DECARBOXYLATION OF PRIMARY AND END-PRODUCTS OF PHOTOSYNTHESIS AT DIFFERENT OXYGEN CONCENTRATIONS

A radiogasometric method for the determination of the rates of intracellular decarboxylation of primary and end products of steady-state photosynthesis in the light has been worked out. On the basis of oxygen dependence of the CO2 evolution, two types of decarboxylation, oxygenase (photorespiration) and oxidase (dark respiration), have been distinguished. Oxygenase decarboxylation is linearly dependent on O-2 concentration in the range from 0 to 210 mmoi mol(-1), which is characteristic of the oxygenation of RuBP, the rate-limiting reaction of the glycolate cycle. Oxidase decarboxylation is saturated at relatively low O-2 concentration (of about 15 mmol mol(-1)), similar to the oxidative reactions associated with glycolysis and the Krebs cycle. In the C-3 plants (tobacco, wheat, barley) we have studied, primary products were decarboxylated predominantly via the oxygenase mechanism; the oxidase component was 3-15% of the total rate of the decarboxylation of primary photosynthates in the light. The oxygenase mechanism was also involved in the decarboxylation of end products of photosynthesis. The rate of this component exceeded by 3 to 5 times the rate of oxidase decarboxylation of end products in the light. It has been suggested that compounds derived from the degradation of end products of photosynthesis are incorporated into the reductive pentose phosphate cycle and, via the cycle, into the glycolate cycle where they are decarboxylated. In this way, four components of plant leaf respiration in the light may be distinguished according to the substrates and mechanisms of decarboxylation. In the dark, only one of them, oxidase decarboxylation of end products, is functioning. The rate of this component has been shown to be partially suppressed by light.