Leaf Chlorophyll Fluorescence as an Indicator of High Light Stress (Photoinhibition) in Cucumis sativus L

Abstract

Chlorophyll fluorescence induction kinetics were measured in detached leaves of cucumber plants (Cucumis sativus L.) grown at low quantum flux density (120 �E m-2 s-1) and then irradiated with high quantum flux density (650 �E m-2 s-1). Short-term irradiations (e.g. 1 min) resulted in loss of the variable component of chlorophyll fluorescence which could be fully recovered over a period of 60-80 min of dark adaptation. Longer irradiations wlth high intensity light (up to 5 h), however, caused an irreversible reduction in fluorescence yield indicative of irreversible photoinhibition. Irradiations as short as 4.5 min produced significant photoinhibition. Similar treatments did not cause irreversible changes in the fluorescence induction kinetics of cucumber leaves grown in sunlight. The capacity of low light grown leaves to photooxidize cytochrome f was not affected by photoinhibitory treatments for as long as 5 h, whereas the slow component of the 518 nm change was almost completely lost. These results support previous findings and provide direct evidence that the sensitive site for photoinhibition in vivo is located on the oxidizing side of photosystem II and that photosystem I mediated reactions do not become inhibited. It is suggested that measurements of chlorophyll fluorescence provide a rapid means of estimating photoinhibitory damage to plants.