Ecophysiology of young stems (cladodes) of Opuntia ficus-indica in wet and dry conditions

The rates of instantaneous net CO2 uptake were measured every 2 h over 24 h periods in young stems (cladodes) of Opuntia ficus-indica (L.) Miller those were well-watered or exposed to drought during the Spring. Stem relative water content was also measured. Young cladodes showed Crassulacean acid metabolism (CAM). Spring drought reduced daily net CO2 uptake and watering sustained it. A dry environment affected the expression of CAM phases in young cladodes, curtailing CO2 uptake in the late afternoon (phase IV of CAM) more than that in the early morning (phase II) or at night (phase I). Young cladodes maintain positive values of carbon gain in the dry spring, notwithstanding the low capacitance of their water-storage parenchyma. Cladodes showed rapid recovery of photosynthesis and relative water content in response to scarce rain that interrupted the drought at the end of May. Both the instantaneous rate of CO2 uptake and the daily carbon gain of young cladodes increased significantly in June, after the plants received considerable rainfall. The photosynthetic efficiency of young cladodes was not far from the values observed in mature cladodes.