Mechanisms of long-distance transport of mineral and organic nutrients in plants

Mechanisms of long-distance transport of substances in a plant

Long-distance transport is the movement of substances between plant organs carried out by a specialized conducting system, which includes xylem vessels and tracheids (ascending flow) and phloem sieve tubes (descending flow).

Water, ions, and a number of organic substances synthesized in the roots enter the above-ground parts of plants via xylem vessels. The solution in the vessels is called xylem sap. Xylem sap has a pH<7. Its value can vary significantly, which is determined by species specificity and changes in the ionic composition and specific selective uptake of ions into the xylem and/or release of ions from it into parenchyma cells.

The composition of xylem sap (qualitative, but especially quantitative) varies and depends on the plant species and age, the status of mineral elements in tissues, the composition and concentration of ions in the medium, their circulation in the plant, and the time of day. As a rule, there is a positive correlation between the ion concentration in the external solution and in the xylem sap. However, the degree of concentration varies depending on the ion and its concentration in the medium.

The amount of mineral substances entering the above-ground organs from the roots depends not only on their concentration in the xylem sap but also on the flow rate of the solution. The water potential gradient can make a significant contribution to the translocation of ions from the roots to the above-ground organs.

Mineral substances absorbed from the soil and the products of their primary assimilation in the roots move along both the vessels and the sieve tubes. Such duplication increases the reliability of the transport system. The ratio of one pathway to the other depends on a number of conditions: the plant's salt status, its water supply, and the concentration of the nutrient solution.

Long-distance transport of organic nutrients in the downward direction is carried out mainly via the phloem.

Comparative characteristics of xylem and phloem flows

The main differences between the flows of substances transported through phloem and xylem vessels can be summarized as follows:

• as a rule, sucrose and other sugars are not transported via the xylem (with the exception of winter and spring transport in trees or in corn after the emergence of silks);
• nitrate and sulfate ions are not transported via the phloem;
• the concentration of phosphorus, organic nitrogen (amine and amide), and many other ions in phloem sap is higher than in xylem sap (calcium is an exception);
• phloem sap has a slightly alkaline reaction, and its pH level fluctuates insignificantly, whereas xylem sap is acidic;
• the velocity of the phloem flow is significantly lower than that of the xylem.

The circulation of mineral substances in the whole plant system ensures the remobilization and rational redistribution of mineral elements during the plant's ontogenesis, especially during the formation of reproductive organs, as well as during periods of exposure to adverse factors, which determines the possibilities for further growth and development (Alekhina N.D., Kharitonashvili E.V., 2005).

Thus, there are three types of transport of absorbed salts and their transformation products in the root: intracellular, short-distance (radial), and long-distance. Movement of mineral nutrition elements throughout the plant is possible in all directions: upward (ascending flow), downward (descending flow), and across (radial transport). Movement within an organ occurs via the apoplast and symplast.