Effect of elevated CO2, temperature and limited water supply on antioxidant status during regrowth of nodulated alfalfa

Abstract

Atmospheric CO2 is a major contributor to the greenhouse effect and is one of the main inducers of climate change. Previous studies with nodulated alfalfa plants have shown that elevated CO2 increased the growth of plants grown under well‐watered or limited water supply conditions. The beneficial effects of atmospheric CO2 enrichment included higher photosynthetic rates, growth and water‐use efficiency and an increase in the root/shoot ratio. However, at the moment, we do not have information on the possible implications of the beneficial effect of elevated CO2 as it may relate to a higher capacity of the violaxanthin–antheraxanthin–zeaxanthin (VAZ) cycle, the dissipation of excess radiation as heat and the effect on photooxidation, and to an improved leaf antioxidant system (Halliwell–Asada cycle). The aim of the present study was to determine the effects of the interaction between CO2 (ambient, around 350 vs 700 μmol mol−1), temperature (ambient vs ambient + 4°C) and water availability (well irrigated vs partially irrigated) on the leaf antioxidant status of nodulated alfalfa during regrowth. Parameters measured in this study included relative growth rate (RGR), H2O2 content, oxidative damage [measured as thiobarbituric acid‐reacting substances (TBARS)], leaf pigment composition (chlorophylls and xanthophylls), ascorbate (ASA) and glutathione pool levels and antioxidant enzymes. Our results revealed that during alfalfa regrowth, the effects of elevated CO2, limited water supply, temperature and their interactions on growth were not related to significant or general changes in leaf antioxidant capacity, H2O2 accumulation or oxidative stress (TBARS concentrations). The beneficial effects of CO2 enrichment in well‐watered and limited water‐subjected plants were not associated with an increase in the capacity of alfalfa leaves to dissipate excess radiation as heat through the VAZ cycle or with an increase in the antioxidant capacity, measured in terms of Halliwell–Asada cycle enzymes and antioxidant compounds. Furthermore, elevated CO2 did not affect RGRs during the last 15 days of regrowth and reduced the activity of several antioxidant enzymes (catalase, superoxide dismutase and glutathione reductase and ASA peroxidase in limited water‐subjected plants), suggesting a lower basal rate of oxygen activation and H2O2 formation, leading to a relaxation of the antioxidant system.