33:67 to 20:80

Table 3.6 The effect of total nitrogen, and the ratio of NO3 to NH4+ on the type of organ produced by alfalfa cells which have been subcultured on a root-inducing medium (Walker and Sato, 1981)

containing glutamine or casein hydrolysate as the sole nitrogen source (Anderson, 1976). Amino acids provide plant cells with an immediately available source of nitrogen, and uptake can be much more rapid than that of inorganic nitrogen in the same medium (Thom et al, 1981). Only the L- form of amino acids is biologically active.

Amino acids can also provide reduced nitrogen in culture media in place of NH4+ and as a supplement to NO3. However they are usually employed as minor additions to media containing both NH4+ and NO3. Uptake of amino acids into cultured tissues causes a decrease in the pH of the medium, similar to that which occurs when NH4+ ions are absorbed.

Sugar-based amines such as glucosamine and galactosamine can also serve as a source of reduced nitrogen in morphogenesis (Margara, 1969a; Margara and Leydecker, 1978).

2.1.12. Biologically-active amino acids

Amino acids are classified according to their stereoisomers and according to the relative positions of the amino group and the acidic radical. Only the L- isomers of the a-amino acids are important for plant tissue culture media. They have the general structure:

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