THE SITE OF PHOTOINHIBITION IN LEAVES OF CUCUMIS-SATIVUS L AT LOW-TEMPERATURES IS PHOTOSYSTEM-I, NOT PHOTOSYSTEM-II

Maximum quantum yields (QY) of photosynthetic electron flows through P SI and PSII were separately assessed in thylakoid membranes isolated f rom leaves of Cucumis sativus L. (cucumber) that had been chilled in v arious ways. The QY(PSI) in the thylakoids prepared from the leaves tr eated at 4-degrees-C in moderate light at 220 mumol quanta . m-2.S-1 ( 400 700 nm) for 5 h, was about 20 30% of that in the thylakoids prepar ed from untreated leaves, while QY(PSII) decreased, at most, by 20% in response to the same treatment. The decrease in QY(PSI) was observed only when the leaves were chilled at temperatures below 10-degrees-C, while such a marked temperature dependency was not observed for the de crease in QY(PSII). In the chilling treatment at 4-degrees-C for 5 h, the quantum flux density that was required to induce 50% loss of QY(PS I) was ca. 50 mumol quanta m-2.s-1. When the chilling treatment at 4-d egrees-C in the light was conducted in an atmosphere of N2, photoinhib ition of PSI was largely suppressed, while the damage to PSII was some what enhanced. The ferricyanide-oxidised minus ascorbate-reduced diffe rence spectra and the light-induced absorbance changes at 700 nm obtai ned with the thylakoid suspension, indicated the loss of P700 to exten ts that corresponded to the decreases in QY(PSI). Accordingly, the dec reases in QY (PSI) can largely be attributed to destruction of the PSI reaction centre itself. These results clearly show that, at least in cucumber, a typical chilling-sensitive plant, PSI is much more suscept ible to aerobic photoinhibition than PSII.