DIURNAL CHANGES OF PHOTOSYNTHETIC CHARACTERISTICS AND CHLOROPHYLL FLUORESCENCE IN CANOPY LEAVES OF FOUR DIPTOCARP SPECIES UNDER EX_SITU CONSERVATION

Abstract

Dipterocarps dominate the canopy of tropical rainforest of Southeast Asia. They are not only the world's main source of hardwood timber, but their canopy leaves are main organs for global carbon sequestration. Due to anthropogenic activities, many species of dipterocarps are threatened. Because of this situation, ex-situ conservation efforts were employed to conserve the genetic resources of several dipterocarps. In this study, four dipterocarp species, Dipterocarpus retusus, Hopea hainanensis, Parashorea chinensis (emergent tree species in the rainforest) and Vatica xishuangbannaensis, were selected as study species that had been transplanted in 1981 to an ex-situ dipterocarp conservation forest in Xishuangbanna Tropical Botanical Garden. We measured the diurnal changes in photosynthetic rates, parameters of chlorophyll fluorescence, and morphological traits of their canopy leaves at 15-21 m height during the rainy season of 2004. The results indicated that the maximum photosynthetic rates (Pmax) per unit leaf area (7.5 to 18.1 μmol·m~-2·s~-1) and mass (89.08 to 150.82 nmol·g~-1 DW·s~-1), dark respiration rates (Rd), light saturation point (LSP), light compensation point (LCP) and leaf morphological traits differed significantly among species. Photosynthesis in the four species was depressed at midday. The results revealed that stomatal closure induced by high leaf-to-air vapor pressure deficit (LAVPD) led to photosynthetic depression at midday. Quantum yields of photosystem II (ΦPSⅡ) in four dipterocarp species decreased significantly at midday, indicating that photoinhibition occurred. However, their PSⅡ values recovered to the early morning value by sunset, indicating that photoinhibition was reversible. The nonphotochemical quenching rate (NPQ) increased significantly in D. retusus, H. hainanensis and V. xishuangbannaensis, indicating that NPQ was used mainly to dissipate excess light energy absorbed by PSⅡ. At midday, the electron transport rate (ETR) in P. chinensis was maintained at high levels, while its photosynthetic rate decreased, suggesting that a large proportion of electrons were allocated to photorespiration. Thus photorespiration was the main mechanism protecting the photosynthetic apparatuses of P. chinensis during the midday photosynthesis depression. Other parameters, such as leaf area, size and density of stomata, and total chlorophyll content, also were measured at the same time. There was a general ranking of Pmax in the following order from highest to lowest: D. retusus, P. chinensis, H. hainanensi, V. xishuangbannaensis. Based on the diurnal changes in chlorophyll fluorescence, both leaf stomatal limitations and non-stomatal effects played an important role to prevent photodamage during the midday depression of photosynthesis brought by the high irradiances, high air temperature, low humidity, and so on. The high midday leaf water potential of the four species showed that water limitations had no influence on photosynthetic rates.