22 Aug Absorption Mechanism of Foliar Applied Nutrients
The first reports on foliar application of mineral nutrients in plant production date back to the second half of the 18th century (after Weinbaum, 1988). Particularly many studies on the uptake of mineral nutrients and their translocation within a plant were carried out after the Second World War. A combined soil and foliar fertilization program should be recommended in plant production to increase both plant productivity and yield quality. Knowledge of nutrient absorption mechanisms by above ground plant parts is crucial to optimize foliar fertilization. Since leaves have a large surface area in relation to other above ground plant parts, mineral nutrient uptake processes in relation to a structure and leaf physiology. Special attention will be given to factors influencing absorption of mineral nutrients by leaves.
Penetration through the epicuticular wax and the cuticular membrane
The epicuticular wax is the outermost and most hydrophobic component on a leaf surface, consisting of ketones, esters of long chain fatty acids, and long chain alcohols placed parallel to one another. Such structure helps to limit penetration of water molecules and ions across the membrane. Even Brown’s movements occurring in the epicuticular waxes do not facilitate the penetration of solutes. “Intracuticular” waxes within the cuticular membrane are considered as more polar than those epicuticular. The cuticular membrane is under the epicuticular waxes and consists of the cuticle proper, and the cuticular and the pectin layer. The cuticular membrane covers not only the leaf surface, but also the mesophyll cells having contact to air spaces. Especially, the cells under the stomata have well developed cuticular membrane. The hydrophobic cuticle proper that lies under the epicuticular waxes contains mainly cutin built of hydroxy fatty acids.
However, cutin contains many free hydroxyl groups which weaken hydrophobic interactions and facilitate penetration of nutrients through the cuticular membrane. The cuticular layer is located under the cuticle proper and consists of cutin, pectin and hemicelluloses. The two last components have dissociated hydroxyl and carboxyl groups causing polar features of the cuticular layer. The pectin layer, situated beneath the cuticular layer, is chiefly composed of negatively charged galacturonic acids. A gradual increase in negative charge from the epicuticular wax to the pectin layer creates an electrochemical gradient that increases the movement of cations and water molecules. The flow of cations through the cuticular membrane is much easier than that of anions. It is estimated that cation ability to penetrate the cuticular membrane is ca. 1000 times higher than for anions. Mineral nutrients do not enter the epidermal cells through the surface of the epicuticular wax, but through the ectodesmata pores with a diameter of less than 1 nm. These pores are readily permeable to solutes such as urea (radii 0.44 nm), but not larger molecules such as synthetic chelates. Ectodestama are lined with fixed negative charges (presumably from polygalacturonic acids) which increases density from the outside of the cuticle to the inside. Accordingly, the permeation of cations along this gradient is enhanced, whereas anions are repulsed from this region. Therefore, the uptake of cations by the leaves is more rapid than that of anions. Ectodesmata are reported to have the highest density on a leaf along the anticlinal walls of the basal cells of hairs, and the epidermal cells surrounding hairs and guard cells. The number of ectodesmata on the adaxial (upper) leaf surface is usually lower than on the abaxial (lower) surface. It is estimated that the number of ectodesmata per cm² of a leaf surface is approximately 1010.
However, the number of ectodesmata is strongly affected by environmental conditions and the physiological state of the leaves. Stresses such as high air temperatures, intense solar radiation, drought, and pathogenic infections, decrease the number of ectodesmata on a leaf. As a leaf develops, the number of ectodesmata per unit of a surface area decreases. Not only the number of ectodesmata affects the leaf ability to absorb ions but also their permeability. Generally, the movement of low molecularweight solutes (e.g. ions, organic acids, amino acids, sugar) from the leaf surface to the epidermal cell wall is a non-metabolic process driven by diffusion and electrochemical potential formed by a negative charge increase across the cuticular membrane.
Penetration through the cell walls
The cell walls of a leaf are continuous and serve as a pathway for freespace (apoplastic) movement of nutrients. Free space occupies 35% of the volume of leaf tissues. The cell wall is mainly built of cellulose, hemicellulose, and pectin. The two last compounds contain large amounts of galacturonic acids rich in free carboxyl groups. At high pH values (>7) resulting in dissociation of carboxyl groups, the cell walls exhibit negative charges active in cation adsorption. The movement of mineral nutrients through the epidermal cell walls takes place in interfibrillar and intermicellar spaces, as well as in ectodesmata and is driven by diffusion and ionic exchange.
Penetration through the plasma membrane
The plasma membrane is mainly composed of proteins and lipids. It is an effective barrier to solutes of high molecular weight. The plasma membrane is the site of selectivity and transport against the concentration gradient of solutes. Selective transport of nutrients across the plasma membrane requires energy and specific carriers, permeases and channels. Nutrient transport through the plasma membrane may also be a passive process driven by diffusion. This “downhill” transport across a membrane is done with the aid of carriers, and/or aqueous pores and is maintained as a result of lowering the ion activity in the cytoplasm by adsorption of ions at charged groups and/or by their incorporation into organic structures.