346 Mineral Nutrition of Plants 



raise the question whether failure of nutrient absorption by roots when 

 their carbohydrate supply is low, may not also be associated with failure 

 of hormone translocation to roots from the shoot. 



During the vegetative phase of growth, there is an intimate connec- 

 tion between carbohydrate and protein synthesis. Not only are carbo- 

 hydrates and nitrogen used in the synthesis of proteins, but a portion 

 of the soluble hexoses evidently provides the respiratory energy neces- 

 sary for the chemical reduction of nitrates as an antecedent to amino 

 acid and protein formation. In fact in young plants, the supply of solu- 

 ble sugars appears largely to condition the rate of protein synthesis 

 (85, 86, 8y, 12J, 753). Obviously, the availability of oxygen also is essen- 

 tial to the respiratory oxidation of a portion of the carbohydrates but 

 this usually is not a limiting factor in early growth as it is later. Mothes 

 (85, 87) has shown that all conditions, such as light, photosynthesis, and 

 open stomata, which tend to raise internal oxygen tension, favor protein 

 formation. Conversely, protein hydrolysis is accelerated by a low in- 

 ternal oxygen tension and by low water content in later development 

 (25). Thus, the rate of photosynthesis as a source of both carbohydrates 

 and oxygen is closely bound up with nitrate reduction and protein syn- 

 thesis. During the later phases of active vegetative growth, the plant 

 rapidly accumulates carbohydrates and appears to become relatively less 

 efficient in protein than in carbohydrate elaboration (//) as shown by 

 increments in the carbon-nitrogen ratio (55). 



Thus from the period of germination to flowering, three fairly dis- 

 tinct stages of metabolism are evident. The conspicuous stages in the 

 nutrition of the vegetative plant comprise an initial anabolic phase (I) 

 in which intake of inorganic nutrients and synthesis of proteins is rapid. 

 In the second phase (II), the accumulation of carbohydrates accelerates 

 while the rate of protein synthesis gradually diminishes. As flowering 

 is approached, a third or catabolic phase (III) becomes evident in which 

 hydrolysis of reserves begins to overbalance synthesis and a general 

 internal redistribution of nutrients is initiated. Though conditions of 

 environment and nutrient supply determine to a considerable degree 

 the exact time of the shift from predominantly anabolic to catabolic 

 activity, the latter transition is characteristically associated with flower- 

 ing and commonly initiated prior to anthesis (40). 



