Irrigation magnifies CAM-photosynthesis in Opuntia basilaris (Cactaceae)

Measurements of acid metabolism and gas exchange were carried out four times during a year to assess the relative importance of temperature and the accompanying seasonal change to the carbon metabolism of Opuntia basilaris Engelm. & Bigel. plants growing in situ under irrigated and natural (control) conditions. Our experiments showed that this cactus did not change its pattern of carbon assimilation when continuously irrigated under field conditions. Non-irrigated cacti had maximum acid accumulation after periods of precipitation. Maximum acid accumulation in irrigated cacti occurred when there was a large difference between day/night temperatures (i.e., 16°C), and when nighttime temperatures were moderate (14C). Irrigated cacti had greater duration of stomatal opening and lower resistance to ¹⁴CO₂ uptake. When temperatures were low, daytime stomatal resistance to ¹⁴CO₂ uptake decreased (to 20-40 s cm^-1), but never to the level of the nocturnal resistances (5-10 s cm^-1). During periods of drought, nonirrigated cycti changed to a pattern in which organic acids fluctuated. Irrigated cacti continued to have ¹⁴CO₂ uptake when nighttime temperatures were as high as 33°C. ¹³C/¹²C isotope composition ratios, determined after two years of irrigation, were near -12‰ in irrigated and non-irrigated plants. Therefore, under conditions of continual irrigation, seasonal and temperature changes affected the degree of dark CO₂ fixation and acid metabolism, but these cacti did not change from CAM to CO₂ fixation in the light.