Utilization of Carbohydrates in Leguminous Symbiosis

Abstract

EXPERIMENTS carried out in this department during the past summer may be of interest in connexion with a recent letter from Allison and his collaborators¹ in which they conclude for field-grown leguminous plants that the associated nodule bacteria consume only a relatively small proportion of plant carbohydrates in their respiration. In these experiments, determinations have been made of the rate of evolution of respiratory carbon dioxide from the root systems of two series of soya bean plants growing in water culture (Crone's solution), one series (of fifteen plants) being nodulated, the other (of ten plants) being kept free of infection by the nodule organism and supplied with sodium nitrate. During the determinations, which were made at the flowering stage and at a temperature of 20°–21° C., air initially free of carbon dioxide was bubbled through the culture solution in which the root systems were growing and then passed into absorption towers containing standard baryta. For the nitrate plants the evolution of carbon dioxide was of the order of 0·8 c.c. (at N.T.P.) per hour per gm. dry weight of root tissues. Allison et al. report an average Qo₂of 2·2 for detached root fragments, equivalent to an absorption of 2·2 c.c. of oxygen per hour per gm. dry weight. They used young roots only, whereas the figure of 0·8 c.c. is an average for the whole root system: again, it is probable that the Washington experiments were conducted at a higher temperature.