Fluorescence Quenching in the Varied Photosynthetic Modes of Portulacaria afra (L.) Jacq.

Abstract

The kinetics of chlorophyll fluorescence were measured in Portulacaria afra (L.) Jacq. when the plants were functioning in either Crassulacean acid metabolism (CAM) or C3/CAM cycling (called cycling) modes, as determined by fluctuation in titratable acidity and gas exchange properties. Cycling plants showed primarily daytime CO2 uptake typical of C3 plants, but with a slight diurnal acid fluctuation, whereas CAM plants showed nocturnal CO2 uptake, daytime stomatal closure, and a large diurnal acid fluctuation. Results from fluorescence measurements indicated no significant differences in photochemical quenching between cycling and CAM plants; however, sizable differences were detected in nonphotochemical quenching (qN), with the largest differences being observed during the middle of the day. Cycling plants had lower qN than CAM plants, indicating altered photosynthetic regulation processes. This qN difference was believed to be related to reduced internal CO2 concentration in the CAM plants because of daytime stomatal closure and reduced deacidification rates in the late afternoon when most of the malic acid has been utilized. Experimentally, higher external CO2 given to plants in the CAM mode resulted in a decline in qN in comparison to that measured in plants in the cycling mode. No changes were observed in photochemical quenching when CO2 was added.Many studies of CAM species have focused on the shift from C3/CAM cycling (called cycling) to CAM photosynthesis during various stresses (3,8,9,17,21,23 The Chl fluorescence transients correlated with increased light scattering. On the other hand, the results of Buschmann and Buchanan-Bollig (4) indicated that Chl fluorescence transients were dependent upon leaf structure, pigment composition, and the organization of the photosystems rather than the CO2 fixation pathway. The question arises as to how well control of the operation of photosynthesis by internal CO2 levels (7) can be measured by fluorescence techniques. The levels of CO2 inside the leaf certainly change between these two photosynthetic modes (5) and so should alter photosynthesis and, concurrently, the fluorescence patterns (18).A fluorescence technique has been developed that enables the separation of qp and qN quenching of Chl fluorescence (20). This technique uses a weak light beam to measure the fluorescence yield and a high intensity actinic beam to drive photosynthesis. The application of a second light pulse of high intensity is used to overwhelm photochemical quenching, thus permitting direct measurement of qN. This technique was used here to monitor changes in qp and qN in Portulacaria afra (L.) Jacq. (a facultative CAM plant) as it shifted from cycling to the CAM mode of photosynthesis under water stress (9, 13) and, in part, to show how intemal CO2 was changing during the metabolic switch. The critical factor in these experiments is that this shift was not accompanied by any changes in pigment composition (11). The effect of extemal CO2 levels on these same fluorescence measurement...

Comparative measurements of chlorophyll a fluorescence, acid accumulation and gas exchange in exposed and shaded plants of Clusia minor L. and Clusia multiflora H. B. K. in the field

“…This effect may be enhanced by decarboxylation of nocturnally accumulated citric acid, which possibly allows the evolution of 3 or 6 mol CO2 per mol citrate (Haag-Kerwer et al 1992). During phase III when malic acid is decarboxylated and internal CO2 concentration is high nonphotochemical quenching is reduced compared to phases II and IV of CAM when internal CO2 concentration is below air level (Winter and Demming 1987;Guralnick et al 1992). During phase III when malic acid is decarboxylated and internal CO2 concentration is high nonphotochemical quenching is reduced compared to phases II and IV of CAM when internal CO2 concentration is below air level (Winter and Demming 1987;Guralnick et al 1992).…”

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“…The NPQ is higher in stressed plants than in control plant, exhibiting enlarged de-exciting by thermal dissipation processes [40,41]. NPQ-value rises as a protective mechanism, however, high value-NPQ may represent a down-regulation of a photosythetic electron transport so that the production of ATP and NADPH would match the decreased CO 2 assimilation.…”