Cladode development, environmental responses of CO2uptake, and productivity for Opuntia ficus-indica under elevated CO2

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Abstract
Opuntia ficus-indica, an extremely productive CAM plant cultivated in many countries, was exposed to 36, 52, and 72–73 Pa CO2 in field plots and open-top chambers. Initiation of new cladodes (stem segments) was monitored until the canopy closed, after which bimonthly harvests maintained the plants for one year at a cladode area per unit ground area that is optimal for biomass production. Doubling the CO2 partial pressure slightly increased the number of first-order daughter cladodes growing on the basal (planted) cladodes after 3 months and nearly doubled the number and area of second-order cladodes. When the C02 level was doubled, cladodes were 5% thicker after a few months and 11 to 16% thicker after one year. Although the productivity enhancement by elevated C02 tended to decrease during the year, the annual above-ground dry-mass gain was 37 to 40% higher when the C02 level was doubled, reaching 65 tons hectare−1 year−1 in a field plot. Well-watered cladodes at day/night air temperatures of 25°C/15°C and a total daily photosynthetic photon flux (PPF) of 15 mol m−2 d−1 in controlled environment chambers had 74% more net CO2 uptake over 24 h at 73 Pa than at 37 Pa CO2. With doubled CO2, the percentage enhancement of net CO2 uptake increased as the PPF was lowered, as the temperature was raised, and during drought. Using an environmental productivity index based on such factors, net CO2 uptake and hence productivity of O. ficus-indica can be predicted for elevated CO2 levels and other variations accompanying global climate change.

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