GAS-EXCHANGE AND GROWTH-RESPONSES TO ELEVATED CO2 AND LIGHT LEVELS INTHE CAM SPECIES OPUNTIA-FICUS-INDICA

Gas exchange and dry-weight production in Opuntia ficus-indica, a CAM species cultivated worldwide for its fruit and cladodes, were studied in 370 and 750 mu mol mol(-1) CO2 at three photosynthetic photon flux densities (PPFD: 5, 13 and 20 mol m(-2) d(-1)). Elevated CO2 and PPFD enhanced the growth of basal cladodes and roots during the 12-week stu dy. A rise in the PPFD increased the growth of daughter cladodes; elev ated CO2 enhanced the growth of first-daughter cladodes but decreased the growth of the second-daughter cladodes produced on them. CO2 enric hment enhanced daily net CO2 uptake during the initial 8 weeks after p lanting for both basal and first-daughter cladodes. Water vapour condu ctance was 9 to 15% lower in 750 than in 370 mu mol mol(-1) CO2. Clado de chlorophyll content was lower in elevated CO2 and at higher PPFD. S oluble sugar and starch contents increased with time and were higher i n elevated CO2 and at higher PPFD. The total plant nitrogen content wa s lower in elevated CO2. The effect of elevated CO2 on net CO2 uptake disappeared at 12 weeks after planting, possibly due to acclimation or feedback inhibition, which in turn could reflect decreases in the sin k strength of roots. Despite this decreased effect on net CO2 uptake, the total plant dry weight at 12 weeks averaged 32% higher in 750 than in 370 mu mol mol(-1) CO2. Averaged for the two CO2 treatments, the t otal plant dry weight increased by 66% from low to medium PPFD and by 37% from medium to high PPFD.