Plant Physiology - M.M. Musiienko 2001

Root Nutrition of Plants
Uptake and Transport of Mineral Elements

Mineral elements are absorbed by the plant Organism along with Water through The ROOT System (Fig. 118).

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Fig. 118. Proposed model of ion transport in plant roots: 1 — total ion uptake; 2 — ion efflux from the root; 3 — symplastic transport; 4 — apoplastic transport; 5 — carrier-mediated transport. V — vacuole; ER — Endoplasmic reticulum, CS — Casparian strips. a — exodermis, b — endodermis, c — xylem parenchyma, d — vessels

They can also enter in small quantities through the leaves; therefore, foliar feeding of plants, especially with micronutrients, has become a common agricultural practice. The apoplastic pathway in the root Functions similarly to that in the leaf.

The protoplast of a plant Cell is surrounded by a fairly strong, dense Cellulose envelope — The Cell wall. It is this wall that serves as the first barrier to The transport of nutrients absorbed by the root system from the soil. The native cell wall consists of two layers: the Primary and secondary walls. The primary wall consists of a loose meshwork of cellulose fibers — microfibrils, which are loosely cemented by an amorphous matrix. This spongy Structure is penetrated by a series of rather large channels, the average radius of which (5-20 nm) is dozens of times larger than the radius of mineral salt ions (0.4-0.6 nm). Consequently, nutrient salts can freely move through these channels.

However, this free transport is hindered by the following circumstances: the matrix that cements the fibrils of the primary wall together is mainly composed of pectin substances — polymers of uronic acids. They have a carboxyl group -COOH, which dissociates in aqueous solution into H⁺ and COO^-, meaning the carboxyl group turns into a COO^- anion, thereby imparting an overall negative charge to the primary cell wall. Therefore, they attract (sorb) positively charged cations, binding them and thus creating an obstacle to further movement. Anions, on the contrary, are repelled due to the negative charge of the primary wall and therefore penetrate such channels with difficulty.

As two adjacent Cells grow, the deposition of the secondary wall gradually occurs on both sides. It consists of the same microfibrils, but they are arranged in more ordered layers that are densely impregnated with matrix, and sometimes with Lignin (lignin is a complex mixture of polyphenols that carry no electrical charges and therefore do not sorb ions). This explains the absence of channels for ions in the dense mass of the secondary wall.

Therefore, there is no doubt that the primary part of The Cell wall is more permeable to substances, and it is this part that serves as the main pathway for transport through the apoplast — the intercellular free space of root and other plant Tissues.

Passive transport. The MAIN MECHANISMS OF ion movement through the cell wall are diffusion and, to a lesser extent, bulk flow. The driving force of diffusion is the solute concentration gradient. Bulk flow is the movement of a solute along with the solvent. The driving force of bulk flow is the hydrostatic pressure gradient.

Diffusion of salts through the cell wall always occurs: indeed, the protoplast on the inner side of the surface constantly absorbs ions, and this continuously maintains their concentration gradient.

Bulk flow (the direct percolation of the soil solution through the wall) occurs due to Transpiration. The pores of the cell wall offer significant resistance to bulk flow because in young roots they are filled with gel, As a result of which its contribution to the total pool of absorbed salts plays a role only during intensive transpiration.

The Role of adsorption in the uptake of mineral salts. Plant cells are characterized by cation-exchange and, to a lesser extent, anion-exchange capacity (Fig. 119). They adsorb on

Fig. 119. Contact Ion Exchange between root cells and soil particles

their surface positively or negatively charged ions of mineral salts, which can be displaced through exchange by other ions of the same charge sign. It has been proven that 90-95% of exchange adsorption of ions occurs on cell walls. Most of the soil cations (Ca²⁺, K⁺, NH₄⁺ Mg²⁺) are not in the free soil solution but are adsorbed On the surface of soil particles (the so-called soil absorbing complex). That is why, to absorb such ions, the root must first detach (desorb) them from The surface of soil particles and bind (sorb) them on its own surface. As S.P. Kostychev explained, the plant opposes the soil absorbing complex with its own absorbing complex. D.A. Sabinin did a lot to explain these processes.

However, the entire diversity of transport processes cannot be reduced to alternating acts of ion adsorption. Indeed, There are two Components of the overall process of ion uptake: those dependent on adsorption and those independent of it.