59-05-032 Proceeding

58 Proceedings of the Princess Maha Chakri Sirindhorn Congress Heat stress was shown to induce deactivation of rubisco and reduce A in various crops such as tomato, rice, wheat and cotton [6], [8]-[11].The results also show that moderate heat stress condition causes reduction of A in all lettuce cultivars, even though these were not statistically significant (Figure 1 a).The reduction of A in ‘Buttercrunch’ and ‘Big Boston’ may not be attributed to stomatal limitation since gs values were not reduced under heat stress condition. On the other hand, ‘Chia-Tia’ and ‘Kan-Tar’ exhibited subtle reduction of gs under heat stress condition which may indicate stomatal limitation of photosynthesis. E of Butterhead is decreased in response to heat stress whereas, in green oak, E is increased. It might be reasonable to speculate that cultivars exhibiting higher E values should be more efficient in dissipating heat from leaf and resulting in lower Tl, but this is not the case of this experiment. ‘Kan-Tar’, which showed highest E value under heat stress condition, had highest Tl values among all lettuce cultivars. This could occur partially because leaf of ‘Kan-Tar’ may contain higher water content than the other cultivars, resulting in accumulation of higher amount of heat energy. Therefore, in case of ‘Kan-Tar’, high E may not be enough to dissipate the larger amount of accumulated heat energy and cool down leaf surface temperature to the similar level as the other cultivars. Figure 1 Effect of heat stress on gas exchange parameters in 4 lettuce cultivars. (a) Net photosynthesis rate. (b) Stomatal conductance. (c) Transpiration rate. (d) Leaf surface temperature. Values are means ± SE (n = 4). Different letters indicate a significant difference (P < 0.05).