The effect of bacterial strain and temperature changes on the nitrogenase activity of Lotus pedunculatus root nodules

After 40 days of growth at 25°C, Lotus pedunculatus cav., cv. Maku plants infected with Rhizobium loti strain NZP2037 displayed similar relative growth rates but had twice the nodule mass and only one third the whole plant dry weight of plants infected with Bradyrhizobium sp. (Lotus) strain CC814s. In the NZP2037 symbiosis, the rate of CO₂ evolution (per g dry weight of nodulated root) was 1.6 times as high as that in the CC814s symbiosis while the rate of C₂H₂ reduction (per g dry weight of nodule) was only 48% of that in the CC814s symbiosis. Studies of the effect of short term temperature changes on the gas exchange characteristics (CO₂ and H₂ evolution, C₂H₂ reduction) of these symbioses revealed wide differences in the optima for C₂H₂ reduction. Nodules infected with NZP2037 displayed maximal C₂H₂ reduction rates [157 μmol (g dry weight nodule)^?1 h^?1] at 12°C, whereas nodules infected with CC814s were optimal at 30°C [208 μmol (g dry weight nodule)^?1 h^?1]. These short term studies suggested that differences in temperature optima for N₂ may have partially accounted for the poorer effectivity, at 25°C, of strain NZP2037 when compared with strain CC-814s. The relative efficiency [RE = 1 – (H₂ evolution/C₂H₂ reduction)] of N₂ fixation varied widely with temperature in the two symbioses, but there was a general trend toward higher RE with lower temperatures. The ratio of CO₂ evolution: C₂H₂ reduction (mol/mol) in nodulated roots infected with CC814s was constant (ca 10 CO₂/C₂H₂) between 5°C and 30°C, whereas in plants infected with NZP2037 it reached a minimal value of 3.3 CO₂/C₂H₂ at 10°C and was 19 CO₂/C₂H₂ at the growing temperature (25°C).