Consequences of Sporangial Development for Nodule Function in Root Nodules of Comptonia peregrina and Myrica gale

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Abstract
Frankia sp., the actinomycetous endophyte in N-fixing actinorhizal nodules, may differentiate 2 forms from its hyphae: vesicles and sporangia. In root nodules of C. peregrina (L.) Coult. and M. gale L., sporangia may be either absent or present. Nitrogenase activity and symbiotic efficiency were constrated in spore(+) and spore(-) nodules of these 2 host genera. Seedlings of C. peregrina nodulated with the spore(+) inoculum showed only 60% of the nitrogenase activity and 50% of the net size of their spore(-) counterparts after 12 wk of culture. Measurements of acetylene reduction (i.e., nitrogenase activity) were coordinated with samplings of nodules for structural studies. Significant differences in acetylene reduction rates were discernible between spore(+) and spore(-) nodules commencing 4 wk after nodulation, concomitant with the maturation of sporangia in the nodule. Spore(+) nodules ultimately reached less than half of the rate of nitrogenase activity of spore(-) nodules. Both types of nodules evolved only small amounts of molecular H, suggesting that both were equally efficient in recycling electrons lost to the reduction of H ions by nitrogenase. Respiratory cost of N fixation, expressed as the quotient of .mu.mol CO2 to .mu.mol ethylene evolved by excised nodules, was significantly greater in spore(+) than in spore(-) nodules. M. gale spore(-) nodules showed variable effectivity, though all had low CO2 to ethylene evolution ratios. M. gale spore(+) nodules resembled C. peregrina spore(+), with low effectivity and high respiratory cost for N fixation.