8 MINERAL SALTS ABSORPTION IN PLANTS 



growth, the reason for this remains largely obscure. It ocxurs mainly 

 in ionic state in cell vacuoles, where it is generally the most abundant 

 cation, and plays a major part in osmotic regulation. In this role it 

 can be replaced at least partly by sodium or other easily absorbed 

 cations. In addition, some potassium exists in bound forms in the 

 cytoplasm where it assists in maintaining the unique structure and 

 activity of protoplasm. It has been shown to activate a number of 

 enzymes in vitro, but its importance as a natural enzyme activator 

 is not yet certain. 



The bulk of the calcium in plants occurs as calcium pectate in 

 cell walls where it may assist in the regulation of growth. Calcium 

 phosphatides are constituents of protoplasmic membranes and may 

 exert an important influence on their properties. Crystals of calcium 

 oxalate are to be found in many cells, and a useful function of calcium 

 may be to neutralize organic acids which might otherwise be toxic. 

 Calcium ions become adsorbed by proteins and exert, in conjunction 

 with other cations, a regulating influence on the hydration of cyto- 

 plasm. Some plants ("calcifuge" species) are usually confined to 

 acid soils where the amount of calcium is low, while others 

 ("calcicole" species) occur only in soils where calcium abounds. 

 The basis of this distinction is probably complex, involving not only 

 varying sensitivity to high and low calcium levels, but also inter- 

 actions with other ions pH effects and competition between species. 

 Magnesium is known to be the only metallic constituent of 

 chlorophylls, and about 10 per cent of the magnesium in green 

 tissues is incorporated in these pigments. In addition, magnesium 

 is the normal activator of a number of important enzymes involved 

 in respiration and photosynthesis. 



Iron occurs in the prosthetic group of several respiratory enzymes, 

 including the cytochromes, and in the ionic state, it activates several 

 others. Boron is the one essential element for which no specific 

 function is yet known. The other micronutrients are either con- 

 stituents of enzymes, as in the case of copper, or serve as specific 

 enzyme activators. 



D. Mechanism of Absorption 



Recognition that mineral salts are absorbed and utilized by 

 plants led naturally to speculation about the mechanism of uptake. 



