Temperature/light dependent development of selective resistance to photoinhibition of photosystem I

Exposure of winter rye leaves grown at 20 degrees C and an irradiance of either 50 or 250 mu mol m(-2) s(-1) to high light stress (1600 mu mol m(-2) s(-1), 4 h) at 5 degrees C resulted in photoinhibition of PSI measured in vivo as a 34% and 31% decrease in Delta A(820)/A(820) (P700(+)). The same effect was registered in plants grown at 5 degrees C and 50 mu mol m(-2) s(-1). This was accompanied by a parallel degradation of the PsaA/PsaB heterodimer, increase of the intersystem e(-) pool size as well as inhibition of PSII photochemistry measured as F-v/F-m. Surprisingly, plants acclimated to high light (800 mu mol m(-2) s(-1)) or to 5 degrees C and moderate light (250 mu mol m(-2) s(-1)) were fully resistant to photoinhibition of PSI and did not exhibit any measurable changes at the level of PSI heterodimer abundance and intersystem e- pool size, although PSII photochemistry was reduced to 66% and 64% respectively. Thus, we show for the first time that PSI, unlike PSII, becomes completely resistant to photoinhibition when plants are acclimated to either 20 degrees C/800 mu mol m(-2) s(-1) or 5 degrees C/ 250 mu mol m(-2) s(-1) as a response to growth at elevated excitation pressure. The role of temperature/light dependent acclimation in the induction of selective tolerance to PSI photoinactivation is discussed. (C) 1998 Federation of European Biochemical Societies.