Foliar penetration by chemicals

Abstract

The unperforated cuticle, breaks in the cuticle, aild openings in the epidermis, such as stomata and hydathodes, are possible sites of entry for foliar-applied chemicals. The cuticle is a barrier to pene-tration, but it is not impenetrable. Cuticular pene-tration has been amply demonstrated by application of chemicals to the stomata-free surface of hypo-stomatous leaves (3. 14, 19) and by applications in the dark when the stomata were closed (10, 16, 20). Yet the exact nature of cuticular entry is not known. Some suggested preferential pathways include epi-dermal cells above veins (18), hairs (7), anticlinal walls of epidermal cells (14), guard cell walls (21), and tears or punctures (13). The problem of stomatal penetration has received much attention in the past, and stomatal entry by oils is well known (5). However, evidence for en-try by aqueous solutions is conflicting. Skoss (15) and others (3, 8, 9) have reported that aqueous solu-tions rapidly enter open stomata. On the other hand, many workers have concluded that there is no entry via this route (2, 16, 19, 20). The intermediate posi-tionl is that of van Overbeek (18) and others (4,12, 17) who state that entry may occur if an efficient surfactant is employed. The consistentlv reported failure of surfactants to enhance uptake of inorganic nutrients (1, 11. 16) contradicts this hypothesis.