Physiochemical mechanisms of cultivarietal differences in the heat tolerance in cucumber

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1.Mechanisms for heat tolerance increases of photosynthetic apparatus (PA) in leaves by environmental stress pretreatments to plants : Water stress and non-lethal high temperature pretreatments caused an increase in heat tolerance of PA as determined by photosynthetic activity and chlorophyll fluorescence of intact leaves before and after exposure to 45ﾟC for 20min. The effect of water stress seems to be mediated by ABA that was increased markedly by water stress, because ABA treatment to non-water-stressed leaves showed the similar effect. A main cause of the increaseed heat tolerance of PA by water stress pretreatment seems to lie on an increased heat stability of electron transport systems in thylakoid membranes, and that by non-lethal high temperature pretreatment on the decrease in fatty acid unsaturation of thylakoid membrane lipids. There were no distinct difference between cultivars in the heat tolerance of PA and the rate of its increase by environmental stress pretreatments.2 … More .Repair of heat injury in the greening of etiolated cucumber cotyledons : When etiolated cotyledons were exposed to various high temperatures for 30 min in the dark and then immediately illuminated at 28ﾟC for 12 hr, greening was more suppressed as the heat temperature was increased When the illumination was prolonged to 38 hr, they were able to green to almost the normal level unless the heat temperature did not exceed 46ﾟC.Cotyledons heat-stressed at 48ﾟC did not green even after 38hr. However, they greened to 80% of the control level when they were placed in the dark at 28ﾟC for 16 hr before illumination for 36 hr. These results suggest that heat injury of greening processes in etiolated cotyledons is repaired by certain mechanisms that are influenced by light and this system is a useful tool for a better understanding of the repair mechansims of heat injury in plants.3.Cultivar differences in heat tolerance ofcucumber roots : The degree of heat tolerance of various cultivars' roots as determined by ion leachage test as well as TTC reduction test did not coincide well with that as determined by regrowth test. High root-zone temperature was less inhibitory to growth but caused a greater inhibition to photosynthesis than high air temperature (both at 37ﾟC). A main cause of photosynthesis inhibition by high root-zone temperature was assumed to be a decrease in stomatal aperture and carboxylation as well ; the activity of photochemical reactions was normal. One of the most important factors responsible for growth inhibition by high root-zone temperatures was assumed to be a marked inhibithion of cytokinin biosynthesis in the roots. Less