BIOCHEMISTRY OF PLANTS 



processes in all living organisms, they possess a peculiar importance in 

 the consideration of the nutrition of higher plants for the reasons already 

 suggested. At one time, the absorption of salts by plant cells would 

 usually not have seemed to belong to the domain of biochemistry. The 

 intake of solutes was regarded as a passive process, in which the per- 

 meability of protoplasmic membranes was chiefly stressed. It is now 

 well recognized that solutes may move into plant cells against concen- 

 tration or activity gradients at the expense of metabolic energy. This 

 kind of absorption of salts by living cells of the root, often referred to as 

 salt accumulation, is dependent on aerobic respiration. The accumula- 

 tion process is inliibited by many respiratory poisons, such as cyanide 

 and iodoacetate, not only in the initial absorption of salts or ions by the 

 root, but also in their polarized movements into the plant's upward 

 conducting system, and their subsequent accumulation in the cells of 

 the leaf or reproductive organs. There are various theories of the 

 mechanisms by which solutes move through the living conducting 

 system of the plant. It is agreed that simple diflTusion cannot explain 

 the movement, and the conclusion cannot be escaped that at some point 

 solutes move against gradients or are accelerated in their movement 

 through coupling with some energy-yielding process. 



Concurrently with the accumulation of some ions by plant cells, 

 various metabolic processes are stimulated, such as synthesis of organic 

 acids, or proteins, and oxidation of sugars. In fact, the biochemistry of 

 salt absorption by plant cells, in its various aspects, appears to offer 

 a profitable branch of research in the plant field. Researches on 

 storage tissues of plants provide eloquent testimony of the value of 

 studying biochemical transformations as part of an investigation of salt 

 accumulation — compare the review by Steward (14). 



In the development of studies on salt absorption or movement, 

 the tool of radioactive isotopes may become of great value. Thus 

 certain metabolic reactions may be related to the movement of a se- 

 lected radioactive ion, which can be detected with extreme sensitivity 

 of measurement. So far, the use of radioactive isotopes in research on 

 higher plants has been limited in the main to simple tracer studies 

 designed to obtain information on rates or direction of movement and 

 to identify tissues through which translocation or accumulation takes 

 place. A much wider field for the application of isotopes, stable and 

 radioactive, awaits development, and with it may come knowledge ol 



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