Non-Autotrophic Carbon Dioxide Metabolism in Cacti

Abstract

Non-autotrophic ("dark") 14CO2 metabolism and associated acid accumulation were studied in the Cactaceae. Carbon dioxide assimilation and oxygen uptake were generally lower than in crassulacean succulents. The primary products were organic acids (malic acid) and amino acids. Little or no 14C activity was detected in lipoidal or neutral components. After a 2-hr dark CO2 fixation period, the first-order rate constant for steady-state decarboxylation in the dark was 1.7 x 10-3 min-1 and in the light was 0.2 x 10-3 min-1. The greater rate in the dark was reflected in the decay of the primary product, malic acid. The slower rate in the light was accounted for by photosynthetic reassimilation of 14CO2. In part, the observed diurnal fluctuation of organic acids may be explained in terms of steady-state decarboxylation. In the dark carboxylation evidently exceeds decarboxylation, and organic acids accumulate. In the light much of the available CO2 would be assimilated by photosynthetic carboxylation reactions so that non-autotrophic carboxylation may be less than steady-state decarboxylation. Thus, a diurnal fluctuation of organic acids would result.