Photo-Protective Mechanisms and the Role of Poly (ADP-Ribose) Polymerase Activity in a Facultative CAM Plant Exposed to Long-Term Water Deprivation

The Crassulacean acid metabolism (CAM) pathway helps plants to alleviate the oxidativestress under drought, but the shift to CAM-idling may expose plants to the overproduction of reactiveoxygen species causing cell damages. The facultative CAM species Portulacaria afra L., was subjectedto long-term water deprivation to assess the photo-protective strategies and the poly (ADP-ribose)polymerase (PARP) activity during water stress and plant capability to recover from the stress.Measurements of titratable acidity, chlorophyll fluorescence emission, and antioxidant activity wereperformed during the stress and rewatering. Under water deprivation, plants shifted from C3 toCAM metabolism, reaching the CAM-idling status at the end of the stress period. The daily variationof the titratable acidity and PARP activity increased at the beginning of stress and declined withstress progression, reaching the lowest value at the end of stress treatment. H2O2 content, superoxidedismutase (SOD), peroxidase (POD), and catalase (CAT) activities increased with the severity of waterstress. The photochemical processes remained high during the entire stress period indicating thepresence of alternative sinks to CO2 fixation. The elevated activity of catalase under severe waterstress suggests the occurrence of photorespiration in sustaining the photosynthetic electron transportunder CAM-idling condition. The overall data indicate that scavenger enzymes, photorespirationand PARP activity modulation contribute to the strong resistance of P. afra to severe water stress,preserving the functioning of photosynthetic apparatus and ensuring plant recovery with rewatering.