Temperature response and photoinhibition investigated by chlorophyll fluorescence measurements for four distinct species of dipterocarp trees

The effects of strong light in combination with elevated temperatures on th e photosynthetic system were examined in 4 dipterocarp tree species with ec ologically different habitats. The 4 dipterocarp tree species were: Shorea platyclados originated from upper dipterocarp forests, Shorea parvifolia lo wland and hill dipterocarp forests, Shorea assamica - lowland dipterocarp f orests, and Dipterocarpus oblongifolius riparian fringes. S. platyclados an d D. oblongifolius have higher growth and survival rates in open sites than S. parvifolia and S. assamica. Tolerance of high temperature among the spe cies was assessed by determining the critical temperatures (T-c) at which t he minimal fluorescence (F-o) began to rise sharply. This was measured by e xposing plants to an increasing temperature of about 1 degrees C min(-1). T he intrinsic thermotolerance of the thylakoid membrane appears to be the hi ghest for D. oblongifolius (T-c = 46.4 degrees C), intermediate for S. plat yclados (45.7 degrees C), and lowest for S. parvifolia and S. assamica (45. 2 and 45.3 degrees C, respectively). The temperature-dependent efficiency o f PSII electron transport (Delta F/F-m'), photochemical quenching (q(P)), a nd the efficiency of light capture of open PSII (F-v'/F-m') were measured a t the photosynthetic steady state at least 10 min after the light exposure (180 mu mol m(-2) s(-1) PFD). Stable temperature responses of Delta F/F-m' and q(P) were observed in S. platyclados and D. oblongifolius, while those in S. parvifolia and S. assamica were more temperature-dependent and severe ly affected at 45 degrees C. Little difference was observed in temperature- dependent F-v'/F-m' among species. Photoinhibitory light exposure (1600 mu mol m(-2) s(-1) PFD) for 2 h at 40 degrees C had little effect on the recov ery kinetics from photoinhibition of S. platyclados and D. oblongifolius co mpared with those at 35 degrees C. In contrast, the recovery from photoinhi bition was retarded in S. parvifolia and S. assamica. These findings sugges t that even at 40 degrees C, a temperature below T-c, an exposure to strong light exacerbated photoinhibition in S. parvifolia and S. assamica corresp onding to the closure of PSII reaction centers, as indicated by the decreas e in q(P) at this temperature. Thus, S. platyclados and D. oblongifolius, w hich occur at uplands and riparian fringes with frequent disturbances, are suggested to have higher photosynthetic tolerance to elevated temperatures contributing to a circumvention of photoinhibition.