Development of chlorophyll bleaching in cucumber leaves after light-chilling stress

Abstract

The relationship between the development of a visible symptom (decrease of chlorophyll content) in chilling injury and the irreversible nature of PSI photoinhibition in cucumber (Cucumis sativus), a chilling sensitive plant, was studied. Changes in the amounts of 1) chlorophyll, 2) photo-oxidizable P-700, a reaction center chlorophyll of PSI, and 3) the reaction center subunits of PSI (PsaA/B protein), were determined after chilling treatment of cucumber leaves at 4oC under the light at 190 mmol m-2s-1. The content of chlorophyll gradually decreased by 30% over subsequent 3 days under growth condition following the treatment while it did not change just after the treatment. Time-course determination of photo-oxidizable P-700 showed that the recovery rate of PSI function was so slow that 50% of the PSI inhibition remained for 6 days following the treatment. The amount of P-700 per chlorophyll, however, was recovered to 90% of the original level after 6 days following the treatment, suggesting that damaged PSI complex was selectively degraded along with its chlorophyll. The residual amount of chlorophyll at 3 days after the treatment closely correlated not only with the amount of functional PSI but also with the amount of PsaA/B protein detected by immunoblotting analysis. These results suggest that the development of visible symptom following the chilling treatment is the consequence of the selective degradation of impaired PSI complex, which reflects the irreversible nature of PS I photoinhibition.