Size exclusion limits and lateral heterogeneity of the stomatal foliar uptake pathway for aqueous solutes and water-suspended nanoparticles

Penetration rates of foliar-applied polar solutes are highly variable and the underlying mechanisms are not yet fully understood. The contribution of stomata especially, is still a matter of debate. Thus, the size exclusion limits of the stomatal foliar uptake pathway, its variability and its transport capacity have been investigated. The size exclusion limits were analyzed by studying the penetration of water-suspended hydrophilic particles of two different sizes (43 nm or 1.1 mu m diameter) into leaves of Vicia faba (L.). To avoid agglutination of the particles, plants were kept in water-saturated atmosphere. Penetration of the larger particles was never detected, whereas after 2 to 9 days, the smaller particles occasionally penetrated the leaf interior through stomatal pores. Permeability of stomata to Na-2-fluorescein along the leaf blade of Allium porrum (L.) was highly variable and not correlated with the position on the leaf. When evaporated residues of the foliar-applied solutions were rewetted repeatedly, approximately 60% of the previously penetrated stomata were penetrated again. The average rate constant of penetration of an individual stoma was in the same order of magnitude as typical rate constants reported for the cuticular pathway. The observed sparseness of stomatal penetration together with its high lateral variability but local and temporal persistency was taken as evidence that stomata contributing to uptake differ from non-penetrated ones in the wettability of their guard cell cuticle. These results show that the stomatal pathway is highly capacitive because of its large size exclusion limit above 10 nm and its high transport velocity, but at the same time the high variability renders this pathway largely unpredictable.