Structural and functional features of the leaves of Ranunculus trichophyllus Chaix., a freshwater submerged macrophophyte

The structural and physiological strategies developed by the leaves of the freshwater macrophyte Ranunculus trichophyllus to adapt to submersed life w ere studied. Photosynthesis is carried out mainly by the epidermis cells of the numerous segments into which the leaf is finely dissected. In these ce lls, containing most of the chloroplasts, a peculiar organization of the wa ll has been identified by cytochemical tests. A thin compact outer region c overs the cell surface and splits up forming large lacunae between adjacent cells. Below it, a thick and loose inner region rich in hydrophilic pectic acids occurs, which grows in along the cell sides giving rise to wide tran sfer areas. In this latter cell wall region, in which the cell/environment contact and exchanges are amplified, the systems for inorganic carbon suppl y to photosynthetic cells operate. The leaves of R. trichophyllus can rely on environmental CO2 and HCO3- as sources of inorganic carbon for photosynt hesis. A mechanism for bicarbonate utilization seems to involve its convers ion to CO2 by an apoplastic carbonic anhydrase, whose activity gains import ance as the availability of environmental CO2 decreases. Interestingly, it has been demonstrated that in this species CO2 can also be obtained from HC O3- by a photodependent increase in plasmamembrane H+-ATPase activity in th e transfer areas of the epidermis cells. This is the first time that such a mechanism has been noted in a nonpolar leaf of a submerged macrophyte.