AgricultureUncategorized

Foliar nutrition: 3 main mechanisms

Although the root is the organ responsible for the uptake of water and minerals from the external environment, the leaves and other organs of the foliage have preserved, throughout plant evolution, the ability to absorb mineral elements in aqueous solution and enhance foliar nutrition.

In fact, the leaves of most plant species are well equipped to absorb water, amino acids, other organic compounds and even whole minerals.

In the course of their evolution, plants have developed multiple survival strategies and the uptake of foliar nutrition is one of these indispensable developed systems.

A good example of the natural process of leaf nutrition can be found in rainforests.

 

In these forests, where large trees cover the entire surface, the plants in the undergrowth receive a very low share of filtering light radiation to carry out photosynthesis. As a result, they are only able to produce a limited amount of sugars for their growth.

Most of them, however, have developed a secondary absorption system.

They succeed in such a harsh environment because they are able to actively absorb amino acids, minerals and even organic humus fractions through their leaves.

These organic molecules come from the upper layers of the forest that cover the undergrowth, where fine (organic) dust particles, dead insects, bird droppings, algae and mosses collect.

 

3 MAIN MECHANISMS      

There are basically three modes of foliar uptake of water and nutrients (minerals, organic molecules, amino acids, etc.):

  • through the glandular hairs (trichomes),),
  • through the wax layers on the leaf (cuticle))
  • by absorption and release through cells (endocytosis)..

Stomata also lend themselves to the uptake of water and mineral salts but, since most of them are concentrated on the underside of the leaf, it is a clear sign that they are not the organs properly dedicated to the uptake of water and nutrients.

 

ABSORPTION THROUGH GLANDULAR HAIRS

  • Trichomes (glandular hairs) are unicellular outgrowths of the epidermis. Minerals in cationic form and amino acids are generally well absorbed through these organs.

       

ABSORPTION THROUGH THE CUTICLE

  • The cuticle is a hydrophobic layer composed of waxes and cutin deposited exclusively on the outside of the cells of the epidermis with a protective function.

The cuticle, due to its greater droplet interception capacity and large contact surface area, is the predominant pathway in the leaf absorption process.

Solutes diffuse by osmosis through the cell layers of the leaf by exploiting a concentration gradient that is established between the high concentration of solution applied to the leaf surface and the low concentration of solute in the cells.

LEAF ANATOMY

Anatomy of the leaf

 

  • High humidity favours the penetration of ionic nutrients as it reduces the hydrophobic properties of the cuticle and increases the diameter of the pores, delaying the dehydration of the sprayed droplets.

 

  • The Light radiation has a positive effect on absorption, promoting flock opening, photosynthetic activity and cuticle permeability in the vicinity of guard cells.

 

  • Also Temperature has an indirect effect on uptake, but it is linked to an increase in the fluidity of the cuticle waxes, which consequently improves the movement of apolar solutes and thus the uptake of the nutrient solution.

 

ABSORPTION BY ENDOCYTOSIS

  • With endocytosis, minerals lying on the leaf are taken up by epidermal cells and transported to so-called storage cells (Vacuoles), which absorb and store the minerals for later use.

The following figure shows the first part of this process.

Anatomy of the leaf 

The principle actually works in the same way as medicinal patches on human skin, slowly releasing the drugs to the body through the skin.

 

Final considerations

Not all crops are equally receptive to foliar nutrition: there is a foliar cation exchange capacity',which differs from species to species.

It is clear that foliar feeding cannot fully replace root feeding, but it can be a very valuable complement to it in order to maximise the production potential of crops.

 

  • Theaddition of specific corroborants and adjuvants foliar fertiliser nutrition improves wetting of the leaf surface as it reduces the surface tension of the droplets and increases the contact angle with the wetted surface.

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