Bioenergetic aspects of competition between nitrogen metabolism and carbohydrate synthesis in small grains

Abstract

Yields of small grains have been significantly increased in recent decades primarily by breeding. No corresponding progress has been made regarding protein content in the grain. That was mostly due to a negative correlation existing between yield level and protein content. Protein synthesis requires a much larger amount of energy that that required for carbohydrate synthesis. A hypothesis explains this negative correlation by competition for energy between nitrogen metabolism and carbohydrate synthesis. The proponents of this hypothesis surmise that nitrate reduction unfolds mostly at the expense of the energy released upon oxidation of carbohydrates and organic acids (heterotrophic), and not at the expense of light energy (autotrophic). In small grains, nitrate reduction takes place mostly in leaves, i. e., it is autotrophic. Under conditions of optimum nitrogen supply, protein content and yield per unit area increase but grain yield does not decrease although it could be expected if there existed a competition for energy between nitrogen metabolism and carbohydrate synthesis. Protein content reduction in consequence to yield increase could be explained by the dilution effect, as evidenced by similar reductions of other substances in the grain, for example mineral elements. When considering the competition for energy between nitrogen metabolism and carbohydrate synthesis, it should not be overlooked that nitrogen stimulates the energy transport in plants indirectly, by stimulating the photosynthetic activity and thus the utilization of light energy by plants