Table 14. Toxicity of copper to Elodea occidentalis 



Cu in Yield of 



culture Elodea , 



soln. , ppm g/21.* 



0.0 1.325 



0.003 1.399 



0.006 1.352 



0.03 1.098 



0.15 0.531 



Average of triplicate cultures. 



Iron Source 



Maintenance of adequate available iron is a critical 

 factor in successful growth of plants in aerated, liquid, 

 culture media. The low solubilities of iron precipitates 

 which form at the slightly acid and alkaline pH values of 

 nutrient media make inorganic iron sources, in general, 

 unsatisfactory. As a result, chelating agents have been 

 employed for a number of years to maintain iron avail- 

 ability. Ferric citrate and particularly FeEDTA have been 

 effective in many culture media. Because of a capacity 

 to maintain iron in a soluble form at higher pH values 

 than does EDTA, EDDHA has recently been the iron source 

 in several culture media (Wallace, 1966). 



The importance of the iron source in the culture of 

 macrophytes is indicated in Table 15. Ferric citrate was 

 completely unsatisfactory as a source of iron for 

 Ceratophyllum demursum. FeEDTA was far superior to ferric 

 citrate but was less satisfactory than FeEDDHA. Because 

 of the increase in the pH of the culture medium associated 

 with autoclaving, it seemed desirable to autoclave the 

 iron source separately and add it to culture media after 

 autoclaving. However, the data indicate separate auto- 

 claving was not beneficial. 



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