Low root temperatures and nitrogenase activity in soybean

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Abstract
A continuously flowing open gas exchange system was used to monitor the rates of CO2 evolution, C2H2 reduction, and H2 evolution from intact nodulated soybean ('Harosoy 63') roots, while root temperatures were dropped from the growing temperature (25 °C) to 10 °C at the rate of 5 °C/h. Four Rhizobium japonicum inoculants were used, three of which (type S (Nitragin Co.), USDA 16, USDA 35) displayed net H2 evolution while the fourth (USDA 110) did not evolve H2. Between 25 and 15 °C, values for Q10 in the H2-evolving symbioses ranged from 2.0 to 2.7 for CO2 evolution. 1.3 to 2.4 for C2H2 reduction, and 3.2 to 3.7 for H2 evolution. Nodulated roots of USDA 110 displayed the highest Q10 values for both CO2 evolution (Q10 = 2.9) and C2H2 reduction (Q10 = 15.2). The temperature profiles of these gas exchange measurements were used to calculate both the relative efficiency (RE = 1 − (H2 evolution/C2H2 reduction)) of N2 fixation and the ratio between CO2 evolution and C2H2 reduction at temperatures between 10 and 25 °C. The effect of short-term changes on the CO2/C2H2 ratio varied with symbiotic association. In the type-S symbiosis, long-term (96 h) treatments at low root temperature (9.5 °C) decreased the amount of CO2 evolved per C2H2 reduced to 35% of the value obtained at 25 °C. This study did not determine whether these observed changes were associated with N2 fixation or growth and maintenance of the nodulated root. In all H2-evolving symbioses, the RE increased from ca. 0.6 to 0.9 in response to a temperature drop from 25 to 10 °C. Studies of net H2 uptake and 3H2 exchange at 25 and 15 °C indicated that the observed changes in relative efficiency were due to variations in electron allocation by nitrogenase rather than a relative increase in uptake hydrogenase activity. The increase in RE above 0.75 suggested that lower temperatures may have altered the minimal nitrogenase electron allocation to less than one H2 per N2 fixed. This observed increase in RE with lower temperatures indicated that, in the symbioses studied, H2 evolution may have provided a buffer which permitted the maintenance of high levels of N2 fixation during short-term or diurnal fluctuations in soil temperature.