Genotypic differences in effects of short episodes of high-temperature stress during reproductive development in sorghum

Recent studies on sorghum [Sorghum bicolor (L) Moench] showed that episodes of high-temperature stress around anthesis could adversely affect grain yield via effects on pollen germination with varying responses among genotypes. The objectives of this study were to determine the reproductive development phase most sensitive to heat stress and to determine whether genotypic differences in high-temperature tolerance were associated with differences in either cardinal temperatures for pollen germination, the duration of the period of sensitivity to high temperatures, or the extent of the response to high temperature per se. To determine cardinal temperatures, pollen from six genotypes, differing in high-temperature tolerance, was incubated on a gradient plate at temperatures between 10 and 40°C. To identify the critical period of high temperature sensitivity, plants of two genotypes with contrasting high-temperature tolerance were grown under optimum (31 vs. 21°C) and high (38 vs. 21°C) day vs. night temperatures, with selected plants reciprocally transferred between the two temperatures for 5-d periods from just before flag leaf full expansion until 20 d thereafter. Pollen germination, seed set, and grain yield were most sensitive to high temperatures immediately around anthesis. Effects on seed mass were smaller and independent of seed set. Genotypes differed in the magnitude of the response to high temperatures, rather than in the duration of the critical period, and the adverse effect of high temperatures increased with duration of exposure during this critical period. These differences were not associated with cardinal temperatures for pollen germination. These results could assist in identifying selection targets for crop improvement programs.