Water Pathways in Wheat Leaves. IV. The Interpretation of Images of a Fluorescent Apoplastic Tracer

Abstract

Sections of wheat leaves fed with the fluorescent apoplastic tracer sulforhodamine G (SR) through the xylem were prepared by freeze-substitution and resin embedding. The distribution of fluorescence intensity (FI) of the tracer was measured by microspectrofluorometry at a resolution of 0.4 µm. SR was found to move within cell walls in restricted paths less than 200 nm wide. The name 'nanopaths' is suggested for these. The highest FI was found around the mestome-sheath / parenchyma-sheath border on the xylem side, and was shown to be due, not to binding of the tracer to wall components, but to the generation of a very high concentration of SR there by the separation of water from the solute. This separation cannot be evaporative but must be osmotic, and is presented as evidence of a major symplastic water movement starting at the parenchyma sheath cell membrane. The main resistance to water loss from the veins is at the mestome sheath and appears to be controlled by the suberised lamellae.