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Photosynthetic Carbon Metabolism of a Marine Grass ¹

^a Department of Plant Sciences Texas A & M University, College Station, Texas 77843

The ¹³C value of a tropical marine grass Thalassia testudinum is –9.04. This value is similar to the ¹³C value of terrestrial tropical grasses. The ¹³C values of the organic acid fraction, the amino acid fraction, the sugar fraction, malic acid, and glucose are: –11.2, –13.1, –10.1, –11.1, and –11.5, respectively. The ¹³C values of malic acid and glucose of Thalassia are similar to the ¹³C values of these intermediates in sorghum leaves and attest to the presence of the photosynthetic C₄-dicarboxylic acid pathway in this marine grass. The inorganic HCO₃^– for the growth of the grass fluctuates between –6.7 to –2.7 during the day. If CO₂ fixation in Thalassia is catalyzed by phosphoenolpyruvate carboxylase (which would result in a –3 fractionation between HCO₃^– and malic acid), the predicted ¹³C value for Thalassia would be –9.7 to –5.7. This range is close to the observed range of –12.6 to –7.8 for Thalassia and agree with the operation of the C₄-dicarboxylic acid pathway in this plant. The early products of the fixation of HCO₃^– in the leaf sections are malic acid and aspartic acid which are similar to the early products of CO₂ fixation in C₄ terrestrial plants.

Electron microscopy of the leaves of Thalassia reveal thick walled epidermal cells exceedingly rich in mitochondria and C₃-type chloroplasts. The mesophyll cells have many different shapes and surround air lacunae which contain O₂ and CO₂. The mesophyll cells are highly vacuolated and the parietal cytoplasm contains an occasional chloroplast. This chloroplast contains grana but the lamellar system is not as developed as the system in epidermal chloroplasts. Extensive phloem tissue is present but the xylem elements are reduced in this aquatic grass. The vascular tissue is not surrounded by bundle sheath cells.

This work does not establish the exact relation between structure and function in Thalassia, but it does show the C₄-type photosynthetic carbon metabolism in this grass involves epidermal and mesophyll cells and internally produced O₂ and CO₂ in the air lacunae.