Field screening for heat-stress tolerance of floret fertility in wheat (Triticum aestivum and T. durum)

Context As global temperatures increase, heat waves of >30 degrees C for at least 3 days are projected to become more frequent during reproductive development of wheat, causing reductions in grain yield. Breeding for tolerance is difficult under field conditions owing to the unpredictable timing and intensity of natural heat events and the narrow developmental windows of sensitivity. Aims We report on a series of experiments designed to explore genetic variability for floret sterility induced by natural heat waves in the field by targeting a sensitive development stage. Methods Our method involved sowing late, and the use of auricle distance to identify main tillers exposed to heat events at a sensitive developmental stage (booting). Key results Substantial genetic variation was found for tolerance of grain set to natural heat waves, with a broad-sense heritability estimate of 0.94 for heat-exposed stems compared with 0.72 for non-stressed controls. An auricle distance of 1-3 cm was established as a phenotypic marker for tagging stems at the sensitive stage of development, and the optimal number of spikes to assess was determined as 10-15 per plot. A validation study with durum lines previously classified as tolerant/intolerant under glasshouse conditions showed agreement in ranking of genotypes at the extremes of tolerance. Conclusions We demonstrate that it is possible to screen wheat genotypes for stage-specific tolerance to natural heat waves in the field. Implications The protocol described could provide a low-cost method for evaluating heat tolerance in wheat, with relevance to commercial field production conditions. The approach allows tolerance to be assessed at a single stage of reproductive development, which should allow greater heritability in field assessments.