Thermoinduction of Chlorophyll Fluorescence and the Age-Related Condition of Higher Plant Leaves

The age-related changes in the temperature dependence curves (TDC) of chlorophyll fluorescence were studied in leaf segments of wheat (Triticum aestivumL.), tomato (Lycopersicum esculentumMill.), and cucumber (Cucumis sativumL.) plants grown under controlled photoculture conditions. Three major TDC patterns of chlorophyll fluorescence were identified within the temperature range of 25–70°C, with each of the patterns corresponding to a certain phase of leaf development. The transition from one type of thermogram to another was a gradual and ordered process. The magnitude of the low-temperature TDC peak increased until leaves completely expanded and declined with leaf senescence. In the course of leaf senescence, the thermograms exhibited an additional shoulder, which further changed into a peak at 55–65°C with increasing magnitude. Our data provide the basis for assessing leaf age from the type of chlorophyll fluorescence thermogram and the changes in the particular indices characteristic of TDC of chlorophyll fluorescence.