| Heat stress is a common abiotic stress factor in many areas where wheat (Triticum aestivum L.) is grown, including the central and southern Great Plains of the U.S.A., and there is demand for heat-tolerant wheat germplasm suited to those areas. Cell membranes are the site for many biological activities of the plant and they play a key role in heat-induced damage to the plant. This damage can be assayed in the lab by the membrane thermostability (MTS) assay which involves the measurement of electrolyte leakage from leaf tissue after exposure to high temperature. Also, the damage can be assessed by the effects on the electron transport system of the mitochondria. The effects can be quantified by measuring the reduction of 2,3,5-triphenyl tetrazolium chloride (TTC) to formazan by dehydrogenase respiratory enzymes in heat-stressed seedlings. Many studies have reported MTS and TTC as criteria for heat tolerance in plants. However, genetic studies on MTS and TTC are not abundant. Therefore, the objectives of this study were: (1) to evaluate the genetic variability of wheat using MTS and TTC assays, (2) to estimate the heritability of MTS and TTC by parent-offspring regression, parent-offspring correlation, and realized heritability using F3 plants and their F4 progeny means, and (3) to determine the genetic control of heat tolerance, as measured by the MTS assay, through diallel analysis of selected wheat germplasm. Results from the two assays were found to be highly associated (r = 0.62, n = 14, P < 0.05). Parent-offspring regression and correlation heritability was intermediate to high (0.50–0.65) for TTC and relatively low (0.32–0.38) for MTS. Realized heritability, based on 15% selection intensity, was intermediate to high (0.49–0.64) for TTC and low to intermediate (0.27–0.47) for MTS. The high heritability of TTC warrants good progress from selection in early generations. The relatively lower heritability of MTS suggests the use of enough replications (> 3) during selection to limit the environmental variation. The diallel analysis revealed that the mean squares of general combining ability (GCA) was four times that of specific combining ability (SCA), indicating the importance of additive gene effects in acquiring tolerance to heat. Maternal effects accounted for 67% of reciprocal variation, suggesting the need for careful choice of the female parents in crossing programs for heat tolerance. Both MTS and TTC assays are simple and fast, and heat tolerance measured by them is heritable. This makes them good traits for screening heat-tolerant wheat genotypes. The next challenge, however, is to establish additional experimental evidence that screening wheat germplasm with these two assays will lead to superior performance in hot environments. |
