Ultrastructural and photosynthetic features of leaves and stems of Elodea canadensis.

Leaves and stems of Elodea canadensis Michx have been characterized morphologically and ultrastructurally. Significant features, such as chloroplast structure and distribution in the different tissues, the transfer cell-like organization of the lower leaf cells, the gas-lacunae of the stem cortical region and the hydrophobic material present in the cell walls, have been described and specially considered in relation to their possible role in fulfilling the photosynthetic demands of the two organs. In order to verify the possibility that the two green organs may adopt different strategies to supply themselves with inorganic C, the ability of leaves and stems to carry out photosynthesis in different environmental conditions, such as different water pH and dissolved inorganic carbon (DIC) availability, has also been studied in this submerged plant. The results show that the leaves in Elodea canadensis are the -truly submerged. organs, well adapted to underwater life. They have ultrastructural features corresponding to an enhancement of the water-cell exchanges and can utilize with high efficiency both the environmental CO2 and HCO-3 as inorganic C sources. The stem, on the contrary, behaves as a «terrestrial organ, whose photosynthetic ability strictly depends on the availability of CO2 entrapped in the wide intercellular spaces of its tissues.