Nitrogen Metabolism, Respiration, and Growth of Cultured Plant Tissue

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Abstract
Critical examination of the amino-acid composition of proteins in fast-growing and slow-growing tissues reveals only very 8maU differences, indicating that some factor other than the amino-acid complement is responsible for, or reflects, the great increase in the mass of protein in the fast-growing tissues. Increases in fresh weight and total protein are exactly parallel, indicating that water uptake is an active process associated with growth. Respiration, on the other hand, increases far more in the fast-growing over the slow-growing tissues than does total protein. A given amount of protein in the fast-growing tissue will support a much greater respiration rate than the same amount in slow-growing tissue. The incorporation of radioactivity into amino-acids of the protein in4icates that there are two distinct types: those in which incorporation is increased in fast-growing tissue much more than the total protein, and to the same ezte as respiration (notably glutarnic acid, aspartic acid, and threonine); and those in which the increased incorporation is much nafler, slightly less than total protein (notably proline and hydroxyproline). It is concluded that there are two n protein fractions: the ‘active’ moiety, which is undergoing rapid breakdown d resynthesis, giving rise to much of the CO through oxidation of its residues; a the ‘inactive’ moiety, which once synthesized is not reutilized or broken down. It is the former, or ‘active‘ protein whose synthesis is greatly increased in the fast. growing tissues, and it is the pace, rather than the kind, of reactions which differ entiates between the fast- and slow-growing tissues. The entire experimental data are discussed with reference to a number of cur rent theories and investigations. A number of experimental observations are noted which admit of interpretation along the lines here developed.

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