128 MINERAL SALTS ABSORPTION IN PLANTS 



a rather sluggish transpiration stream. In a few cases, e.g. the peanut 

 {Arachis hypogaea) where transpiration is still further reduced 

 because the fruits develop underground, some calcium and other 

 salts are taken up directly from the soil via specialized absorptive 

 structures, the gynaphores. 



The idea that mineral salts circulate in plants, travelling upwards 

 in the xylem and downwards in the phloem was suggested by Mason 

 and Maskell (1931) in the course of their investigations of solute 

 movement in the cotton plant. They proposed that phosphorus and 

 potassium are sometimes transported downwards to the roots at a 

 greater rate than they are utilized there, so that they re-enter the 

 xylem, and are recirculated. In an attempt to demonstrate circulation 

 of ions, Biddulph et al. (1958) supphed calcium, phosphate and 

 sulphate labelled with radioactive isotopes to the roots of intact 

 bean {Phaseolus vulgaris) plants for 1 hr, and then examined the 

 subsequent migration of the radioactivity over a period of several 

 days, while the plant was growing in an non-radioactive solution. 

 Labelled phosphate was carried up the stem and into the leaves as a 

 discrete unit. Later it moved into the roots again, and continued to 

 circulate until the end of the experiment. Circulation of sulphate 

 was rapidly curtailed by metabolic incorporation in young leaves, 

 and calcium did not circulate at all. 



Salts accumulated by storage tissues are redistributed during a 

 subsequent phase of the life cycle. During the early stages in the 

 development of seeds, for example, salt is supplied to the young 

 radicle and plumule from the cotyledons. Biddulph (1951) observed 

 a movement of iron from the cotyledons of germinating bean seeds 

 (Fig. 44a) which closely parallels that of carbohydrates. Roots of 

 seedlings grown under conditions of sulphur deficiency were found 

 to receive higher proportions of the sulphur leaving the cotyledons 

 than they did when sulphur was supplied in the medium. In pea 

 plants, grown in potassium-deficient media in the dark, potassium 

 was transferred from the cotyledons during the first 10 days, about 

 equally into roots and shoots (Fig. 44b). Movement into roots 

 became very slow after about 2 weeks, at a time when they also 

 ceased to grow. When aeration of the roots was reduced, the 

 amount of potassium absorbed, and of growth, decreased without 

 affecting either the salt content or growth of the shoot. Removal 

 of the shoot at an early stage leaving the cotyledons intact did not 



