Photosynthesis in Longan and Mango as Influenced by High Temperatures under High Irradiance.

Abstract

Leaves of 2-year-old seedlings of longan (Dimocarpus longan Lour.)and mango (Mangifera indica L.) were exposed to high irradiance (2000mu mol . m(-2) . s(-1)) provided by metal halide lamps and to 70+/- 5 % of relative humidity. Temperature was raised by incrementsof 3 degrees C each, at every 1.5 hr interval, from 30 degrees Ctill 36 degrees C. Net assimilation rates decreased for both speciesas the temperature was increased, but the decline in net assimilationrates was more in longan than in mango. As the temperature increased,stomatal conductance decreased and intercellular CO2 concentrationincreased for both species, especially in longan. A positively significantcorrelation (r = 0.91**) was found in longan between stomatal conductanceand net assimilation rate at 30 degrees C and 33 degrees C, but r= 0.27(NS) at 36 degrees C. In mango, however, the correlation coefficientwas consistently significant, r ranging from 0.79** to 0.92**, atthe three temperatures. Intercellular CO2 concentration and net assimilationrate were not closely related at 30 degrees C and 33 degrees C, buta strong negative association was found at 36 degrees C in both longan(r= -0.89**) and mango (-0.70**). These results indicate that thedecline in net assimilation rate was caused by nonstomatal limitationsat high temperatures. The extent of decline in the chlorophyll fluorescenceratio [variable (F-v) to maximum fluorescence (F-m)] by high temperaturetreatment in the dark was greater in longan than in mango, suggestingthat mango leaves are more tolerant of high temperatures than arelongan leaves. [References: 25]