A comparative study on the effect of chilling treatment in the light and in the dark on subsequent photosynthesis in cucumber

Abstract

The adverse effect of chilling treatment (4°C) in the light and dark on the subsequent photosynthesis was compared by measuring O ₂ evolution and modulated chlorophyll (Chl) fluorescence using leaf discs taken from chilled cucumber (Cucumis sativus L.) plants. Weak light (50 mol m ^–2 s ^–1) was adopted for light chilling to discount the photoinhibitory effect, which was substantiated by no decline in the ratio variable/maximal fluorescence after dark adaptation (F_v /F_m) after light chilling. The inhibitory pace was faster in the light. Decrease in photosynthesis was not prominent for the initial 2 h of chilling in the light or 12 h of chilling in the dark, but was manifested as an abrupt drop thereafter. Approximately 50 and 90% inhibition of O ₂ evolution was observed after chilling for 4 and 6 h, respectively, in the light, but the same degree of inhibition was shown only after 24 and 48 h of chilling in the dark. Chl fluorescence measurement showed no significant change in initial fluorescence (F _o ) and F_v /F_m but notable change in quenching parameters. Chloroplasts isolated from either 4-h light-chilled or 24-h dark-chilled plants showed roughly 50% reduction in CO ₂ fixation. Results with reconstituted chloroplasts revealed damage in thylakoids from light-chilled plants. In contrast, stroma from dark-chilled plants was less functional. Electron transport was hampered only in light-chilled plants with most lesions residing in photosystem I (PSI). Separation of photosynthates showed changes in metabolite distribution, but more substantially in the dark-chilled chloroplasts, indicating that stromal enzymes were affected. These results suggest that chilling-stress in the light primarily interferes with thylakoidal function while that in the dark mostly affects stromal function within the chloroplasts.