Abstract
Heat tolerance is measured at tissue level by cellular membrane thermostability (CMT) and at the whole plant level by the heat tolerance index (HTI). Eight upland cotton cultivars and 15 crosses were used to determine the type and extent of genetic variability associated with the expression of these traits between and within environments. Heat stress and non-stress conditions were used as the CMT environments and years for HTI. The wide variation in heterotic expression and combining ability effects observed for CMT and HTI suggest multigenic inheritance of these traits. Significant genetic variability across environments was evident but the traits were not highly heritable because of substantial environmental interaction. The available genetic variability included both additive and non-additive components, but the proportion of additive genetic variability was high for HTI. The parental cultivars CRIS-19 and CIM-448 were good donor parents for high CMT under heat-stressed conditions, and MNH-552 and N-Karishma under non-stressed conditions. Cultivar FH-634 was a good donor parent for HTI. The results show two types of general combining ability (GCA) inheritance among high CMT parents: positive GCA inheritance expressed by CRIS-19 in the presence of heat stress and MNH-552 and N-Karishma in the absence of heat stress; and negative GCA inheritance expressed by FH-900 in the presence of heat stress. It was also evident that genes controlling high CMT in cultivar CRIS-19 were different from those present in the MNH-552, N-Karishma and FH-900 cultivars. Similarly, among high HTI parents, FH-634 showed positive and CIM-443 negative GCA inheritance. No significant relationship due to genetic causes existed between tissue and whole plant heat tolerance, diminishing the likelihood of simultaneous improvement and selection of the two traits.
Highlights
Heat stress primarily affects photosynthetic activity (Berry and Bjorkman, 1980) and cellular membranes (Raison et al, 1980), but its ultimate economic effect is reduced yield
The objectives of the present study were to evaluate the effect of the environment on the available genetic variability associated with tissue and whole plant heat tolerance to decide on their potential as breeding objectives, and to evaluate the relative worth of upland cotton cultivars in terms of their combining ability effects for the two traits
The parents x temperature regime interaction was non-significant (p > 0.05), indicating that the relative ranking among parental cultivars for cellular membrane thermostability (CMT) remained consistent across temperature regimes in Source df relative cell injury (RCI) mean squares heat tolerance index (HTI) mean squares
Summary
Heat stress primarily affects photosynthetic activity (Berry and Bjorkman, 1980) and cellular membranes (Raison et al, 1980), but its ultimate economic effect is reduced yield. The cellular membrane thermostability (CMT) assay (Sullivan, 1972; Sullivan and Ross, 1979) is an indirect screening technique for heat tolerance and provides a reliable measure of tissue tolerance to heat stress (Raison et al, 1980, Blum and Ebercon, 1981). The CMT assay has been successfully used to identify heat tolerant and susceptible genotypes in several crop species, including cotton (Rahman et al, 2004). Information on the genetic behavior of CMT and HTI in upland cotton has not been established but is imperative to understanding the genetic bases of the two traits and providing theoretical grounds for applied cotton breeding programs
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