Photoinhibition of Photosystem I in Chilling Sensitive Plants Determined in vivo and in vitro

Abstract

In chilling sensitive plants such as cucumber, tomato and common bean, the site of chilling damage under moderate light is Photosystem I (PSI) [1,2]. Upon the treatment of cucumber leaves at 4°C for 5 h under the photon flux density at 100–200 µmol/m2s, the PSI activity determined in vitro as electron transfer from an artificial electron donor, DAD, to NADP+ decreased to about 20% while more than 80% of the PSII activity remained [3]. The damage, which was induced by the reactive oxygen species produced at the acceptor side of PSI [4,5], involved the destruction of iron-sulfur centers [6] and the degradation of a PSI reaction center subunit, PsaB protein [4,5,7]. Similar selective photoinhibition of PSI was observed also in another chilling sensitive plant, common bean [8]. Although PSI in chilling tolerant plant can he also photoinhibited after the isolation of thylakoid membranes [9], extent of PSI photoinhibition in vivo was relatively small in chilling tolerant plants unless the leaves were poisoned with KCN [10]. One exception was potato, in which absorption change due to P-700, the reaction center chlorophyll of PSI, determined in vivo was reported to decrease with only a small change in PSII [11]. To understand the relation between chilling sensitivity of plants and photoinhibition of PSI, we compared the absorption change of P-700 determined in vivo and in vitro after the chilling treatment of cucumber leaves. It was concluded that apparent decrease of P-700 determined in vivo, possibly duc to the enhanced cyclic electron flow around PSI, can be induced upon chilling treatment with little change in the extent of P-700 determined in vitro.