INFLUENCES OF SOIL VOLUME AND AN ELEVATED CO2 LEVEL ON GROWTH AND CO2EXCHANGE FOR THE CRASSULACEAN ACID METABOLISM PLANT OPUNTIA-FICUS-INDICA

Effects of the current (38 Pa) and an elevated (74 Pa) CO2 partial pre ssure on root and shoot areas, biomass accumulation and daily net CO2 exchange were determined for Opuntia ficus-indica (L.) Miller, a highl y productive Crassulacean acid metabolism species cultivated worldwide . Plants were grown in environmentally controlled rooms for 18 weeks i n pots of three soil volumes (2 600, 6 500 and 26 000 cm3), the smalle st of which was intended to restrict root growth. For plants in the me dium-sized soil volume, basal cladodes tended to be thicker and areas of main and lateral roots tended to be greater as the CO2 level was do ubled. Daughter cladodes tended to be initiated sooner at the current compared with the elevated CO2 level but total areas were similar by 1 0 weeks. At 10 weeks, daily net CO2 uptake for the three soil volumes averaged 24% higher for plants growing under elevated compared with cu rrent CO2 levels. but at 18 weeks only 3% enhancement in uptake occurr ed. Dry weight gain was enhanced 24% by elevated CO2 during the first 10 weeks but only 8% over 18 weeks. Increasing the soil volume 10-fold led to a greater stimulation of daily net CO2 uptake and biomass prod uction than did doubling the CO2 level. At 18 weeks, root biomass doub led and shoot biomass nearly doubled as the soil volume was increased 10-fold; the effects of soil volume tended to be greater for elevated CO2. The amount of cladode nitrogen per unit dry weight decreased as t he CO2 level was raised and increased as soil volume increased, the la tter suggesting that the effects of soil volume could be due to nitrog en limitations.