The C-4 pathway in Pennisetum purpureum : III. Structure and photosynthesis

The anatomical structure of leaf tissue of P. purpureum, and the short term labelling pattern following exposure to (14)CO2 in the lighht, have been investigated. Both the arrangement of photosynthetic tissue in two layers around the vascular tissue, and the early labelling of malate and aspartate, characteristic of C-4 plants were observed. The structure of the epidermis and the arrangement of stomata is such that CO2 must pass through non-chloroplast-containing tissue before reaching the chloroplasts. At 0.05% CO2 in air the rate of photosynthetic CO2 assimilation was about 70 μmoles/mg chl·h. This increased to over 700 μmoles/mg chl·h at saturating concentrations of CO2. At 0.05% CO2 negative slopes were obtained from percentage plots for malate, which was the major product. As the CO2 concentration was increased, sugar phosphates became the major product. At saturating concentrations of CO2, both malate and aspartate had positive initial slopes and a negative slope was observed for phosphoglyceric acid. These results are discussed in relation to the contribution of C-4 metabolism to photosynthesis in P. purpureum.