59-05-032 Proceeding

57 Proceedings of the Princess Maha Chakri Sirindhorn Congress condition (39°C/29°C) was used in order to mimic natural heat stress condition. The results from this study should provide some useful information for developing of breeding program for heat tolerant lettuce cultivar in the future. II. Materials and Methods Plant materials and growth conditions Lettuce seeds of ‘Buttercrunch’, ‘Big Boston’ ‘Chia-Tia’and ‘Kan-Tar’ were used for the whole experiment. These seeds were germinated and grown in pots placed in a tray. The growth medium consisted of peat moss, coarse sand and coconut coir (1:1:1, v/v). The growth conditions were 25°C/25°C (day/night), 16 hours of light per day and photosynthetic photon flux density of 200 µmol•m-2•s-1 (generated by light emission diodes 3000K and 5000K (1:1)). Plants were supplied with 0.5 × Hoagland solution [7]. At 21 days after sowing, seedlings of all cultivars were transferred to either 25°C/25°C or 39°C/29°C conditions for 6 days before all the physiological parameters were measured. Measurement of gas exchange and chlorophyll fluorescence parameters All themeasurementswere performed following the protocols of Sorwong and Sakhonwasee (2015) [7]. Measurement of leaf surface temperature Leaf surface temperature (Tl) was measured by using infrared thermometer. In each plant, Tl was measured 3 times on 3 different leaves and the average value was used to represent the Tl value of individual plant. Experimental design and statistical analysis A randomized complete block design was used for the experiment. All statistical analyses were performed by using SPSS V.22 (2013). Data are reported as mean ± standard error (SE). Significant differences among means were determined by one-way ANOVA with Duncan’s post hoc test at P < 0.05. III. Results and Discussions Moderate heat stress condition did not significantly affect net photosynthetic rate (A), stomatal conductance (gs) and transpiration rate (E) of all lettuce cultivars, except ‘Buttercrunch’ (Figure 1). It was found that heat stress condition causes significant higher of gs value and lower of E value as compared to control in ‘Buttercrunch’. Leaf surface temperatures (Tl) of all lettuce cultivars are higher than control when grown under heat stress condition. ‘Kan-Tar’ exhibited the largest difference between Tl in control and heat stress condition whereas ‘Buttercrunch’ exhibited the least.