59-05-032 Proceeding

60 Proceedings of the Princess Maha Chakri Sirindhorn Congress To further understand the impact of moderate heat stress condition on photosynthesis, cholophyll fluorescence parameters were studied. It was found that heat stress condition resulted in lower PSII operating efficiency ( Φ PSII) values in butterhead lettuce (Figure 2 a). In contrast, Φ PSII values of green oak lettuce were higher in plants grown under heat stress. The values of maximum quantum efficiency of PSII (Fv/Fm) were slightly lower in all lettuce plants grown in heat stress as compared to those of control (Figure 2 b). These lower Fv/Fm values were attributed to an increasing of Fo values in larger extent than an increasing of Fm in all cultivars. One of the major consequences of heat stress is an increasing of reactive oxygen species amount which impedes repairing process of PSII [12].This could lead to an increasing of damaged PSII complex and eventually resulted in photoinhibition [5]. In this experiment it was found that heat stress causes photoinhibition in all lettuce cultivars, as indicated by reduction of Fv/Fmvalues. Interestingly, in case of green oak lettuce, these reductions are contradicted with an increasing of Φ PSII values under heat stress. One possible explanation is that both ‘Chia-Tia’ and ‘Kan-Tar’ are bred and selected under Thailand climate condition which is relatively hot. Therefore, they might be more adaptable to high temperature condition than ‘Buttercrunch’ and ‘Big Boston’. However, damages of PSII under heat stress are unavoidable, but these damages may not be severe enough to negatively affect PSII operating efficiency in short term in these ‘Chia-Tia’ and ‘Kan-Tar’. An increasing of Fo value under stress condition has been shown to associatedwith damage of PSII and chloroplast [13]. In this study, the Fo values are increased in all lettuce cultivars in response to heat stress which may indicated the negative impact of heat stress on PSII. The extent of increasing of Fo values seems to be relatively the same in all lettuce cultivars. This may imply that damaging of PSII is a common theme in lettuce grown under heat stress condition. Growths of all lettuce cultivars were suppressed under heat stress condition as seen by reduction of fresh and dry weight (Figure 3 a and b). Fresh weight was more affected than dry weight indicating that loss of water is the major cause of reduction of fresh weight. Reduction of dry weight is statistically significant only in ‘Buttercrunch’ and ‘Big Boston’ varieties.This seems to be contradicted to the fact that net photosynthesis rates of both ‘Chia-Tia’ and ‘Kan-Tar’ cultivars are reduced more than ‘Buttercrunch’ and ‘Big Boston’ (Figure 1). Because net photosynthesis rates were measured at 6 days and dry weights were measured at 10 days after exposure to heat stress condition, it is possible that photosynthesis of butterhead varieties may exhibit better heat tolerance only in at the beginning period of heat stress condition. Time course measurement of photosynthesis rate should be performed in future in order to clarify this issue. Acknowledgement This work is supported by The Office of Agricultural Research and Extension Maejo University, project number MJU.1-57-002/58-015.