Photosynthetic Carbon Metabolism of a Marine Grass

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Abstract
The .delta.13C value of a tropical marine grass Thalassia testudinum is -9.04.permill.. This value is similar to the .delta.13C value of terrestrial tropical grasses. The .delta.13C values of the organic acid fraction, the amino acid fraction, the sugar fraction, malic acid and glucose are -11.2.permill., -13.1.permill., -10.1.permill., -11.1.permill. and -11.5.permill., respectively. The .delta.13C values of malic acid and glucose of Thalassia are similar to the .MU.P13C 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 and -2.7.permill. during the day. If CO2 fixation in Thalassia is catalyzed by phosphoenolpyruvate carboxylase (which would result in a -3.permill. fractionation between HCO3- and malic acid), the predicted .delta.13C value for Thalassia would be -9.7 to -5.7.permill.. This range is close to the observed range of -12.6 to -7.8.permill. 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. EM 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 shows the C4-type photosynthetic C metabolism in this grass involves epidermal and mesophyll cells and internally produced O2 and CO2 in the air lacunae.