LIGHT, CHLOROPHYLL, CARBOXYLASE ACTIVITY AND CO2 FIXATION AT VARIOUS DEPTHS IN THE CHLORENCHYMA OF OPUNTIA-FICUS-INDICA (L) MILLER UNDER CURRENT AND ELEVATED CO2

Mature cladodes of Opuntia ficus-indica (L.) Miller have a thick chlor enchyma (about 4 mm) with a relatively high chlorophyll content (0.65 g m(-2)), suggesting that light may be greatly attenuated and hence CO 2 fixation negligible in the inner part of this tissue. Indeed, blue l ight (400-470 nm) and red light (670-685 nm) were 99 % attenuated in t he outer 2 mm of the chlorenchyma when the cladodes developed under bo th current and elevated CO2 concentrations. Nevertheless, the nocturna l acidity increase and C-14 accumulation following a brief exposure to (CO2)-C-14 night decreased only 22 to 47 % for a layer 2-3 mm deep in the chlorenchyma of this CAM plant. Under a particular growth CO2, th e activities of both ribulose-1,5-bisphosphate carboxylase/oxygenase a nd phosphoenolpyruvate carboxylase were similar for each of the outer three 1-mm-thick layers of the chlorenchyma. Therefore, although the l ight level and total chlorophyll decreased sharply with depth and the chlorophyll a/b ratio also decreased, substantial CO2 fixation apparen tly occurs throughout most of the chlorenchyma. When O. ficus-indica w as grown under 720 mu mol CO2 mol(-1), the chlorenchyma nas 20 % thick er but contained 11 % less chlorophyll and had a lower absorptance tha n under the current CO2 concentration (370 mu mol(-1) ). Greater noctu rnal acidity increases and C-14 accumulation following exposure to (CO 2)-C-14 at night occurred at the doubled CO2 concentration despite 29- 39% reductions in the activities of the two carboxylating enzymes, the lower absorptance, and a 24 % increase in the cladode reflectance fro m 400 to 700 nm.