Light, chlorophyll, carboxylase activity and CO 2 fixation at various depths in the chlorenchyma of Opuntia ficus–indica (L.) Miller under current and elevated CO 2

summary Mature cladodes of Opuntia ficus‐indica (L.) Miller have a thick chlorenchyma (about 4 mm) with a relatively high chlorophyll convent (0.65 gm −2 ), suggesting that light may be greatly attenuated and hence CO 2 fixation negligible in the inner part of this tissue. Indeed, blue light (400–470 nm) and red light (670–685 nm)were 99% attenuated in the outer 2 mm of the chlorenchyma when the cladodes developed under both current and elevated CO 2 Concentrations. Nevertheless, the nocturnal acidity increase and 14 C accumulation following a brief exposure to 14 CO 2 at night decreased only 22 to 47% for a layer 2–3 mm deep in the chlorenchyma of this CAM plant. Under a particular growth CO 2 , the activities of both ribulose‐1,5‐bisphosphate carboxylase/oxygenase and phosho‐enolyruvate carboxylase were similar for each of the outer three 1‐mm‐thick layers of the chlorenchyma. Therefore, although the light level and total chlorophyll decreased sharply with depth and the chlorophyll a/b ratio also decreased. Substantial CO 2 fixation apparently occurs throughout most of the chlorenchyma. When O. ficus‐indica was grown under 720 μmol CO 2 mol −1 , the chlorenchyma was 20% thicker but contained 11% less chlorophyll and had a lower absorptance than under the current CO 2 concentration (370μmol mol −1 ). Greater nocturnal acidity increases and 14 C accumulation following exposure to 14 CO 2 at night occurred at the doubled CO 2 concentration despite 29–39% reductions in the activities of the two carboxylating enzymes, the lower absorptance, and a 24% increase in the cladode reflectance from 400–700 nm.