Effect of Nitrate in the Rooting Medium on Carbohydrate Composition of Soybean Nodules

Abstract

Nodulated soybean plants (Glycine max [L.] Merr) [inoculated with Rhizobium japonicum] were grown in sand culture. Carbohydrate composition of nodules, roots and leaf blades was determined and related to the effects of nitrate in nutrient solution on nodule growth and on nitrogenase activity of nodules. When plants were grown without nitrate for 6 wk and then supplied with 150 mg NO3-N/l for 4 or 7 days, there was an 80% decline in acetylene reduction activity of nodulated roots, relative to the 0-N control. The 80% decline in nitrogenase activity was accompanied by a decline in nodule glucose concentration of .apprx. 0.15 mg/g fresh wt and an increase in nodule sucrose concentration of .apprx. 0.23 mg/g fresh wt. Plants were grown with 0, 30 or 100 mg NO3-N/l for 5 or 6 wk to study long-term effects of nitrate on nodule growth. The 100-N treatment reduced nodule weight/plant by 70% but reduced the sum of sucrose + glucose + fructose concentration in nodules by only 12%. The ratios of [sucrose] in nodules/[sucrose] in roots and [fructose] in nodules/[fructose] in roots increased slightly in response to nitrate, indicating that nitrate affects sugar concentration in roots more than sugar concentration in nodules. The effect of nitrate on glucose concentration of nodules was consistently negative. If it is assumed that sucrose, glucose and fructose are equivalent in their ability to support nodule functions, then the overall results are not consistent with the hypothesis that nitrate inhibits nodule growth and activity by reducing the accumulation of carbohydrate in nodules.