Terrestrial rhizophytes and H + currents circulating over at least a millimetre: an obligate relationship?

summary Circulating ionic currents are apparently ubiquitous in growing and differentiating eukaryotes. In many the current is carried by H + , in which case the medium around the apex of growing organs is alkalinized while the medium adjacent to subapical portions is acidified. These two zones may help in, respectively, molybdenum acquisition and reduction of aluminium toxicity, and phosphorus and iron acquisition, by rhizophytic plants. An analysis of the taxonomic distribution of H + ‐borne circulating current shows that all rhizophytes (terrestrial, freshwater and, probably, marine) except those in the green algal class Ulvophyceae have such currents; the Ulvophyceae have circulating currents carried by Cl − . The Ulvophyceae do not seem to have any alternative means of causing pH zonation in the rhizosphere. Their marine habitat, with the possibility of molybdenum acquisition from the alkaline bulk water phase, may mean that the Ulvophyceae are not disadvantaged by the absence of rhizosphere acid‐alkaline zonation as would be a land plant; similarly, they are less likely to suffer aluminium toxicity to the growing zone of rhizoids. Ulvophyceae do not occur as terrestrial plants; this may relate not only to the absence of rhizosphere pH zonation in these organisms but also to mechanical problems related to their coenocytic habit, which in turn results from their inability to produce plasmodesmata.