Respiration of barley plants III—Protein catabolism in starving leaves

Abstract

In an earlier paper (Yemm 1935) the results of analytical and gasometric experiments with starving barley leaves were described. These experiments indicated that the respiratory mechanism was considerably modified under conditions of extreme starvation: the rapid breakdown of carbohydrates, conspicuous at first, may play only a minor part in the later stages. It was suggested that protein catabolism is important in the respiration of highly starved and yellow leaves. The evidence for this conclusion was drawn primarily from observations of the respiratory quotient; such data are notoriously difficult to interpret and alone they do not lead to any satisfactory certainty. In the present case further information could be gained by biochemical examination of the transformation of proteins and other nitrogenous compounds occurring during progressive starvation. By this means the preliminary analysis of the living system by gasometric methods could be extended. It is known that loss of protein occurs in plants under conditions of carbohydrate shortage. Onslow (1931) has collected together analytical data which show this in both etiolated seedlings and isolated organs, while the researches of Chibnall (1922a, 19246), Mothes (1926), and Vickery, Pucher, Wakeman and Leavenworth (1933) have provided information especially for detached leaves. However, any detailed comparison of the results available with the starving barley leaves is obviously hazardous, since differences, both of the leaf type and of starvation conditions, may lead to confusion. Therefore, biochemical data have been sought for barley leaves starved under the controlled conditions previously used: a description of the analytical methods and results is given in this paper.

Glutamine in the Metabolism of Barley Plants

“…before subjecting it to analysis. Controlled conditions of starvation and analytical methods were similar to those already described (Yemm, 1935(Yemm, , 1937. The results are summarized graphically in Fig.…”

“…For example, the two amide fractions, now known to be mainly asparagine and glutamine, show consistent differences in relation to catabolism of proteins and respiration of starved barley leaves. Glutamine accumulates most rapidly at first in the freshly detached leaves, while asparagine is only formed at later phases of starvation (Yemm, 1937). In other species distinctive features have been observed and discussed by Vickery (1939), Mothes (1940), Steward (1946) and their collaborators.…”

“…In an earlier discussion of the respiration of barley leaves, attention was drawn to the close relationship between glutamine and the production of carbon dioxide during the later phase of starvation (Yemm, 1937). There was at that time some evidence to show that glutamic acid, as distinct from almost all other amino-acids, could be oxidized readily in many living tissues.…”

Studies on the Nitrogen Metabolism of the Barley Plant (Hordeum Sativum)

“…(ii) Extraction of Soluble Nitrogenous Compounds.-Chibnall's ether-water method, as modified by Yemm (1937) was used. Material (5-10 g.) was cut finely, allowed to stand 15 minutes with 2 m!.…”

“…INTRODUCTION There is strong evidence (Yemm 1937) that much of the carbon dioxide produced in the respiration of detached barley leaves arises, especially in the later stages of starvation, from nitrogenous compounds. The present paper is mainly concerned with transformation of nitrogenous compounds in detached leaves and developing seedlings of barley.…”