concentrations as a result. This could give a false 

 indication that a specific element was growth limiting. 

 Therefore, the critical concentration of each element was 

 established in a plant part (index segment) which more 

 accurately reflected environmental supply than did the 

 entire plant. 



Two procedures were employed in determining the critical 

 concentration of nitrogen and phosphorus. One approach 

 was similar to the procedure routinely used in agricul- 

 tural applications of plant analysis. The plants were 

 grown in nutrient cultures similar in all respects except 

 for the concentration of the element under investigation. 

 Concentrations of that element varied from suboptimal to 

 above optimal. The plants were harvested after a culture 

 period of approximately four weeks, when ranges of growth 

 and of element concentration in the plants were represented. 

 All treatments were at least in duplicate and often in 

 triplicate. 



To establish the most suitable index segment, at harvest 

 plant material from each culture was divided into three 

 or four segments: the terminal one inch of growth on 

 main branches and laterals, the second inch of growth, 

 the third inch of growth (in some cases), and the remainder 

 of the plants. Samples were oven-dried to constant 

 weight at 65-70 °C, and analyzed for the element under 

 study. A critical concentration was established from 

 curves relating average oven-dry yield and tissue content 

 of the element under study in the plant segments from 

 each treatment of an experiment. The first one-inch 

 portion cut from all main shoots and laterals was found 

 to be the most suitable index segment for elements which 

 are relatively immobile in plants, that is, they are not 

 re-exported from older to younger tissues. The second 

 one-inch was the index segment for mobile elements. 



A second procedure was developed for establishing critical 

 concentrations in Elodea . Individual plants were grown in 

 34 x 20 x 5 cm Pyrex trays containing 1200 ml of the 

 appropriate nutrient medium. Each tray was covered with a 

 glass plate which was sealed to the tray with Apiezon 

 sealing compound (Associated Electrical Industries, Ltd.) 

 after the culture assembly had been autoclaved and inoculated. 

 This was necessary to minimize algae contamination. Two 



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