J. Plant Physiol. 157. 169-176 (2000) O Urban & Fischer Verlag http:/lwww.urbanfischer.deljournals/jpp JOURNAL OF PLANT PHYSIOLOGY Photosynthetic activity during high temperature treatment of pea plants Katya Georgieva*, Tsonko Tsonev, Violeta Velikova, Ivan Yordanov Institute of Plant Physiology, Bulgarian Academy of Sciences,Acad. G. Bonchev Str., BI. 21, 1113 Sofia, Bulgaria Received August 8, 1999 . Accepted April 25, 2000 Summary The functional activity of the photosynthetic apparatus of pea plants (Pfsurn safivurn, L.) under high temperature conditions was investigated. The rates of C 0 2 uptake and O2 evolution declined after 1h exposure to 45°C and their activity did not change significantly up to the 8'h h of treatment, which could be due to some acclimation to the unfavourable temperature. Photosynthetic activity sharply decreased after 24 h at 45 "C. Information about the functional activity of PSI1 was derived from analyses of in vivo chlorophyll fluo- rescence. It was recorded at 25 "C and also at 45 "C, immediately after the respective time of 45 "C treatment of pea plants. Our results showed that when whole plants were exposed to 45 "C up to 24 h the C 0 2 assimilation and O2 evolution rate were more inhibited than the yield of primary photochemistry (estimated by the ra- tio F,/F,). These values started to decrease after 5 h of high temperature exposure and was reduced to a greater extent after 24 h of treatment. The higher PSI1 thermostability in vivo could be due to the protective effect of low light intensity and the low air humidity during the heat treatment. Key words: Pisum sativum - high temperature stress - acclimation - photosynthetic function - chlorophyll fluorescence Abbreviations: PSI and PSI1 photosystem I and photosystem II - C h chlorophyll - Fo initial Chl fluo- rescence - F , and F , ' maximal Chl fluorescence in dark and light-acclimated leaves, respectively - F , variable Chl fluorescence - F, steady state Chl fluorescence Introduction structural and functional changes. At temperatures higher than 30 "C both the capacity and the quantum yield of C02 Photosynthesis is one of the most heat-sensitive functions in assimilation start to decline (Berry and Bjorkman 1980). plant cells ( B ~ ~ ~ ~ and 6jorkman 1 ~ 8 0 , yordanov et al, ,986). These decreases could in part result from the inhibition of The damage due to heat stress includes a wide range of Photos~stem 11 (PSII) activity. It has been recognised that PSI1 is the most thermally labile component of the electron * E-mail corresponding author: katya@obzor.bio21,bas.bg transport chain (Quinn and Williams 1985, Havaux et al.