Abstract
Leaves of 2-year-old seedlings of longan (Dimocarpus longan Lour.) and mango (Mangifera indica L.) were exposed to high irradiance (2000 μmol m-2•S-1) provided by metal halide lamps and to 70 ± 5 % of relative humidity. Temperature was raised by incre ments of 3 °C each, at every 1.5 hr interval, from 30 °C till 36 °C. Net assimilation rates decreased for both species as the temperature was increased, but the decline in net assimilation rates was more in longan than in mango. As the temperature increased, stomatal conductance decreased and intercellular CO2 concentration increased for both species especially in longan. A positively significant correlation (r=0.91**) was found in longan between stomatal conductance and net assimilation rate at 30 °C and 33 °C, but r = 0.27NS at 36 °C. In mango, however, the correlation coefficient was consistently significant, r ranging from 0.79 ** to 0.92**, at the three temperatures. Intercellular CO2 concentration and net assimilation rate were not closely related at 30 °C and 33 °C, but a strong negative association was found at 36 °C in both longan (r= --0.89**) and mangc (-0.70**). These results indicate that the decline in net assimilation rate was caused by nonstomatal limitations at high temperatures. The extent of decline in the chlorophyll fluorescence ratio [variable (Fv) to maximum fluorescence (Fm)] by high temperature treatment in the dark was greater in longan than in mango, suggesting that mango leaves are more tolerant of high temperatures than are longan leaves.
