Photosynthetic Carbon Metabolism of a Marine Grass

Abstract

The 813C value of a tropical marine grass Thalassia testudinum is -9.04%.. This value is similar to the 813C value of terrestrial tropical grasses. The 813C values of the organic acid fraction, the amino acid fraction, the sugar fraction, malic acid, and glucose are: -11.2%., -13.1%o, -10.1%o, -11.1%., and -11.5%., respectively. The 813C values of malic acid and glucose of Thalassia are similar to the 813C 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%o during the day. If CO2 fixation in Thalassia is catalyzed by phosphoenolpyruvate carboxylase (which would result in a -3%o fractionation between HCO3- and malic acid), the predicted 813C value for Thalassia would be -9.7 to -5.7%o. 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 fiation in C4 terrestrial plants. Electron microscopy of the leaves of Thalassia reveal thick waled epidermal cels exceedingly rich in mitochondrin and C3-type chloroplasts. The mesophyil cells have many different shapes and surround air lacunae which contain 02 and CO2. The mesophyHl cells are highly vacuolated and the parietal cytoplasm contains an occasional chloroplast. This chloroplast contains grans but the lameUlar 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 grss involves epidermal and mesophyl cells and internally produced 02 and CO2 in the air lacunae.