Leaves are the major site of food production for the plant.
Structures within a leaf convert the energy in sunlight into chemical energy that the plant can use as food.
Besides that, leaves and some other plant organs have developed the ability to absorb water, minerals, organic matter and gasses.
There is no essential difference in the mechanism of uptake through the leaves and roots.
In both organs, nutrient uptake is based on the diffusion, ion exchange and active transport through the membrane. The only difference between leaves and roots in regards to nutrient uptake, is that the leaves are covered by a cuticle.
Cuticle
Cuticle, a lipophilic layer synthesized by epidermal cells and composed of a cutin polymer matrix and waxes, is a protecting film covering the epidermis of leaves, young shoots and other aerial plant organs.
Intermolecular spaces inside the cuticle are large enough for the transport of ions and molecules. Substances that have passed through the cuticle encounter the next obstacle, the cellular wall.
The cellular wall, which is predominantly composed of cellulose and hemicellulose, has hydrophilic properties and a large cation exchange capacity.
Passing dissolved substances through the cell wall and their transport to the interior of the leaf is associated with a hydrophilic microchannels.
Ions and molecules can pass through the cuticle and cell wall if there is a concentration gradient between the outside and inside of the cuticle and the inside of the cell walls, whereby the mechanism of movement of particles is based on diffusion.
Plants can also uptake nutrients and other elements in the gaseous state (SO2, NO2, NH3 and H2S) via stomatal apparatus.
Nutrient uptake through the leaves
The possibility of absorption of matter through leaves has found practical application in the form of fertilization (foliar nutrition) and application of pesticides.
Nutrient uptake through the leaves is affected by many internal and external factors.
Mobility of nutrients absorbed by the leaves has a great practical importance. In the case of foliar application of medium or low mobility ions in phloem, treatments should be repeated during the growing season—since this is the only way to provide good nutrition to all plant parts.
Foliar fertilization provides fast action of applied minerals or other substances. Their exploitation is much higher than in the case of soil application.
This kind of fertilization can be applied throughout the growing season and is considered an important supplemental measure in plant nutrition.
Plants also release mineral matter
Plants can release the mineral matter in the external environment in the form of ions, molecules or gaseous form. Absorption and release of the matter occur simultaneously.
(continued next month)
See this article in Tree Fruit May 2017
