Photosynthetic Carbon Metabolism in Seagrasses 14 C-Labeling Evidence for the C 3 Pathway

Abstract

The 813C values of several seagrasses were considerably less negative than those of terrestrial C3 plants and tended toward those of terrestrial C4 plants. However, for Tbalassia hemprichil (Ehrenb.) Aschers and Hal-ophils spinulosa (R. Br.) Aschers, phosphoglycerate and other C3 cycle intermediates predominated among the early labeled products of photosyn-thesis in 14C-labeled seawater (more than 90% at the earliest times) and the labeling pattern at longer times was brought about by the operation of the C3 pathway. Malate and aspartate together accounted for only a minor fraction of the total fixed label at all times and the kinetic data of this labeling were not at all consistent with these compounds being early intermediates in seagrass photosynthesis. Pulse-chase 14C-labeling studies further substantiated these conclusions. Significant labeling of photores-piratory intermediates was observed in all experiments. The kinetics of total fixation of label during some steady-state and pulse-chase experi-ments suggested that there may be an intermediate pool of inorganic carbon of variable size closely associated with the leaves, either externally or internayll. Such a pool may be one cause for the C4-like carbon isotope