Plant leaves absorb leaf-applied nutrients mainly through the cuticle, stomata, trichomes; the actual contribution of the stomatal versus the cuticular pathway remains unclear. Substances with low points of deliquescence will favour the penetration of nutrients supplied as foliar sprays. The occurrence of two distinct penetration pathways in the cuticle, for hydrophilic and lipophilic compounds has been suggested. Neutral, non-charged molecules penetrate the cuticle by dissolving and diffusing in lipophilic domains made of cutin and cuticular waxes (lipophilic pathway), while ionic species may penetrate the leaf through aqueous pores (hydrophilic pathway). The response to foliar fertilization increases when tree mineral status is insufficient and if the nutrients are supplied in available forms. The sprayed solution is retained in the target tissues, then it penetrates the plant surface, it enters into the apoplast, and it is transported to the tissues were it is demanded. Foliar application is recommended: 1) to apply micronutrients such as Fe, Mn, Zn, B, etc. (because of their low plant requirement); 2) when soil constraints like high pH, calcium carbonate, moisture, low temperature, clay, etc. limit the availability of soil nutrients; 3) to prevent or overcome transient nutrient deficiencies of nutrients. Factors with positive effects on leaf nutrient absorption are: air relative humidity, that promotes the salt hydration; moderately high temperatures, that increase wax fluidity and non-ionic molecules mobility; photosynthetic radiation, that stimulates stomatal opening and photosynthesis; the use of surfactants, that lower surface tension and contact angles. Young, partially expanded leaves are more penetrable than fully-expanded leaves, potentially because of their higher penetration efficiency through stomata, higher presence of trichomes, thinner cuticle and lower wax deposits. Since stomata are often present in the abaxial leaf side, this surface is normally more efficient in nutrient uptake than the upper one. In the case of iron, a nutrient often supplied through the leaves, deficiency reduces leaf size and surface smoothness, increases leaf transpiration rate and promotes stomata closure with a final reduction of nutrient uptake. The plasma membrane absorption rate is fast at the beginning because of the high concentration gradient that promotes a passive movement across membrane channels or by specific transporters. In the cytoplasm nutrients are metabolized, and only mobile nutrients (N, P, K, B) may move to other plant organs such as reproductive or storage tissues.
Keywords: leaf cuticle, fertilizers, iron nutrition, macronutrients, stomata