The device used by Schonherr (2006) to measure foliar transport of chemicals applied to the surface of a cuticular membrane.

What are the merits of foliar fertilization in turf-management programs? Foliar fertilization, not to be confused with liquid applications or fertigation, has been proposed as an efficient and environmentally safe method of supplying nutrient elements to turf, especially intensively managed turf (Middleton, 2001; Liu et al, 2008; Totten et al., 2008). Foliar fertilization specifically targets turfgrass leaves as the site for nutrient application and absorption, while other liquid methods deliver substantial nutrient to the thatch and soil ultimately available for root absorption.

While there are ample opportunities for nutrient loss from all liquid and granular delivery methods, there seems to be a growing sense that foliar fertilization losses can be minimized if appropriate application methods and materials are used (Middleton, 2001). This may be true, but it depends entirely upon how effectively foliar-applied materials can cross the cuticle of leaf epidermal cells and become assimilated within the living protoplasts of those cells.

Foliar-applied materials are sometimes thought to be at least partially absorbed into leaves through the open stomate pores (stomata) but there is really little direct evidence to support that notion. True, the underside of leaves, where most stomata normally are present, absorbs more foliar-applied nutrients than the astomatous (lacking stomata) upper surface, but absorption rates are greatest in darkness when stomata are closed. Also, the guard cells bordering stomatal pores and the outer portion of the substomatal chambers are both covered by a waxy cuticle making wetting and penetration of aqueous solutions highly unlikely. Adding surfactants (wetting agents) to a spray solution increases stomatal penetration but only involving a small percentage of stomata. Due to the limitation of space, this review will focus only on the primary barrier to nutrient uptake by leaves: the surface cuticle.

Diagram of a leaf epidermal segment showing the sites of greatest concentration and activity of aqueous cuticular pores.

A wax-coated cuticle covers the outer cell walls of upper- and lower-leaf epidermal cells. The primary function of this cuticle is to minimize the evaporative loss of water from the leaf. Leaves are designed for gaseous exchange (carbon dioxide, oxygen and water vapor) between a leaf's interior and the atmosphere to be controlled by the opening and closing of stomata, not by movement across the cuticle. The cuticle's multilayered composition suits it well to function as a hydraulic barrier (Riederer and Muller, 2006).

The outer-most layer consists of epicuticular wax composed of straight-chain alkanes, long-chain (25-35 carbon atoms) fatty acids and alcohols and esters formed by them. Just beneath the surface wax is a lamellate-structured cuticle proper composed of wax and cutin (polymerized hydroxy fatty acids). Still deeper is a thicker cuticular layer composed of wax, cutin, suberin and long cellulose (glucose polymers) microfibrils that are carbohydrate and have a high affinity for water. Finally, the cell wall proper is reached composed almost entirely of highly hydrated carbohydrate polymers (cellulose and hemicelluloses) and glycoproteins. This carbohydrate cell wall is bonded to the cuticular layer by a pectin (polymers of galacturonic acid and sugars) layer. Through calcium linkages and borate esters, pectin binds cell wall carbohydrates with cellulose in the cuticular layer. The protoplasts of epidermal cells abut the cell walls at their plasma membranes, the final barrier that must be crossed before anything applied to a leaf surface can be useful to the plant.