Effect of Cold Hardening on the Components of Respiratory Decarboxylation in the Light and in the Dark in Leaves of Winter Rye.

In the dark, all decarboxylation reactions are associated with the oxidase reactions of mitochondrial electron transport. In the light, photorespiration is also active in photosynthetic cells. In winter rye (Secale cereale L.), cold hardening resulted in a 2-fold increase in the rate of dark respiratory CO2 release from leaves compared with nonhardened (NH) controls. However, in the light, NH and cold-hardened (CH) leaves had comparable rates of oxidase decarboxylation and total intracellular decarboxylation. Furthermore, whereas CH leaves showed similar rates of total oxidase decarboxylation in the dark and light, NH leaves showed a 2-fold increase in total oxidase activity in the light compared with the dark. Light suppressed oxidase decarboxylation of end products of photosynthesis 2-fold in NH leaves and 3-fold in CH leaves in air. However, in high-CO2, light did not suppress the oxidase decarboxylation of end products. Thus, the decrease in oxidase decarboxylation of end products observed in the light and in air reflected glycolate-cycle-related inhibition of tricarboxylic acid cycle activity. We also showed that the glycolate cycle was involved in the decarboxylation of the end products of photosynthesis in both NH and CH leaves, suggesting a flow of fixed carbon out of the starch pool in the light.

Photorespiratory and respiratory decarboxylations in leaves of C3 plants under different CO2 concentrations and irradiances.

We used an advanced radiogasometric method to study the effects of short-term changes in CO2 concentration ([CO2]) on the rates and substrates of photorespiratory and respiratory decarboxylations under steady-state photosynthesis and in the dark. Experiments were carried out on Plantago lanceolata, Poa trivialis, Secale cereale, Triticum aestivum, Helianthus annuus and Arabidopsis thaliana plants. Rates of photorespiration and respiration measured at a low [CO2] (40 micromol mol(-1)) were equal to those at normal [CO2] (360 micromol mol(-1)). Under low [CO2], the substrates of decarboxylation reactions were derived mainly from stored photosynthates, while under normal [CO2] primary photosynthates were preferentially consumed. An increase in [CO2] from 320 to 2300 micromol mol(-1) brought about a fourfold decrease in the rate of photorespiration with a concomitant 50% increase in the rate of respiration in the light. Respiration in the dark did not depend on [CO2] up to 30 mmol mol(-1). A positive correlation was found between the rate of respiration in the dark and the rate of photosynthesis during the preceding light period. The respiratory decarboxylation of stored photosynthates was suppressed by light. The extent of light inhibition decreased with increasing [CO2]; no inhibition was detected at 30 mmol mol(-1) CO2.

Reassimilation of carbon dioxide by Flaveria (Asteraceae) species representing different types of photosynthesis.

The capability to reassimilate CO2 originating from intracellular decarboxylating processes connected with the photorespiratory glycolate pathway and-or decarboxylation of C4 acids during C4 photosynthesis has been investigated with four species of the genus Flaveria (Asteraceae). The C3-C4 intermediate species F. pubescens and F. anomala reassimilated CO2 much more efficiently than the C3 species F. cronquistii and, with respect to this feature, behaved similarly to the C4 species F. trinervia. Therefore, under atmospheric conditions the intermediate species photorespired with rates only between 10-20% of that measured with F. cronquistii. At low oxygen concentrations (1,5%) the reassimilation potential of F. anomala approached that of F. trinervia and was distinct from that found with F. pubescens. The data are discussed with respect to a possible sequence of events during evolution of C4 photosynthesis. If compared with related data for C3-C4 intermediate species from other genera they support the hypothesis that, during evolution of C4 photosynthesis, an efficient capacity for CO2 reassimilation evolved prior to a CO2-concentrating mechanism.