Dark Release of 14CO2 from Higher Plant Leaves

Abstract

A study was conducted with 48 species of the amount of ¹⁴CO₂ released during the first minute of dark following fixation of ¹⁴CO₂ in the light. Light fixation periods varied from 5 to 60 seconds. The species examined included both monocots and dicots and represented C₄, C₃, and Crassulacean acid metabolism (CAM) photosynthetic types. Total CO₂ uptake was determined as the sum of soluble ¹⁴C (ethanol and water), insoluble ¹⁴C, and ¹⁴CO₂ released during the first minute of dark. Insoluble ¹⁴C made up a very small percent of the total. The percent of soluble ¹⁴C increased with fixation time in C₄ species, while the percent of ¹⁴CO₂ released in the dark decreased. C₃ species released a very small percent of ¹⁴CO₂ in the dark, and this percent remained relatively constant with time of ¹⁴CO₂ fixation whereas C₄ species released a higher percent initially and this percent declined with time of fixation. CAM species were similar to C₄ species. The species are grouped according to the ¹⁴CO₂ released during the first minute of dark with light labeling times of 5 and 60 seconds. These comparisons are shown to separate C₃ from C₄ and CAM species. Further, the data suggest that the origin of the CO₂ in the dark release of C₄ plants is quite different from that of C₃ plants.