14 MINERAL SALTS ABSORPTION IN PLANTS 



Study of salt uptake, because roots are the major absorbing organs 

 in most angiosperms. The system is less complex than is a whole 

 plant, inasmuch as any influences of the shoot on uptake by the 

 root are removed, and it is somewhat less variable in behaviour. 

 The material is usually prepared by growing plants (barley, Hordeum 

 vulgar e, and wheat, Triticum spp., have often been used) in media 

 containing a low concentration of salts for several days, under 

 uniform conditions of light and temperature. The "low salt" roots 

 so obtained have a high sugar content, and after excision are 

 capable of absorbing salt rapidly for a limited time, in a reproducible 

 manner. 



Various objections can be raised to the claim that excised roots 

 are ideal material for salt absorption studies: 



{a) The shoot may exert an important influence on absorption 

 by roots, e.g. by virtue of transpiration, and this obviously 

 cannot be investigated adequately in excised roots. 



{b) When roots are separated from the rest of the plant in the 

 manner normally employed for salt absorption studies, they 

 rapidly stop growing and are clearly in an abnormal 

 physiological state which may well affect the uptake process. 

 Techniques are now available (White, 1954) for culturing 

 excised roots in nutrient media in such a way that they 

 continue to grow, more or less normally, but these have not 

 yet been applied extensively to investigations of salt uptake, 



(c) Excised roots are not much less complex structurally than is 

 an entire plant, and results obtained from experiments with 

 them are thus difficult to interpret at the cell level. 

 Absorption occurs simultaneously into a heterogeneous 

 mixture of meristematic, expanding and mature cells, and 

 in addition there is transport across the cortex into the 

 stele (Fig. 3). When the roots are entirely immersed in a 

 solution, the possibility exists that salts absorbed from the 

 medium are released again from the cut ends of the xylem 

 vessels, so that the true rate of absorption may be difficult 

 to ascertain. Unfortunately, many investigators have 

 attempted to interpret their observations as if the organ 

 consisted of a homogeneous mass of parenchyma, and have 

 ignored its manifest complexity. 



