迁地保护的4种龙脑香冠层叶光合速率和叶绿素荧光参数的日变化

DIURNAL CHANGES OF PHOTOSYNTHETIC CHARACTERISTICS AND CHLOROPHYLL FLUORESCENCE IN CANOPY LEAVES OF FOUR DIPTOCARP SPECIES UNDER EX-SITU CONSERVATION

龙脑香科植物是东南亚热带雨林冠层的优势树种,在生态和经济上具有重要地位,而我国西南地区被认为是龙脑香科植物分布的最北端.该文于2004年雨季测定了西双版纳热带植物园迁地保护区内引种的4种国产龙脑香科植物:望天树(Parashorea chinensis)、云南龙脑香(Dipterocarpus retusus)、版纳青梅(Vatica xishuangbannaensis)和海南坡垒(Hopea hainanensis)冠层叶片的光响应曲线、光合速率的日进程、叶绿素荧光参数、叶绿素含量及叶片平均面积、气孔保卫细胞长度和分布状况等.结果表明,4种植物的最大光合速率(Pmax)(7.5～18.1 μnol·m-2·s-1,用单位叶面积表示;89.1～150.8 nmol·g-1DW·s-1,用单位于重表示)、暗呼吸速率(Rd)、光补偿点(LSP)、光饱和点(LCP)、表观量子效率和叶片特征差异显著.在正午时4个树种均出现光合作用显著降低的现象,其中海南坡垒光合速率下降最少.虽然高的叶片温度对光合作用有一定的影响,但正午光合作用的下降主要是由于正午高的叶片-空气的水蒸气压力差(LAVPD)引起的气孔关闭造成的.4个树种正午的光系统Ⅱ线性电子传递的量子效率φPSⅡ显著降低,表明遭受了强烈的光抑制.云南龙脑香、版纳青梅和海南坡垒的非光化学猝灭系数(NPQ)在正午时升高的幅度最大,而望天树的NPQ上升较少,表明热耗散是前3个树种的主要光保护机制.中午前后望天树的表观电子传递速率(ETR)一直保持在很高的水平,但其光合速率显著降低,表明大量的电子被分配到光呼吸上,也说明望天树主要通过光呼吸来保护光合机构.

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 （Pmax） per unit leaf area （7.5 to 18. 1μmol·m^-2·s^-1） and mass （89.08 to 150.82nmol·g^-1DW·s^-1）, dark respiration rates （Rd）, 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 （IAVPD） led to photosynthetic depression at midday. Quantum yields of photosystem Ⅱ （ФPSⅡ） in four dipterocarp species decreased significantly at midday, indicating that photoinhibition occurred. However, their PS Ⅱ 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. xishuangbarmaensis, indicating that NPQ was used mainly to dissipate excess light energy absorbed by PS Ⅱ . 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 plaotosynthesis 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 Pmax in the following order from highest to lowest ： D. retusus, P. chinertsis, H. hainanertsi, 7 V. xishuangbannaertsis. 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.