The Respiratory Costs of Nitrogen Fixation in Soyabean, Cowpea, and White Clover

Abstract

Soyabean, cowpea, and white clover, inoculated with effective rhizobia, were grown singly with a standard mineral nutrition and light regime in controlled environments until seed maturation (in soyabean and cowpea) or late vegetative growth (white clover). Day/night temperature regimes were 23/18, 30/24, and 20/15 °C in soyabean, cowpea, and white clover, respectively. The respiratory losses of CO₂ from the nodulated root systems were studied in relation to the concurrent rate of fixation of atmospheric nitrogen. Despite differences in development, the effectiveness of the symbioses, and the temperature of growth, all three legumes exhibited similar respiratory losses from nodulated roots per unit of nitrogen fixed. During intense nitrogen fixation, the average respiratory losses for the three legumes varied between 6·3 and 6·8 mg C mg⁻¹ N; within each species, the losses varied more widely at different stages of development. These respiratory burdens reflect the total cost to the plant of the nodule/nitrogen fixation syndrome including the subtending roots. The results are discussed in relation to the respiratory effluxes from nodules and roots, and to biochemical investigations of the costs of nitrogen fixation.