The cause of PSI photoinhibition at low temperatures in leaves of Cucumis sativus, a chilling‐sensitive plant

In a chilling-sensitive plant, cucumber, chilling of leaves in the light results in irreversible damage to PSI. Recent in vitro studies suggested that hydroxyl radicals, which are formed in the presence of H₂O₂ and reduced Fe-S centers, are involved in the PSI inhibition. We therefore examined this possibility in vivo. Chilling of leaves at 5°C in the light caused a temporary increase in H₂O₂ concentration, which was probably due to the net H₂O₂ production in vivo. The activity, measured at 5°C, of the thylakoid ascorbate peroxidase (APX), a key enzyme of the H₂O₂-scavenging system, was about 20% of that measured at 25°C. The isolated thylakoids retaining high thylakoid APX activity did not show light-dependent net H₂O₂ production at 25°C. However, at 5°C, net production of H₂O₂ was observed. Since the rate of electron flow to molecular oxygen in the isolated thylakoids was ca 5 mmol e^? mol^?1 Chl s^?1 at 5°C, the H₂O₂-scavenging capacity was below this level. When intact leaves were illuminated at 5°C at an irradiance of 100 µmol m^?2 s^?1, the rate of electron transport through PSII was ca 20 mmol e^? mol^?1 Chl s^?1 and more than 80% of Q_A was in the reduced state. Since thylakoids are uncoupled in cucumber leaves at 5°C in the light. ATP is not formed and energy dissipation in the form of heat is suppressed. Therefore, the electron flow to molecular oxygen would be greater than 5 mmol e^? mol^?1 Chl s^?1. Moreover, under such conditions, components in the electron transport chain, including Fe-S centers in PSI, were probably reduced. These features indicate that, when cucumber leaves are chilled in the light, hydroxyl radicals can be produced by the Fenton reaction and cause damage to PSI.