Temperature and water regulation of gas exchange of Opuntia polyacantha.

Opuntia polyacantha was collected from the shortgrass prairie in Colorado. Carbon dioxide and water vapor exchange was monitored in plants pretreated and analyzed under cool temperatures (20/15°C) and warm temperatures (35/15°C). Well watered plants under a 35/15 thermoperiod supported the fixation of atmospheric CO2 during the night, early morning, and late afternoon. Plants under a 20/15 thermoperiod exhibited CO2 uptake only during the afternoon. The fixation of CO2 at night could be stimulated or induced by decreasing the night temperature. Plants from which water was withheld two or four weeks showed a decline in CO2 fixation with the uptake at night exhibiting the greatest sensitivity. Under conditions of water stress the enhancement of CO2 uptake at night by cool night temperatures was largely lost. Plants water stressed for 4 weeks recovered rapidly upon rewatering under warm or cool temperatures. Rates of CO2 fixation were comparable to well watered plants within 24 h. The effects of temperature and water stress on gas exchange are compared to the in situ growth pattern of O. polyacantha and contrasted with the regulation of gas exchange observed in C3 and C4 grasses of the shortgrass prairie.