Importance of Carbohydrate Supply in Legume Symbiosis

Abstract

EXPERIMENTAL evidence, which has been accumulating for a number of years, but especially more recently, emphasises the importance of carbohydrate in nodule development and nitrogen fixation in leguminous plants inoculated with effective bacterial strains. There are, however, certain facts well known to plant physiologists which deserve greater emphasis in this connexion: (a) An abundant supply of nitrogen is conducive to top growth while a wide carbohydrate-nitrogen ratio favours root growth. The carbohydrate-nitrogen relations may be altered by changing the conditions for photosynthesis, by varying the nitrogen additions to the culture medium, and by supplying sugars to the roots. (b) During active photosynthesis a sugar concentration gradient¹, decreasing downward, commonly exists between the leaves and roots of higher plants. These synthesised sugars usually² do not remain long in this form; a considerable portion, 20 per cent or more, is required for respiration, while the remainder is either used for growth or is stored largely as starch.