Equivalent pore radii of hydrophilic foliar uptake routes in stomatous and astomatous leaf surfaces – further evidence for a stomatal pathway

Foliar uptake pathways for hydrophilic solutes were studied by the analysis of co-uptake of 15N-labelled urea, NH4+ or NO3- and 13C-labelled sucrose across leaf surfaces of various plant species. Uptake of N (y) and sucrose (x) were strongly correlated. Curvilinear regression revealed significantly positive intercepts with the y-axis indicating the involvement of a sucrose-excluding pathway consisting of small pores with radii <0.5 nm. Depending on plant species, N source, leaf side and aperture of stomata, these small pores accounted for 6-62% of total N uptake. Regression analysis revealed that in stomatous leaf surfaces of Vicia faba L., Coffea arabica L. and Prunus cerasus L., the remaining N uptake occurred via another pathway with an estimated average pore radius (r(P)) greater than 20 nm. This is two orders of magnitude greater than previous estimations of cuticular r(P), indicating that this pathway, which was only found in stomatous leaf surfaces, was probably not located in the cuticle but at the surfaces of the stomatal pores. In astomatous leaf surfaces of C. arabica and Populus x canadensis Moench, average r(P) was 2.0 and 2.4 nm, respectively, which is four to eight times larger than previous estimations of cuticular r(P). These results indicate that for polar solutes, the size exclusion limits of plant surfaces can be considerably larger than previously estimated. The far-reaching implications of these findings are discussed.