PLANT CELL GROWTH AND NUTRITION 477 



Other events— protein synthesis, growth of the cells, ion accumulation- 

 may stem from this fact. 



Therefore it is essential to interpret the facts of salt accumulation, 

 recognizing that it is the factors controlling the growth of the cells, 

 first by cell multiplication, later by cell enlargement, that will ulti- 

 mately control the course of absorption and the final concentrations 

 obtained. This inevitably means that the problem is one of organization 

 and of the specific agents and factors that control the pace and kind of 

 operations it can carry out. But the resting, quiescent cells may diflFer 

 from active ones in another important respect. 



Metabolism and "turnover" in quiescent and growing cells. Cells 

 in their resting state may be expected to exist in a state of nitrogen 

 balance. Any breakdown of protoplasmic protein could be made good 

 by a minimal resynthesis of protein from stored, soluble nitrogenous 

 reserves. But the net effect of this on the carbon balance sheet of the 

 tissue should be negligible. Thus the principal fate of absorbed sugar 

 could well be the conversion of sugar to CO2 and water by one or 

 another of the standard respiratory routes (glycolysis and Krebs's 

 cycle, etc. ) . 



But when metabolic activity and protein synthesis are stimulated 

 by good aeration and minimal concentration of carbon dioxide, it has 

 seemed necessary to visualize a different possibility. Under these cir- 

 cumstances a principal use for sugar is to furnish carbon for protein 

 synthesis. Moreover, the ready use of the products of glycolysis and the 

 normal operation of Krebs's cycle may be limited for lack of free CO2 

 and by fostering decarboxylations. In this way, metabolism gets di- 

 verted to the resynthesis of protein from the nitrogen-rich compounds 

 of the tissue, using sugar as the main source of extra carbon; mean- 

 while the deaminated and deamidated products of the erstwhile stor- 

 age nitrogen compounds are then respired away in the form of keto 

 acids which can enter Krebs's cycle, as it were, by the "back door" 

 ( Steward, Bidwell, and Yemm, 1956, 1958 ) . This view gains credence 

 from the observation that a prime effect of coconut milk, in the stimula- 

 tion of growth, is not only to promote the net synthesis of protein but 

 also protein turnover. The evidence for "turnover" is that glutamic 

 acid, formed from sugar, becomes labeled in the protein much more 

 rapidly than would be expected from the direct incorporation of the 

 labeled free glutamic acid of the cell. Thus again one should consider 

 the cell, in its growing and non-growing states, not merely as a bag of 

 enzymes that recapitulate their properties as if in homogenous solu- 

 tion but as a highly integrated, coordinated system, capable of doing 

 quite different things in its separate parts and in its separate states. 

 The balance between these various activities— the emphasis given to 

 one or the other— obviously may be controlled and regulated by the 



