The occurrence of haemoglobin and hydrogenase in nodules of twelve Casuarina-Frankia symbiotic associations

Seedlings of Casuarina cunninghamiana Miq., C. equisetifolia L. and C. glauca Sieber growing in N-deficient potting mix were inoculated with Frankia sp. from each of 4 different sources. After ca 4 months, plants were harvested and nodules from the 12 Casuarina-Frankia combinations evaluated for (1) concentrations of haemoglobin (measured as CO-reactive haem) and (2) occurrence of hydrogenase. The aim was to determine if these factors were related to nitrogen accumulation and biomass production. There were marked host-Frankia source interactions with up to 10-fold differences in plant dry weight and 50-fold differences in the efficiency of nitrogen fixation (as estimated by N₂ accumulated mg^?1 nodule dry weight). Differences in plant growth and nitrogen accumulation were apparently related to nodule specific activity, because the 12 associations had similar nodulation characteristics, e.g. time for nodulation to occur. The concentration of haemoglobin in Casuarina nodules ranged from 0 to 27 nmol haem (g FW)^?1. There was a strong linear correlation between concentrations of haemoglobin and dry weights of the whole plants (r=0.77, 0.92 and 0.97, P≤0.05) for C. cunninghamiana, C. equisetifolia and C. glauca symbiotic associations, respectively. However, the linear correlation between concentration of haemoglobin and nitrogen content of whole plant was lower (r=0.60, 0.64 and 0.71, P≤0.05) for the three Casuarina symbioses, respectively, and there was only a poor correlation between haemoglobin concentration in nodules and the rate of nitrogen accumulation on nodule weight basis. This indicates that haemoglobin concentration is not the sole physiological determinant of nitrogen fixation in Casuarina. All the Casuarina-Frankia symbiotic associations studied also showed the presence of a hydrogen uptake enzyme. The activity of the enzyme ranged from 5.1 to 34.1 μmol H₂ (g FW)^?1 h^?1, and hydrogen uptake was not correlated with plant dry weight, nitrogen content or the rate of nitrogen fixation. Hydrogen evolution could not be detected in any of the associations.