124 MINERAL SALTS ABSORPTION IN PLANTS 



in the xylem. Within rather wide Hmits, the rate at which sahs are 

 deHvered to the leaves is determined by the rate at which they are 

 supplied to the conducting elements in the roots. At slower rates 

 of flow, the concentration of xylem sap is increased (Table 12, 

 p. 121) so that the total quantity of salt transported remains approx- 

 imately the same. One effect of an increasing concentration of salts 

 in the xylem sap is to reduce transport into the stele from the cortex 

 of the root (cf the effect of internal concentration on vacuolar 

 accumulation. Chapter 4, pp. 65-6). In this way transpiration rate exerts 

 an indirect influence on salt absorption into the root (see p. 115). 



Anderssen (1929) found that the total concentration of electro- 

 lytes in xylem sap from the outer annual rings of pear and apricot 

 trees is almost twice that in the inner rings. Since water is also 

 transported most rapidly in young xylem, it is likely that in trees 

 most of the mineral salts are carried through the most recently 

 formed wood. The amounts of salt transported also vary with 

 season. Bollard (1953) found that the amount of nitrogen, 

 phosphorus, potassium and magnesium in the tracheal sap of apple 

 trees in New Zealand is low during the winter. This is presumably 

 the result of low rates of absorption of salts by the roots, since the 

 movement of water is also slow. The concentration of the xylem sap 

 increases rapidly during spring, reaching a maximum several weeks 

 after flowering, and then gradually declines again until the low 

 winter level is attained (Fig. 41). 



In addition to inorganic salts, xylem sap contains a variety of 

 organic substances, including nitrogenous and phosphorus com- 

 pounds. Bollard (1957) demonstrated the presence of a variety of 

 organic nitrogen compounds, including amino acids and amides in 

 the xylem sap of a wide range of plants, and the possibility must be 

 considered that an appreciable part of the nitrogen transported into 

 the shoot is carried in organic forms produced in the roots following 

 reduction of nitrate. Phosphoryl-choline and glyceryl phosphoryl- 

 choline are among the substances detected in sap exuding from 

 excised barley roots, and it has been claimed that about 20 per cent 

 of the phosphorus transported in the xylem occurs in the form of 

 these substances (Tolbert and Wiebe, 1955). Sulphur seems to 

 move upwards in the stem entirely as the sulphate (Thomas et al. 

 1944), and there is no doubt that the bulk of the metallic cations 

 are also transported as free ions. 



