Carbon Costs Associated with N₂ Fixation in Vicia faba L. and Pisum sativum L. over a 14-Day Period

Abstract

Long-term (14 days) carbon costs of N₂ fixation were studied in pot trials. For this purpose the CO₂ release from the root space of nodulated and non-nodulated (urea nourished) Vicia faba L. and Pisum sativum L. plants was compared and related to the amount of fixed or assimilated N. Additional measurements of shoot CO₂ exchange and dry matter increment were carried out in order to calculate the overall carbon balance. The carbon costs for N₂ fixation in Vicia faba L. (2.87 mg C/mg N_fix) were higher than in Pisum sativum L. (2.03 mg C/mg N_fix). However, the better carbon efficiency in Pisum sativum L. did not lead to a better growth performance compared to Vicia faba L. Vicia faba L. compensated for the carbon and energy expenditure by more intensive photosynthesis in the N₂-fixing treatment. This was not the case with Pisum sativum L., where the carbon balance indicates that the carbon costs of N₂ fixation restricted root growth. It is proposed that low carbon costs for N₂ fixation indicate an adaptation to a critical carbon supply of roots and nodules, e.g., during the pod-filling of grain legumes.

Key words

Dinitrogen fixation - carbon costs - Pisum - Vicia - legumes