Date of this Version
New Phytologist, Vol. 128, Issue 2, October 1994, Pages: 315–322
Mature cladodes of Opuntia ficus-indica (L.) Miller have a thick chlorenchyma (about 4 mm) with a relatively high chlorophyll content (0-65 g m-2), suggesting that light may be greatly attenuated and hence CO2 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 CO2 concentrations. Nevertheless, the nocturnal acidity increase and 14C accumulation following a brief exposure to 14CO2 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 CO2, the activities of both ribulose-l,5-bispbospbate carboxylase/oxygenase and phosphoenolpyruvate carboxylase were similar for each of the outer three 1-mm-thick layers of tbe chlorenchyma. Therefore, although the light level and total chlorophyll decreased sharply with depth and the chlorophyll a/b ratio also decreased, substantial CO2 fixation apparently occurs throughout most of the chlorenchyma. When O. ficus-indica was grown under 720 μmol CO2 mol-1, the chlorenchyma was 20% thicker but contained 11% less chlorophyll and had a lower absorptance than under the current CO2 concentration (370 μmol mol-1). Greater nocturnal acidity increases and 14C accumulation following exposure to 14CO2 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 from 400 to 700 nm.