Comparative studies on the photosynthesis of higher plants

Abstract

Abstract 1) CO2 compensation points of the plants tested correlate well with the leaf anatomy. Low CO2 compensation plants had well-developed VBS containing large and specialized chloroplasts but no plant with a high CO2 compensation point possessed chloroplasts in the VBS. 2) CO2 Compensation Points Closely Correlated With The Major Carboxylation Pathway In Photosynthesis. Low Compensation Plants Fixed CO2 Via The C-4 Pathway (C-4 Plants) While High Compensation Plants Carried Out CO2 Fixation By The Calvin Cycle (C-3 Plants). 3) Close correlations could be established for the CO2 compensation point, the major carboxylation pathway, and glycolate oxidase activity. Glycolate oxidase activity was much higher in C-3 plants than in C-4 plants. On the other hand, dark respiration in C-4 plants was higher than that in C-3 plants. 4) TCA cycle activity in detached leaves was not inhibited to any large extent by illumination. In C-3 plants, the release of 14CO2 from alanine-1-14C increased with an increase in the ambient O2 concentration; whereas, radioactivity in the sugar fraction was quite small at all O2 concentrations. In C-4 plants the release of 14CO2 was little affected by the ambient O2 concentration while sugar formation was stimulated at high O2 concentrations. This indicates that in C-3 plants CO2 fixation is blocked at a high O2 concentration, therefore, internal 14CO2 is released from the leaf without being refixed, but in C-4 plants internal 14CO2 can be efficiently refixed and metabolized to sugar by a combination of active PEP carboxylase and the ‘Kranz type’ of leaf anatomy.