Light Adaptation and Senescence of the Photosynthetic Apparatus. Changes in Pigment Composition, Chlorophyll Fluorescence Parameters and Photosynthetic Activity

This chapter deals with pigment composition, activity and chlorophyll (Chi) fluorescence signatures of the photosynthetic apparatus as modified by light adaptation, stress events and autumnal senescence. It starts with a description of the regular pigment composition and function of the different photosynthetic pigments (Chls a and b and several carotenoids) in the chloroplasts. Light adaptations of the photosynthetic apparatus with differential pigment composition, Chi fluorescence and higher C0₂ assimilation rates of sun-type and high-light chloroplasts as compared to shade-type and low-light chloroplasts are contrasted. The Chi fluorescence decrease ratio (R_Fd) as non-destructive indicator of the C0₂-fixation rates (P_N ) and as vitality index of the photosynthetic apparatus is presented. The R_Fd values are determined from the Chi fluorescence induction kinetics. They are defined as ratio of the Chi fluorescence decrease Fd to the steady state Chi fluorescence Fs after onset of photosynthesis: R_Fd = Fd/Fs = (Fm- Fs)/Fs. Possible modifications in pigment composition and photosynthetic activity (changes in R_Fd values and P_N rates) during the vegetation period and stress events are indicated.

The pigment breakdown, paralleled by measurements of Chi fluorescence parameters, during autumnal senescence is characterized by a differential breakdown of Chi a and b and of total carotenoids, xanthophylls (x) and carotenes (c). Despite an extensive breakdown of pigments, the Chi fluorescence decrease ratio, R_Fd ‘normal’, indicates that the remaining Chi of leaves is photosynthetically active during the major part of the autumnal senescence process. We emphasize the highly flexible reactivity of the photosynthetic apparatus and show that certain pigment ratios [Chi a/b and (a + b)/(x + c)] as well as Chi fluorescence parameters (R_Fd-values) are good indicators of ongoing stress and senescence processes. Finally, the great advantage of the recently developed Chi fluorescence imaging techniques of leaves, as an excellent means for screening changes in the functionality of the photosynthetic apparatus during adaptation and stress events, is briefly discussed. Fluorescence imaging of leaves with video Charge Coupled Device (CCD) cameras, that simultaneously provides the Chi fluorescence information of more than 100,000 leaf pixels, is much superior to the data on Chi fluorescence measurements applied thus far using conventional fiuorometers which yield average fluorescence information of only one leaf area point per measurement.