Responses of five cultivars of cotton (Gossypium hirsutum L.) B-557, CIM-70, MNH-93, NIAB-78 and S-12 to high temperature stress were assessed at germination and a later growth stage under controlled environmental conditions. Although all five cultivars did not germinate at 50°C, B-557 and MNH-93 had relatively higher percentage germination at 40°C than the other cultivars. In the growth experiment, the cultivars were subjected to 30°C (control) or 48°C for a 21-day period after 3 weeks of initial growth at 30°C. Cultivars B-557 and MNH-93 had lower relative injury (higher membrane thermostability) and were higher in fresh and dry mass production than the other three cultivars. The cultivars did not differ in leaf water potential but CIM-70 and S-12 had significantly greater leaf turgor potential than the others. Of the four different low molecular weight organic osmotica determined in this study, leaf soluble proteins, soluble sugars and proline were significantly higher in B-557 and MNH-93 as compared with those in the other three cultivars. It is concluded that degree of heat tolerance in cotton at germination and later growth stage does not vary. Membrane thermostability is a successful measure of heat tolerance in cotton. Leaf soluble proteins, proline and soluble sugars are important adaptive components of heat tolerance of this crop.
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