Photosynthesis in longan and mango as influenced by high temperatures under high irradiance.
Fukamachi H., Yamada M., Komori S., Hidaka T.
Author Affiliation: Okinawa Subtropical Station, Japan International Research Center for Agricultural Sciences, Maesato, Kawarabaru, Ishigaki, Okinawa, 907-0002, Japan.
JIRCAS Working Report : 77-88
Abstract : Leaves of 2-year-old seedlings of longan (Dimocarpus longan) and mango (Mangifera indica) were exposed to high irradiance (2000 µmol?m-2?s-1) provided by metal halide lamps and the temperature was raised in 3° increments every 1.5 h from 30° to 36°C. Net assimilation rates decreased for both species as the temperature was increased, but the decline in the net assimilation rates was more pronounced in longan than in mango. As the temperature increased, stomatal conductance decreased and intercellular CO2 concentration increased for both species, especially in longan. A significant positive correlation was found in longan between the stomatal conductance and net assimilation rate at 30° and 33°, but no significant correlation was found at 36°. In mango, however, the correlation was consistently significant at the three temperatures. Intercellular CO2 concentration and net assimilation rate were not closely related at 30° and 33°, but a strong negative correlation was found at 36° in both longan and mango. These results indicate that the decline in the net assimilation rate was caused by nonstomatal limitation at high temperatures. Within the temperature range of 15°-35° when vapour pressure deficits were kept within 1.5 kPa, photosynthesis rates increased with the rise of temperature. However, the photosynthesis rate decreased when the temperature exceeded 35° at the same vapour pressure deficit. Intercellular CO2 concentration decreased with the rise of temperature in the range of 15°-35°, but it increased when the temperature exceeded 35°. Vapour pressure deficits varied with the changes of the leaf temperature within the temperature range of 25°-35°. At a leaf temperature of 25°, the photosynthesis rate decreased sharply when the vapour pressure deficit increased from 0.8 kPa. On the other hand, at leaf temperatures of 30° and 35° a sharp decrease in the photosynthesis rate started when the vapour pressure deficit increased from 1.5 kPa. Stomatal closure was considered to be the main limiting factor of the photosynthesis rate. However, when the leaf temperature was 35° and the vapour pressure deficit exceeded 3.0 kPa the photosynthesis rate decreased and intercellular CO2 concentration increased. It is suggested that the photosynthetic activity decreased with the decrease of the leaf water content, because high leaf temperature and high vapour pressure deficit caused high transpiration, leading to a low leaf water content. Thus, it is suggested that the suitable temperature range for photosynthesis of mango was about 30°-35°. The extent of the 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.