CO2 uptake, water vapor conductance, and biomass production of Opuntia ficus-indica, a Crassulacean acid metabolism species, were studied at CO2 concentrations of 370, 520, and 720 pL 1-' i n open-top chambers during a 23-week period. Nine weeks after planting, daily net CO2 uptake for basal cladodes at 520 and 720 pL L-' of CO2 was 76 and 98% higher, respectively, than at 370 p L 1-'. Eight weeks after daughter cladodes emerged, their daily net CO2 uptake was 35 and 49% higher at 520 and 720 pL L-' of CO2, respectively, than at 370 M L 1-'. Daily water-use efficiency was 88% higher under elevated CO2, for basal cladodes and 57% higher for daughter cladodes. l h e daily net CO2 uptake capacity for basal cladodes increased for 4 weeks after planting and then remained fairly constant, whereas for daughter cladodes, it increased with cladode age, became maximal at 8 to 14 weeks, and then declined. The percentage enhancement in daily net CO2, uptake caused by elevated CO2 was greatest initially for basal cladodes and at 8 to 14 weeks for daughter cladodes. l h e chlorophyll content per unit fresh weight of chlorenchyma for daughter cladodes at 8 weeks was 19 and 62% lower i n 520 and 720 p L 1-' of CO2, respectively, compared with 370 pL L-'. Despite the reduced chlorophyll content, plant biomass production during 23 weeks in 520 and 720 p L 1-' of CO2 was 21 and 55% higher, respectively, than at 370 pL 1-l. The root dry weight nearly tripled as the CO2 concentration was doubled, causing the root/shoot ratio to increase with CO2 concentration. During the 23-week period, elevated CO2 significantly increased CO2 uptake and biomass production of O. ficus-indica.
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