Metabolism and mode of action 



300 foot-candles) and temperature (25°C) in a complete nutrient solution 

 (pH5-l) containing varying concentrations of the growth regulator (for 

 details see Blackman and Robertson-Cunninghame, 1954). Under such 

 conditions control samples of both species will grow at a constant and high 

 relative growth rate {ca. 22-27 g/g/day) and the contents of nitrogen, 

 phosphorus, and potassium are largely independent of time. 



In the Lemna minor experiments a wide range of concentrations of 2:4- 

 dichlorophenoxyacetic acid were selected (0-1 25-48-0 p. p.m.) such that, 

 even with the highest concentration, the growth rate, although exhibiting a 

 cumulative depression below that of the control, is still considerable at the 

 end of 12 days. After 12 days concentrations of 3 and 12 p. p.m. also cause 

 significant reductions in the growth rate but no significant depressions 

 occur at the end of 4 days. 



At concentrations above 3 p. p.m. the total amounts of nitrogen, 

 phosphorus, and potassium absorbed are progressively depressed with both 

 time and concentration, the redvictions being greatest for potassium. On 

 either a fresh- or dry-weight basis the content of nitrogen and phosphorus is 

 augmented by 3-48 p. p.m. and for 3 and 24 p. p.m. the gains increase up to 

 12 days. The results for potassium are in marked contrast. On a dry-weight 

 basis the content is depressed and the order of the depression is positively 

 correlated with both time and concentration. On a basis of fresh weight the 

 content is initially depressed but returns to or above the level of the control 

 at the end of 12 days. 



When the concentrations of nitrogen, phosphorus, and potassium in the 

 nutrient solution are varied there is little evidence of an interaction between 

 the effects of the growth regulator and variations in the total amount of 

 each nutrient, absorbed. Likewise, for both nitrogen and phosphorus content 

 the interaction is not significant but for potassium content the depressive 

 effect induced by the growth regulator is magnified when the external 

 concentration is reduced. 



For Salvinia nutans the pattern of experimentation was on similar lines but, 

 in this instance, the range of physiological concentrations which only partially 

 suppressed growth at the end of 12 days did not exceed 24 p. p.m. At a 

 lower concentration of 12 p. p.m. the relative growth rate of both the fresh 

 and dry weights can be significantly increased (about 17 per cent) in the first 

 three days. Subsequently, the growth rate falls off and by 9 days is below that 

 of the control. Over all concentrations (6-24 p. p.m.) there is a cumulative 

 depression in the uptake of nitrogen, phosphorus, and potassium and the 

 depression is most marked for nitrogen. At 12 p. p.m., compared to the 

 control, the absorption of potassium may be greater at 3 days, equal at 6, 

 and less at 9 and 12 days. 



On the criteria of dry and fresh weight the growth regulator during the 

 12 days of the experiment progressively reduces the nitrogen content. With 

 phosphorus at and below 12 p.p.m. the content at first rises and then tends 

 to return to the control level after 12 days; at a higher concentration of 

 24 p.p.m. the final content is below that of the control. The trends for 

 potassium content resemble those for phosphorus. Initially, in the first 

 6-7 days at all concentrations the potassium content increases and on a 

 fresh-weight basis this gain in content is maintained during the rest of 



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