I02 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



simply solutions of three or four salts containing the major nutrients and in- 

 clude, in addition, the micronutrient elements essential in small amounts, 

 though frequently toxic if present above certain concentrations. 



In the classical investigations in plant nutrition conducted by the late Dennis 

 R. Hoagland and his associates at Berkeley (21), a solution of the following 

 composition was widely used, tomato and barley being the favorite objects of 

 experimentation (22, 23): KNO3, 0.0025 M/l.; Ca(N03)2, 0.0025 m/1.; ]\IgS04 

 o.ooi m/1.; KH2PO4, 0.0005 m/I- To support sustained growth of plants, the 

 solution must be renewed at appropriate intervals and be supplemented by 

 small concentrations of iron, boron, manganese, zinc, copper and molybdenum 

 (3, 4). Recent findings (10, 12) indicate that chlorine must be added to the list 

 of plant micronutrients. Aeration of the solutions is necessary for optimal 

 growth. 



A severe deficiency in any one of the essential nutrient elements will spec- 

 tacularly manifest itself in the development of a more or less characteristic 

 syndrome (5), preceded or accompanied by changes in the enzyme complement 

 of the plant (6-8, t,^, 35, 36). In the case of the micronutrient ions, elaborate 

 purification of the nutrient solutions and rigid precautions to prevent in- 

 advertent contamination may be required in order to induce a deficiency (41). 



For experiments on the mechanism of absorption, it usually is desirable to 

 simplify the system still further. Prolonged culture of plants in complete 

 nutrient solution involves progressive changes in the organism studied, through 

 growth and differentiation, the various ions being absorbed are subject to com- 

 plicated mutual interrelations, rates of transport are not constant, and a two- 

 way traffic occurs between the root and the shoot. One common way of dealing 

 with these difficulties is to grow plants in a relatively low-salt medium for a 

 time varying from a few days to a few weeks, following which the roots are 

 excised and used in short-term absorption experiments lasting from a few 

 minutes to a few hours. It need hardly be pointed out that radioisotopes have 

 proven invaluable in experiments of this kind. In fact, the first biological ap- 

 plication of radioactive tracer technic dealt with plant nutrition: in 1923, 

 Hevesy studied the uptake of lead by plants using Pb-^- as a tracer. 



RESPIRATION AND ABSORPTION OF IONS 



When a salt such as KCl is absorbed by plant roots, indications are that 

 nearly all of it appears in solution in the aqueous phase of the tissue, probably 

 largely in the central vacuoles which are characteristic of plant cells. After a 

 period of active absorption the internal concentration of salt may vastly ex- 

 ceed the concentration of the external medium. Metabolic energy expenditure 

 is obviously involved in that case, but even when the diffusion gradient is in- 

 ward absorption of salt is slight or negligible under conditions of arrested me- 

 tabolism. Direct evidence concerning the linkage between metabolic activity 



