Physiological responses of <i>Opuntia ficus‐indica</i> to growth temperature

Abstract

The influences of various day/night air temperatures on net CO 2 uptake and nocturnal acid accumulation were determined for Opuntia ficus‐indica , complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO 2 uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO 2 residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO 2 uptake at all growth temperatures. Nocturnal CO 2 uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO 2 uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non‐optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO 2 uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus‐indica and presumably for other CAM plants.