Photosynthetic Carbon Metabolism in Seagrasses 14C-Labeling Evidence for the C3 Pathway

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Abstract
The .delta.13C values of several seagrasses were considerably less negative than those of terrestrial C3 plants and tended toward those of terrestrial C4 plants. However, for Thalassia hemprichii (Ehrenb.) Aschers and Halophila spinulosa (R. Br.) Aschers, phosphoglycerate and other C3 cycle intermediates predominated among the early labeled products of photosynthesis in 14C-labeled seawater (more than 90% at the earliest times) and the labeling pattern at longer times was brought about by the operation and 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 photorespiratory intermediates was observed in all experiments. The kinetics of total fixation of label during some steady-state and pulse-chase experiments suggested that there may be an intermediate pool of inorganic carbon of variable size closely associated with the leaves, either externally or internally. Such a pool may be 1 cause for the C4-like carbon isotope ratios of seagrasses.