BIOLOGIA PLANTARUM 38 (4): 501-509, 1996 Effect of short-term heat treatment of rice seedlings on sensitivity of thylakoid membranes to photoinhibition B. VANI*, P. PARDHA SARADHI* and PRASANNA MOHANTY** Centre for Biosciences, Jamia Millia lslamia, New Delhi - 110 025, lndia* School of Life Sciences, Jawaharlal Nehru University, New Delhi - 110 067, lndia** Abstract The after-effects of 24 h high temperature (35 or 45 ~ treatment on the photochemical activities and photooxidative lipid peroxidation, subsequent to their irradiation were studied in 7-d-old etiolated rice (Oryza sativa) seedlings. Photosystem (PS) 1 and PS 2 mediated photoreactions of thylakoids isolated from the seedlings exposed to high temperature did not differ significantly from the thylakoids isolated from control seedlings (25 ~ Hence, all kinds of tested thylakoids were equally efficient in capturing and utilizing radiant energy. The high irradiance induced loss in PS 2 activity and lipid peroxidation measured in terms of malondialdehyde production was more rapid in thylakoids isolated from stressed seedlings as compared to that of control seedlings. Thus the thylakoids isolated from the stressed seedlings were more prone to photodamage than those from the control seedlings. Additional key words: chlorophyll, lipidperoxidation, malondialdehyde, Oryza sativa, photosystem 1, photosystem 2. Introduction When leaves or isolated chloroplasts are exposed to high temperatures for short periods, their photosynthetic apparatus shows characteristic changes such as irreversible inhibition of CO 2 fixation, 02 evolution and photophosphorylation (for reviews see Berry and Bjtrkmann 1980, Quinn and Williams 1985). In contrast, the PS 1 mediated electron transport is stimulated by high temperature treatment (Armond et al. 1978, Stidham et al. 1982, Thomas et al. 1986, Sabat and Mohanty 1989). These changes occur over a wide temperature range, depending upon the plant species (Berry and Bj6rkmann 1980, Quinn and WiUims 1985). Received 17 November 1995, accepted 19 January 1996. Acknowledgements: Financial supports from DST ILTP B 6.1 and University Grants Commission are gratefully acknowledged. 501