Pigment Degradation in Discs of the Thermophilic Cucumis sativus as Affected by Light, Temperature, Sugar Application and Inhibitors

Chlorophyll degradation in Cucumis leaf discs was measured at different temperatures between 1 and 25°C in the light and in darkness, and in the presence or absence of sucrose. Two different processes of chlorophyll degradation could be distinguished, a light-requiring process operating at 1 and 5°C and another, light and sucrose enhanced degradation process which was evident at 25°C. Degradation of leaf pigments at low temperatures was of a photo-oxidative nature since there was no degradation in the dark. The chlorophyll a/b ratio was decreased, carotene was degraded at a faster rate than chlorophyll, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and triphenyltetrazolium chloride (TTC) which prevent photo-oxidation, protected against chlorophyll degradation. The light and sucrose enhanced chlorophyll degradation at 25°C was of an enzymatic nature since it occurred in the dark as well as in the light. The chlorophyll a/b ratio was not affected, and carotene and chlorophyll degradation occurred at the same rate. Since DCMU completely inhibited the light enhancement at 25°C and experimentation in a low oxygen atmosphere also protected chlorophyll against the effect of light and sugar application, it is suggested that the enhancement of chlorophyll degradation by light and sucrose at 25°C may be due to increased sugar uptake of the chloroplasts and consequently excessive starch formation in the organelles.