Advanced radiogasometric method for the determination of the rates of photorespiratory and respiratory decarboxylations of primary and stored photosynthates under steady-state photosynthesis

An advanced radiogasometric method for the study of plant leaf CO₂ exchange is presented. The method enables determination of the rates of CO₂ fixation, photorespiration and respiration in the light under steady‐state photosynthesis and discrimination between primary and stored photosynthates as substrates of photorespiratory and respiratory decarboxylations. The method is based on the analysis of the time curves of ¹⁴CO₂ evolution from labeled primary and stored photosynthates in leaves previously exposed to ¹⁴CO₂. The molar rates of different decarboxylation reactions are calculated from the initial slopes of the curves taking into account the specific radioactivity of CO₂ fed to leaves and/or evolved from leaves. To estimate the contribution of primary and stored photosynthates, the measurements of ¹⁴CO₂ evolution are performed after feeding plant leaves for different periods with ¹⁴CO₂. Photorespiration and respiration are distinguished on the basis of data obtained from measurements of ¹⁴CO₂ evolution under normal (210 ml l⁻¹) and low (15 ml l⁻¹) concentrations of oxygen. A principally new method for the determination of the rate of intracellular refixation of respiratory CO₂ has been developed. The method is based on the measurements of ¹⁴CO₂ evolution from leaves into the medium of very high concentrations (30 ml l⁻¹) of ¹²CO₂, where the probability of refixation of ¹⁴CO₂ evolved inside the cell is close to zero. The results obtained were comparable with the data derived from parallel refixation measurements by means of gasometric methods. As an example of application, the data on CO₂ exchange in leaves of two contrasting groups of C₃‐species, differing in the ability of starch accumulation, are presented.