The F685/F730 Chlorophyll Fluorescence Ratio as a Tool in Plant Physiology: Response to Physiological and Environmental Factors**

Summary The effect of chlorophyll concentration, light intensity and leaf temperature on the chlorophyll fluorescence ratio F685/F730 of intact leaves was evaluated. Fluorescence reabsorption that affects mainly the F685 band increases with chlorophyll concentration. This phenomenon was studied on an aurea mutant of tomato and its wild type, with very different chlorophyll content. Fluorescence spectra of the two genotypes were corrected for reabsorption using their transmittance and reflectance properties. The correction removes most of the differences in the two fluorescence spectra. The F685/F730 decreases during the declining phase of the fluorescence induction kinetics. We demonstrated that when red light is used to induce the fluorescence kinetics the variation of F685/F730 is not due to a change in the leaf absorption, as proved by the simultaneous measurement of leaf transmittance. This evidence suggests that the F685/F730 ratio is sensitive to changes in the photosynthetic activity of the leaf. Under natural conditions, the F685/F730 ratio markedly decreases as light intensity and leaf temperature increase during a daily cycle. This behaviour can be due to photoinhibitory and heat stresses. In controlled laboratory conditions, the F685/F730 ratio was seen to decrease under high light intensity (> 1000 μmol m-2 s-1) at constant leaf temperature. It decreases also when leaf temperature was decreased from 25°C to 14°C at low light intensity (150 μmol m-2 s-1). A possible interpretation of these experimental data relies on a non-negligible contribution of PSI to the total fluorescence at physiological temperatures with respect to - PSII fluorescence. Changes in the photosynthetic activity of the two photosystems may induce variation in the F685/F730 ratio. Our results indicate that light intensity and leaf temperature are important parameters to take into account when the F685/F730 ratio is used as stress indicator.

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