Critical Phosphorus Concentration in Elodea 



Yield and tissue phosphorus concentrations of Elodea 

 grown in nutrient media varying in phosphorus content are 

 presented in Table 4. As with nitrogen, the youngest 

 tissues, that is the terminal one-inch of growth, had the 

 highest phosphorus concentration. This was anticipated 

 because both phosphorus and nitrogen are readily re- 

 exported from younger to older tissues. The second one- 

 inch segment was considered the most satisfactory index 

 segment in which to establish the critical phosphorus 

 concentration . 



Ory-weight yields did not level off at a constant maximum 

 as sharply as they did in the nitrogen experiments. There- 

 fore, it was more difficult to establish the critical 

 phosphorus content with certainty. When data of Table 4 

 were graphed, there was a marked decrease in the yield 

 response per unit of phosphorus made available to the 

 plants at tissue concentrations above approximately 0.14 

 percent phosphorus in the second one-inch segment. This 

 was considered the critical phosphorus concentration in 

 that segment. The 0.14% value is supported by the data in 

 Table 5 obtained from an experiment in which the critical 

 level of phosphorus was established using the tray 

 procedure. At the time of harvest, the plants were 

 severely phosphorus deficient as indicated by the small 

 amount of growth in the low phosphorus treatment. The 

 phosphorus concentration in the index tissue of these 

 severely deficient plants, 0.12 percent, was slightly 

 lower than the comparable value obtained with the batch 

 procedure. In an earlier study (Gerloff and Krombholz, 

 1966) , the critical phosphorus content for Elodea occidentalis 

 on a whole plant basis was 0.14 percent. It is not sur- 

 prising that the values based on the entire plant and the 

 second one-inch segment should be the same, because 

 analysis of entire plants represents an average of younger 

 tissues with a higher phosphorus concentration than the 

 second one-inch segment and older tissues with a lower 

 concentration . 



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