Changes in laser-induced chlorophyll fluorescence ratio F690/F735 in the poikilochlorophyllous desiccation tolerant plant Xerophyta scabrida during desiccation

The degradation of chlorophylls, carotenoids and the photosynthetic apparatus in photosynthetically active leaves of Xerophyta scabrida was studied during slow desiccation using chlorophyll fluorescence induction kinetics measured at two wavelengths, at 690 nm (F690) and 735 nm (F735). Breakdown of the photosynthetic apparatus in X. scabrida was indicated by a decrease in both the chlorophyll a+b and carotenoid x+c contents and by decreasing values of the ratio chlorophylls/carotenoids (a+b)/(x+c). During desiccation, the total carotenoid content decreased less than the total chlorophyll content (63% and 78% loss respectively). The rising value of the chlorophyll fluorescence ratio F690/F735 during desiccation was inversely proportional to the decrease in chlorophyll content and to the decline of the ratio of chlorophylls/carotenoids (a+b)/(x+c). Our results indicate that the fluorescence ratio F690/F735 is a reliable parameter for monitoring and following the loss of chlorophylls and the breakdown of the photosynthetic apparatus in poikilochlorophyllous desiccation tolerant (PDT) plants in a non-invasive manner. The results also provide evidence that slow chlorophyll fluorescence induction kinetics and its parameters (fluorescence ratio F690/735 and the variable fluorescence decrease ratio Rfd) can provide a useful alternative to gas exchange measurements for screening the photosynthetic activity in the PDT plants and other desiccation tolerant plane during desiccation.