The antioxidative protection during the C3-CAM shift induced by
water stress was investigated in the temperate succulent Sedum album L. The C3-CAM shift was characterized in terms of CO2 exchange, titratable acidity and
phosphoenolpyruvate carboxylase activity. Well-watered plants displayed C3-like patterns of gas exchange and exhibited a mild day-night
acid fluctuation indicating that those plants were performing CAM-cycling metabolism. Imposed drought highly stimulated CAM cycling, decreasing the net CO2 uptake during the day, eliminating net CO2 efflux at night and stimulating tissue
acid fluctuations. As water deficit developed,
chlorophyll fluorescence measurements showed a decrease in the Fv/Fm ratio, indicating that photoinhibition could follow after severe drought. Protection might be performed by the increased activity of
enzymes involved in the destruction of
free radicals and
oxidants, but their response depended on the water status of the plant.
Ascorbate peroxidase and
superoxide dismutase activities increased in plants subjected to mild stress but declined during severe
water stress.
Catalase activity, however, was quite stable under mild
water stress and was clearly inhibited under severe
water stress. At this stage,
glutathione reductase and
monodehydroascorbate reductase seemed to be very important in the protection against
oxidants, both increasing considerably their activities under severe
water stress. Even if recycling has been shown to alleviate photoinhibition, our results clearly demonstrate that antioxidative
enzymes play an important role in the protection of plants from
oxidants during the C3-CAM shift induced by
water stress.