Nature of Metabolites in Sieve Tubes

The phloem sap contains three major classes of organic compounds - organic acids, amino acids and sucrose besides some cations, anions and hormones. Their concentrations are represented in the histogram shown below. Sucrose - the major energy source in plants is the major metabolite transported from leaves to sinks. Rarely unusual sugars such as raffinose or mannitol are transported in some species. of the total solute content in phloem, sucrose content ranges from 50 to 90%. The reason why sucrose is the major transporter of energy is not known.

Figure:  Ionic and organic compounds of phloem sap of Ricinus. In general phloem sap is alkaline .

It is loaded by active transport because of its high concentration in sieve tubes. Amino acids and organic acids, mainly malate, are also substantial components of phloem sap. Among cations, the concentration of K⁺ is much higher. In the xylem sap the concentrations of NO^-₃ and K⁺ are about the same, but in phloem concentration of NO^-₃ is very small. Can you think where the NO^-₃ could disappear? Nitrate is reduced in the leaves and used for the synthesis of amino acids and several other nitrogen containing compounds. Now, can you guess why there is so much K⁺ in the phloem sap? Unlike NO^-₃ K⁺ is not incorporated into organic molecules. Due to transpiration there is a persistent influx of K⁺ in the leaves. Because water evaporates leaving K⁺ behind, its enhanced levels in the shoot require recycling in the xylem.

The presence of excess of ions in root tissues in turn affects the uptake of these mineral nutrients from soil by influencing the ionic relationships of the cells of the root. Here, we would like to emphasise that ions that accumulate in excess in the leaf apoplasm are passed on to the sieve elements via the transfer cells. The presence of K⁺ is important as a counter ion to organic acids and amino acids within the cytoplasm of all cells and it is gradually assimilated in the cells of growing plant tissues.