Physiological Responses to Irrigation in Opuntia basilaris Engelm. & Bigel.

Natural stands of Opuntia basilaris Engelm. & Bigel, growing under irrigated and nonirrigated (control) conditions, were used to test the hypothesis that this CAM plant was capable of a "shift" in its method of gas exchange from a typical CAM mode to a C3 mode of carbon uptake. Gas exchange data were accumulated in the field with a dual-isotope (3H2O and 14CO2) porometer, which allowed simultaneous estimates of carbon uptake and transpiration on the same plant sample. Water availability (through irrigation) increased the response of the cacti to nighttime uptake of 14CO2 (9-10 mg dm-2 h-1 in irrigated plants vs 6-7 mg dm-2 h-1 in control plants) and did not result in appreciable daytime opening of stomata as reported for other succulents. Young stems responded to irrigation to a greater extent and with greater duration than mature stems. Increased rates of nocturnal 14CO2 were related to decreased stomatal (3-4 s cm-1 for irrigated plants, 7-8 s cm-1 for control plants) resistance to gas exchange. The nocturnal efficiency of water use and CO2 uptake, as measured by the transpiration ratio, was higher than the diurnal gas exchange efficiency. Opuntia basilaris, a stem succulent, did not show a shift in metabolism in response to irrigation as commonly observed in leaf succulents.