PHOTOSYNTHETICA 41 (4): 505-511, 2003 Photosynthetic performance of Ginkgo biloba L. grown under high and low irradiance S. PANDEY, S. KUMAR, and P.K. NAGAR * Division of Biotechnology, Institute of Himalayan Bioresource Technology, Palampur – 176 061 (H.P.), India Abstract Diurnal variation in net photosynthetic rate (P N ) of three-year-old plants of Ginkgo biloba was studied under open, O (receiving full sunlight), net-shade, NS (40 % of photosynthetically active radiation, PAR), or greenhouse, G (25 % PAR) conditions. In all three conditions, P N was higher in morning along with stomatal conductance (g s ), and intercel- lular CO 2 concentration (C i ), while leaf temperature and vapour pressure deficit were low. The O-plants exhibited a typical decline in P N during midday, which was not observed in NS-plants. This indicated a possible photoinhibition in O-plants as the ratio of variable to maximum fluorescence (F v /F m ) and photosystem 2 (PS2) yield (Φ PS2 ) values were higher in the NS- and G-plants. On the contrary, stomatal density and index, chlorophyll a/b ratio, leaf thickness, and density of mesophyll cells were greater in O-plants. Further, higher P N throughout the day along with higher relative growth rate under NS as compared to O and G suggested the better efficiency of Ginkgo plants under NS conditions. Therefore, this plant species could be grown at 40 % irradiance to meet the ever-increasing demand of leaf and also to increase its export potential. Additional key words: chlorophyll a fluorescence; leaf anatomy; leaf mass; net photosynthetic rate; photoinhibition; solar irradiance; stomatal conductance; transpiration rate. Introduction Irradiance is the energy source for all photosynthetic organisms, which are finely tuned to harvest it efficiently. On the other hand, excess irradiance captures result in photoinhibition of photosynthesis (Long et al. 1994). As a result, plant has devised some sophisticated mecha- nisms to adapt them in irradiance environment that pre- vails. The adaptation of photosynthetic apparatus to the prevailing irradiance is known as irradiance acclimation of photosynthesis (Anderson et al. 1995). Photosynthetic irradiance acclimation involves a variety of responses, including changes in leaf anatomical (Taiz and Zeiger 1998), morphological (Boardman 1977), biochemical and photosynthetic (Chazdon and Kaufmann 1993) charac- teristics. There is a close relationship between the leaf characteristics and the mean irradiance experienced by the leaves. Hence irradiance distribution can be used as a predictor for spatial variation of leaf properties (Evans 1993, Pons et al. 1993, Anten and Werger 1996). The empirical relationships between leaf irradiance and leaf characteristics are widely used to scale up photosynthesis from leaf to canopy level (Sinquet et al. 2001). Leaf photosynthetic characteristics remarkably adapt to irradi- ance (Evans 1989) and generally, leaves developed at high irradiance exhibit higher photosynthetic capacity per unit leaf area as compared to shade leaves (Niinemets and Tenhunen 1997). However, exposure of leaves to excessive irradiance is a well-known cause of photoinhibition, which de- creases the capacity for photosynthesis in many plants (Baker and Bowyer 1994). Photoinhibition is caused by damage to the photosynthetic components and it may be short term and reversible or long term and irreversible, and is related to increased leaf temperature and to high irradiance when electron transport to acceptors is limited ——— Received 1 July 2003, accepted 17 September 2003. * Author for correspondence; fax: 91-01894-230433, e-mail: nagar_pk2001@yahoo.co.uk Abbreviations: Chl – chlorophyll; C i – intercellular CO 2 concentration; E – transpiration rate; F v /F m – ratio of variable to maximum fluorescence; g s – stomatal conductance for CO 2 ; LMA – leaf mass per leaf area; PAR – photosynthetically active radiation; P N – net photosynthetic rate; VPD – vapour pressure deficit; Φ PS2 – quantum efficiency of photosystem 2. Acknowledgements: The authors thank Dr. P.S. Ahuja, Director, IHBT for providing necessary facilities. Financial assistance from CSIR, New Delhi to SP under pool scientist scheme is gratefully acknowledged. We also thank Dr. Alok K. Sinha for critically read- ing the manuscript. IHBT Communication No. 0325. 505