Twenty-one L-amino acids and the amides L-glutamine and L-asparagine have been tested for their effects on the growth of excised tomato roots, by adding them singly to a medium of pH 4.8 containing the standard amount of nitrate. L- and D-cystine were growth-stimulatory. The amides, L-aspartic acid and L-arginine, did not significantly affect growth in concentrations up to 25 mg/1. The other amino acids were, to very differing extents, inhibitory to growth. The inhibitory activity of certain amino acids was greater at pH 6.8 than at 4.8. At this pH low concentrations of L-arginine enhanced growth. The L- and D- forms of the amino acids differed in their inhibitory activity.
With the exception of L-glutamine and, to a lesser extent, of L-asparagine and the natural forms of the dicarboxylic amino acids, all the amino acids tested as sole nitrogen sources for root growth were completely ineffective at both pH 4.8 and 6.8. This applied also to the ‘balanced’ mixtures of amino acids which had only low inhibitory activity when added to a nitrate-containing medium.
“Bacto’ casamino acids (an acid-hydrolysed casein) was more effective than ammonium as a nitrogen source over the range pH 4.8 to 7.0, but was only as effective as nitrate at pH values close to neutrality.
At pH values of 5.3 or above, the growth of excised tomato roots on ammonium was markedly stimulated by the addition of amino-acid mixtures whose composition was based upon amino-acid analyses of hydrolysed casein. A mixture of the basic amino acids only was almost as effective as more complex mixtures. At pH 6.8 ammonium plus a suitable amino-acid mixture supported growth equal to that with the casamino acids. The very poor growth of excised tomato roots supplied with ammonium at pH 4.8 was, however, not significantly enhanced by the amino-acid additions which were markedly stimulatory at higher pH values, and was inferior to growth with casamino acids at this pH. A high level of root growth cannot yet be achieved at pH 4.8 in the absence of nitrate.