Metabolism of Amino Acids in Plants. I. Changes in the Soluble Amino Acid Fractions of Bushbean Seedlings (Phaseolus vulgaris L.) and the Development of Transaminase Activity
The total nitrogen content of seedlings grown under light or dark conditions remained relatively constant during the first 2 weeks of growth, but there was considerable redistribution of nitrogen from cotyledons to the seedling tissues and appreciable changes occurred in the ratio between insoluble and soluble nitrogen in the different organs. Soluble nitrogen increased in etiolated seedlings to reach 55% of the total nitrogen at 14 days, while in light-grown seedlings the soluble nitrogen reached a peak on the 8th day and thereafter rapidly decreased to 25% of the total nitrogen by the end of 2 weeks. The quantitative composition of the soluble nitrogen fraction, with respect to free protein amino acids, was found to differ considerably, not only between seedlings at different stages of growth, but also between organs taken from seedlings at the same stage of growth under the two light conditions. Asparagine, serine, valine, alanine, and threonine were the amino acids that showed the greatest change in amounts in both types of seedlings. Of the six transaminases investigated, only aspartate, alanine, and asparagine aminotransferase systems showed high total activity. Valine aminotransferase had low activity, and the serine and threonine transaminases were not detected. Development of the different transaminase activities was quite similar in seedlings grown under light or dark conditions. No correlation was observed between the development of transaminase activities and the changes in concentrations of the corresponding free amino acids in these seedlings.