Effects of Water Stress on the Respiratory and Nitrogen-fixing Activity of Soybean Root Nodules

Abstract

Seventy-five per cent of the N₂-fixing activity (measured as the reduction of C₂H₂ to C₂H₄) and 50 per cent of the respiratory activity of detached soybean root nodules was lost when the water potential (Φ) of the nodules was lowered from approximately −1 × 105 Pa (turgid nodules) to −9 × 105 Pa (moderately stressed nodules). Severely stressed nodules (Φ = −1.8 × 10⁶ Pa) showed almost total loss of N₂-fixing activity and up to 80 per cent loss of respiratory activity. Increasing the oxygen partial pressure (PO₂) from 10⁴ to 10⁵ Pa completely restored both N₂-fixation and respiration in moderately stressed nodules, but only partial recovery was possible in severely stressed nodules. The activity of the stressed nodules was very low at low PO₂ (5 × 10³ and 10⁴ Pa). The C₂H₂-reducing activity of nodule slices, nodule breis, and bacteroids from turgid and moderately stressed nodules was almost identical but some activity was lost in the breis and bacteroids from severely stressed nodules. Calculations showed that at low PO₂ (10⁴ and 2 × 10⁴ Pa), the rate of O₂ diffusion into severely stressed nodules was ten times lower than that for turgid nodules, but only four times lower at a higher PO₂ (4 × 10⁴ Pa). Carbon monoxide inhibition of C₂H₂ reduction was slower in stressed nodules than in turgid nodules. The results are discussed in view of the possible development of a physical barrier to gaseous diffusion and/or the possible altered affinity of the nodule leghaemoglobin for O₂ in the water-stressed nodules.