130 Journal of Forestry Research, 14(2): 130-134 (2003) Effect of overnight temperature on leaf photosynthesis in seedlings of Swietenia macrophylla King ZHANG Cheng-Jun 1, Carlos Henrique B. de A. Prado 2, ZU Yuan-Gang 1, GUO Jia-Qiu 3, Carlos Cesar Ronquim 2, Leonnardo Lopes Ferreira 2 1 Key Laboratory of Forest Plant Ecology, Northeast Forestry University, Harbin 150040, P. R. China 2 Laboratory of Plant Physiology, Department of Botany, Federal University of Sao Carlos, SP, 13565-905, Brazil 3 The Second Affiliated Hospital of Harbin Medical University, Harbin 150086, P. R. China Abstract: After exposure of one-year old seedlings of Swietenia macrophylla to an overnight temperature (13 ~ 19 ~ 25 ~ 31 ~ or 35 ~ the leaf net photosynthetic rate (P.) was researched through measuring photosynthetic light-response curves at 360 p.mol,mol1 CO2, and photosynthetic CO2-response curves at light-saturated intensity (1500 p.moi-m 2 -sl). The optimal temperature for photosynthesis measured at 360 p.mol mol ~ CO2 was from 25 ~ to 31 ~ but which was from 31~ to 35 ~ at saturating CO2 concentration. At temperature of below 25 ~ the decline in Pn was mainly due to the drop in carboxylation effi- ciency (Co), while as temperature was over 31 ~ the reduction in Pn resulted from both decrease in Co and increase in leaf respiration. The CO2-induced stimulation of photosynthesis was strongly inhibited at temperatures below 13 ~ The results showed that, the leaf photosynthesis of tropical evergreen plants should not be accelerated at low temperature in winter season with elevated CO2 concentration in the future. Keywords: Apparent quantum yield; Carboxylation efficiency; CO2-induced stimulation; Swietenia macrophylla King; Leaf temperature CLC number: Q945.11 Document code: A Article ID: 1007-662X(2003)02-0130-05 Introduction Leaf photosynthesis is one of the most thermal sensitive processes providing an indicator for functional limitations imposed particularly by air temperature (Harley 1995). At temperatures too high or too low the photosynthetic yields decrease steadily until CO2 uptake ceases (Larcher 1994). Plants are exposed to only a narrow range of temperatures at most tropic regions, so even smaller changes in tem- perature might be equal or more important in the tropics than at temperate latitudes (Hogan et aL 1991). However, very little attention has been directed to the photosynthetic response of tropical trees to the interactions between temperature and irradiance, as well as between tempera- ture and CO2 concentrations (Hogan et aL 1991). Big-leafed Mahogany (Swietenia macrophylla King (Ma- liaceae)) is distributed naturally from the south of Mexico throughout Central and South America to Bolivia and Brazil, including large portions of the Amazon Basin. It is one of the most valuable plants on the international market be- cause of the beauty and durability of its wood. Increasing demand for this valuable timber has resulted in a severe decline in the wild population of this species (Schmidt et aL 2000). Thus, it is necessary to determine its ecophysi- ological properties, especially to predict its possible re- sponse in the future with increasing CO2 and temperature. Biography: ZHANG Cheng-Jun (1968-), male, Lecturer in Key Laboratory of Forest Plant Ecology, Northeast Forestry University, Harbin 150040, P. R. China. Received date: 2003-03-11 Responsible editor: Zhu Hong This work was to determine the response of net photo- synthetic rate in Swietenia macrophylla seedlings to leaf temperature, through measuring photosynthetic ligh-response curves and photosynthetic CO2-respons curves at light-saturated intensity, following an overnight temperature (13 ~ 19 ~ 25 ~ 31 ~ or 35 ~ which is similar to the annual air temperature scope in the natural habitat of this species). Considering CO2-temperature in- teraction we predicted that the optimal temperature of photosynthesis in this tropical species might also be shifted upward at high CO2 concentration like most temperate plants. Materials and methods The one-year-old seedlings of Swietenia macrophylla were cultivated in polystyrene foam boxes containing 32 kg sifted cerrado soil. Five boxes of seedlings were used for experiment, two seedlings per box. They were grown in a greenhouse with diurnal temperature (25+5) ~ night temperature (18_+3) ~ and relative humidity (RH, (60• During the experimental period the seedlings cultivated in the soil was irrigated well once a week. Preliminary experiment showed that photosynthesis measured at similar conditions was similar for leaves with similar age in these seedlings. Before temperature treat- ment, two boxes were wrapped with a thick synthetic man- tle in order to maintain the soil temperature constant. Thus, only the aerial parts of plants were exposed to the desired temperature. The two boxes were placed into an incubator (Model NT-708, Piracicaba, Brazil) at a temperature set as