59-05-032 Proceeding

56 Proceedings of the Princess Maha Chakri Sirindhorn Congress THE EFFECT OF MODERATE HEAT STRESS ON PHOTOSYNTHESIS OF FOUR LETTUCE CULTIVARS Siriwat Sakhonwasee Department of Horticulture, Faculty of Agricultural Production, Maejo University Abstract : Lettuce is a popular leafy vegetable consumedworldwide. Lettuce is normally produced in cool area because high temperature induces rib discoloration, stimulates bolting and inhibits growth of lettuce. Understanding the effect of high temperature on photosynthesis of lettuce is necessary for evaluating the large-scale impact of heat stress on lettuce production. In this study, the effects of heat stress on photosynthesis were studied in 4 lettuce cultivars namely, Buttercrunch, Big Boston, Chia-Tia and Kan-tar. Lettuce plants were grown under 25°C/25°C (Day/Night) for 3 weeks before transferred to moderate heat stress condition at 39°C/29°C for 6 days. The result revealed that moderate heat stress condition did not significantly affect CO 2 assimilation rate of all lettuce cultivars. However, heat stress caused significant reduction in maximum quantum efficiency of PSII in some lettuce cultivars. Fresh and dry weights of all cultivars were also reduced after exposure to heat stress. These results suggested that light reaction may be more sensitive to prolonged moderate heat stress than carbon reaction in lettuce. Moreover, it was found that the relative reduction of fresh and dry weights under heat stress condition is larger in Buttercrunch and Big Boston suggesting that these two cultivars may be more sensitive to heat stress than the other two cultivars Keywords : Lettuce, Heat stress, Photosynthesis, Butterhead, Green Oak, Light reaction I. Introduction An increasing of average global temperature induces heat stress and negatively affects production of several crops including lettuce [1]. Generally, temperature higher than 29°C is likely to induce early bolting, rib discoloration, tip burn and growth inhibition which reduce quality of lettuce [2]-[4]. To mitigate the effect of heat stress on lettuce production, it is important to establish breeding program in order to obtain heat tolerant lettuce cultivar. However, heat tolerant is a complex trait involving many pathways and genes [5]. Therefore, profound understanding of plant physiological response to heat stress is necessary for developing a proper screen for selection of heat tolerant crop strains. Crop growth is usually suppressed under heat stress conditionmainly due to the inhibition of photosynthesis. It has been found that heat stress reduce an activity of PSII complex and resulted in photoinhibition [5]. Heat stress condition also reduce activity of rubisco activase, an enzyme required for reactivation of ribulose 1.5-bisphosphate carboxylase/oxygenase [6]. Hence, parameters involving photosynthesis are among the major indicators used for selection of heat tolerant cultivar in crop plants. In this study, photosynthesis of lettuce in different varieties and cultivars, moderate heat stress condition. The prolonged (6 days) and moderate heat stress