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-ind ica, were monitored to determine the effects of a doubled atmospheric CO2 concentration on their development and mature form. For daughter c ladodes developing in controlled environment chambers for 60 d, maxima l elongation rates were similar under a photosynthetic photon flux den sity (PPFD) of 6 mol m(-2) d(-1) and a CO2 concentration of 370 mu l l iter(-1), an increased PPFD (10 mol m(-2) d(-1)), and an increased PPF D and a doubled CO2 concentration. These maximal rates, however, occur red at 20, 15, and 12 d, respectively. The maximal relative growth rat e under the doubled CO2 concentration was about twice that under the o ther 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 fin al length or width. At 4 mo, cladodes under doubled CO2 were 27% thick er, perhaps allowing the earlier production of second-order daughter c ladodes. The chlorenchyma was then 31% thicker and composed of longer cells. At 16 mo, the difference in cladode thickness diminished, but t he chlorenchyma remained thicker under doubled CO2, which may contribu te to greater net CO2 uptake for O. ficus-indica under elevated CO2 co ncentrations. Two other persistent differences were a 20% lower stomat al frequency and a 30% thicker cuticle with more epicuticular wax for cladodes under doubled CO2, both of which may help reduce transpiratio nal water loss.