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.
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.
The difference between these two processes represents the sum of absorption of mineral substances.
If the concentration of ions in external environment is too low, ion release and absorption are similar.
Leaves release minerals through special organs called hydathode and salt glands. Rainfall washes significant amounts of organic and mineral matter from the aerial parts of the plant.
Salt secretion through salt glands is considered an adaptive strategy to regulate plant tissue ion concentration. It is considered to be an important mechanism which contributes to the resistance of plants to increased salinity levels.
Response to foliar fertilizing
The effect of crop phenology on response to foliar fertilization is related to both physical and biological effects.
Physical effects include changes in leaf structure and composition that may alter the penetration and subsequent utilization of foliar nutrients; and changes in canopy size and architecture which directly influence the surface area available to intercept the foliar spray.
Unfavourable climate such as cool, wet springs, before the onset of warm conditions, can limit nutrient uptake from soils.
This condition is described as spring fever and it causes deficiencies of the immobile elements, B, Cu and Zn which are critical for bud break, pollen tube development, flowering and vegetative expansion.
Therefore, foliar application of above mentioned nutrients is crucial in such years.
See this article in Tree Fruit June 2017
