CO2 fluxes of Opuntia ficus-indica Mill. trees in relation to water status

Gas exchange pattern in O. ficus-indica(OFI), refers to the Crassulacean Acid Metabolism (CAM); trees have nocturnal stomata opening, so net CO2 uptake and water loss occur during the cooler part ofthe 24-hour cycle. Succulent cladodes skip severe periods of drought through their water storer tissue (parenchyma). To study carbon fluxes in stress and no stress conditions, an experiment was carried out on 3-year-old irrigated and non-irrigated OFI potted trees; whole tree gas exchange was measured continuously with a balloon system made up by a portable Infrared Gas Analyzer. Continuous measurements(nighttime) during the summer season were useful to assess differences in carbon uptake under stress and no stress conditions. There was a gradual increment (5 μmol m2 s-1in June, 7 μmol m2 s-1 in July and 8.8 μmol m2 s-1 in August) in terms of CO2 uptake in irrigated trees from June to August 2010. The uptake was lower in stressed trees than in irrigated ones in each measurements date. Measurements carried out on non-irrigated trees showed carbon gain even 60 days after irrigation was stopped, with less than 2% of soil water content, far below the wilting point. Considering an average of 6.9 μmol CO2 m2 s-1, for well watered trees, from June to August, and a stem area index (SAI) of 2, a daily amount of 21.8 kg ha-1 d-1 of CO2 was accumulated in irrigated trees in that period, corresponding to a carbon assimilation of 0.54 T ha-1.