or long durations of photosynthetic activity, high kernel weights, and high harvest indices, which are potential selection criteria.