Cladode development for Opuntia ficus-indica (Cactaceae) under current and doubled CO2 concentrations

Morphological and anatomical changes for first-order daughter cladodes (flattened stem segments) of a prickly pear cactus, Opuntia ficus-indica, were monitored to determine the effects of a doubled atmospheric CO₂ concentration on their development and mature form. For daughter cladodes developing in controlled environment chambers for 60 d, maximal elongation rates were similar under a photosynthetic photon flux density (PPFD) of 6 mol m^?2 d^?1 and a CO₂ concentration of 370 μl liter^?1, an increased PPFD (10 mol m^?2 d^?1), and an increased PPFD and a doubled CO₂ concentration. These maximal rates, however, occurred at 20, 15, and 12 d, respectively. The maximal relative growth rate under the doubled CO₂ concentration was about twice that under the other conditions. For cladodes at 60 d as well as after 4 and 16 mo in open-top chambers, doubling the CO₂ concentration had no effect on final length or width. At 4 mo, cladodes under doubled C0₂ were 27% thicker, perhaps allowing the earlier production of second-order daughter cladodes. The chlorenchyma was then 31% thicker and composed of longer cells. At 16 mo, the difference in cladode thickness diminished, but the chlorenchyma remained thicker under doubled CO₂, which may contribute to greater net CO₂ uptake for O. ficus-indica under elevated CO₂ concentrations. Two other persistent differences were a 20% lower stomatal frequency and a 30% thicker cuticle with more epicuticular wax for cladodes under doubled CO₂, both of which may help reduce transpirational water loss.