Apoplastic water and solute movement: new rules for an old space
Annual Review of Plant Physiology, (1995)Abstract
The history of the apoplast, both the word and the concept, is traced from its invention by Mtinch (1930), through its adoption by the translocation physiologists as a gateway by which to feed pesticides, and the independent evolution of the free-space concept by ion-uptake physiologists. Both usages helped spread the idea that the cell walls were freely permeable to flowing solutions. Recent work has produced six contradictions to the prevailing notions of the rules that operate in the apoplast: (a) The flow of the apoplast leaky, and the balance of the flow and leaks depends on vessel diameter. (b) Water leaves the vessels and enters the symplast faster than do some solutes, resulting in the accumulation of high concentrations of solutes at places called sumps. (c) Solutes diffuse away from sumps in the cell-wall apoplast at rates much slower than diffusion in water. (d) Ion concentrations in leaf vessels may be as high as 200 mM. (e) The intercellular-space apoplast of roots often contains solution with high concentrations of ions. (f) The threshold of cavitation of the flow-apoplast appears to be in the range of i to 2 bar below atmospheric, much less than the tensions required by the Cohesion Theory.