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

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 nonphoto-chemical quenching (q_n), with the largest differences being observed during the middle of the day. Cycling plants had lower q_n than CAM plants, indicating altered photosynthetic regulation processes. This q_n 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 q_n in comparison to that measured in plants in the cycling mode. No changes were observed in photochemical quenching when CO₂ was added.