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

¹ Australian Institute of Marine Science, PMB No. 3, M.S.O., Townsville, Queensland 4810, Australia

The 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 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 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 one cause for the C₄-like carbon isotope ratios of seagrasses.