Abstract
The Crassulacean acid metabolism (CAM) pathway helps plants to alleviate the oxidative stress under drought, but the shift to CAM-idling may expose plants to the overproduction of reactive oxygen species causing cell damages. The facultative CAM species Portulacaria afra L., was subjected to 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 were performed during the stress and rewatering. Under water deprivation, plants shifted from C3 to CAM metabolism, reaching the CAM-idling status at the end of the stress period. The daily variation of the titratable acidity and PARP activity increased at the beginning of stress and declined with stress progression, reaching the lowest value at the end of stress treatment. H2O2 content, superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities increased with the severity of water stress. The photochemical processes remained high during the entire stress period indicating the presence of alternative sinks to CO2 fixation. The elevated activity of catalase under severe water stress suggests the occurrence of photorespiration in sustaining the photosynthetic electron transport under CAM-idling condition. The overall data indicate that scavenger enzymes, photorespiration and 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.
Highlights
Crassulacean acid metabolism (CAM) represents a carbon fixation pathway typical of plants adapted to arid environments
Plants of Portulacaria afra (L.) Jacq. of three-years old were grown in a greenhouse under natural sunlight in pots of 5 L filled with a mixture of soil: peat (1:1 v:v) and irrigated to field water capacity
Our results showed that P. afra plants counteract the water stress by defense mechanisms operating both at the physiological and biochemical level
Summary
Crassulacean acid metabolism (CAM) represents a carbon fixation pathway typical of plants adapted to arid environments. In CAM plants the stomata are open during night and the CO2 uptake occurs with organic acids synthesis The CO2 fixation is mediated by phosphoenolpyruvate carboxylase, PEPC (phase I). In the morning, during the early light period the simultaneous CO2 fixation by PEPC and Rubisco occurs while stomata start to close (phase II). Stomata are closed, and organic acids stored during night are decarboxylated. The released CO2 is re-fixed by Rubisco and assimilated in C3 pathway (phase III). In the late light period, stomata reopen and Plants 2020, 9, 1192; doi:10.3390/plants9091192 www.mdpi.com/journal/plants
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