Heat stress constitutes a major threat to crop production, and according to climatic projections, night temperatures are expected to increase faster and to a greater extent compared to day temperatures. While extensive research has been dedicated to the effects of higher than optimum day temperatures on cotton physiology, metabolism, and yield, and while heat-tolerant cotton cultivars have been introduced, the responses of such heat-tolerant cultivars to high night temperatures have not been evaluated. The objective of this study was to assess the efficiency of heat-tolerant cultivars to high night temperatures stress by monitoring the physiological and biochemical responses of two cotton cultivars, differing in thermotolerance, subjected to higher than optimum night temperatures, during anthesis. To that end, growth chamber experiments were conducted using two cotton cultivars differing in thermotolerance, namely ST5288B2RF (thermosensitive) and VH260 (thermotolerant). Treatments consisted of normal day/night temperatures (32/24 °C) and high night temperatures (32/30 °C) for 2 weeks at flowering (approximately 8 eight weeks after planting). The results indicated that VH260 was more thermotolerant than ST5288 even under conditions of high night temperature stress, as it managed to maintain its net photosynthetic rates, cell membrane integrity, as well as pistil carbohydrate contents and ultimately achieved higher total reproductive weight. It was concluded that heat tolerance of thermotolerant cultivars selected under conditions of high day temperatures is also conserved under high night temperatures, while net photosynthetic rates and cell membrane integrity can be utilized as selection traits for heat tolerance under either high day or night temperatures.
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