Chemical Salts 



Solubility and effect on the pH of the medium throughout 

 the growth period are primary factors in the selection 

 of salts to provide essential elements in a nutrient 

 medium. The four salts Ca (N0 3 ) 2 • 4H 2 0, KN0 3 , MgS0 4 .7H 2 0, 

 and Kl^POit have been widely used in plant culture media, 

 including the modified Hoagland's solution from which the 

 macrophyte medium was derived. The NaNC>3 in the recom- 

 mended medium is to provide additional nitrogen without 

 increasing the concentration of another essential element. 



As will be apparent in data to be presented, the pH of 

 the macrophyte solution immediately after preparation and 

 autoclaving was approximately 5.0, and after equilibrium 

 with the 1% C0 2 in air mixture it was 4.9. As plants 

 absorbed ions , the pH of the medium increased so that at 

 harvest it correlated to a degree with plant yields. In 

 general, pH values at harvest were in the range of 6.0 to 

 7.5 for Elodea and Ceratophyllum and somewhat higher for 

 Myriophyllum , as high as 10.0. Differences in yield and 

 absorption of ions account for these variations. 



Trace Element Concentrations 



The primary difference in the culture medium used in the 

 critical concentration experiments of Section IV and the 

 medium derived from those experiments is the concentrations 

 of the trace elements which were reduced to 1/5 to 1/10 

 the concentrations in the original medium. This seems 

 desirable because, except for chlorine, the range in 

 concentration between adequacy and toxicity of the trace 

 elements can be narrow as illustrated in the data of 

 Table 14 showing the response of Elodea to copper. The 

 average yield from triplicate cultures provided with 0.006 

 ppm copper was 1.352 g; with 0.03 ppm copper, yield was 

 reduced to 1.098 g; and with 0.15 ppm, yield was only 

 0.531 g. The adverse effects of concentrations of trace 

 elements close to the toxicity level probably are primarily 

 on the inoculum and on the initiation of growth in a 

 culture. If the concentration of an element is not so 

 high that the inoculum is killed, the amount of the ele- 

 ment per unit of material is reduced as growth develops 

 and initial toxicity effects in a culture no longer will 

 be evident. 



42 



