THE F685 F730 CHLOROPHYLL FLUORESCENCE RATIO AS A TOOL IN PLANT PHYSIOLOGY - RESPONSE TO PHYSIOLOGICAL AND ENVIRONMENTAL-FACTORS/

The effect of chlorophyll concentration, light intensity and leaf temp erature on the chlorophyll fluorescence ratio F685/F730 of intact leav es was evaluated. Fluorescence reabsorption that affects mainly the F6 85 band increases with chlorophyll concentration. This phenomenon was studied on an aurea mutant of tomato and its wild type, with very diff erent chlorophyll content. Fluorescence spectra of the two genotypes w ere corrected for reabsorption using their transmittance and reflectan ce properties. The correction removes most of the differences in the t wo fluorescence spectra. The F685/F730 decreases during the declining phase of the fluorescence induction kinetics. We demonstrated that whe n 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 s uggests that the F685/F730 ratio is sensitive to changes in the photos ynthetic activity of the leaf Under natural conditions, the F685/F730 ratio markedly decreases as light intensity and leaf temperature incre ase 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 mu mol m (-2) s(-1)) at constant leaf temperature. It decreases also when leaf temperature was decreased from 25 degrees C to 14 degrees C at low lig ht intensity (150 mu mol m(-2) s(-1)). A possible interpretation of th ese experimental data relies on a non-negligible contribution of PST t o 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 param eters to take into account when the F685/F730 ratio is used as stress indicator.