Light, chlorophyll, carboxylase activity and CO2 fixation at various depths in the chlorenchyma of Opuntia ficus-indica (L.) Miller under current and elevated CO2.

Abstract

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 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 14 C accumulation following a brief exposure to 14 CO2 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-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 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 (370μmol mol-1 ). Greater nocturnal acidity increases and 14 C accumulation following exposure to 14 CO2 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 400-700 nm.