Seasonal variation of net CO2 uptake for cactus pear (Opuntia ficus-indica) and pitayo (Stenocereus queretaroensis) in a semi-arid environment

The objective of this work was to evaluate the effects of seasonal variatio n in temperature, irradiation, and soil moisture content on the photosynthe tic rates of Opuntia ficus-indica and Stenocereus queretaroensis. The lowes t values of positive dairy net CO2 uptake were recorded during the summer r ainy season and the highest values during autumn and winter. These optimum values are likely to be the result of a prolonged period of CO2 assimilatio n caused by decreased cloudiness, and high rates of nocturnal CO2 assimilat ion due to moderate day/night air temperatures. In spring, when drought was exacerbated, daily net CO2 uptake was negative for both O. finus-indica an d S. queretaroensis. Stem photosynthesis allowed both O. ficus-indica and S . queretaroensis to obtain carbon during the fall and winter, when soil wat er was at its lowest. (C) 2000 Academic Press.