Growth, nodule morphology, and nitrogenase activity of Myrica gale with roots grown at various oxygen levels

Abstract

Myrica gale L. plants were inoculated with Frankia strain HFP M+g15 and grown for 28 days with root systems exposed to 2, 5, 10, 21, and 40 kPa O2. Plant growth was similar under all treatments, except for a small decrease in final size of plants at 2 kPa O2. At the end of the experiment nitrogenase activity of representative plants was analyzed over a variety of Po2 levels using an open-flow cuvette. Optimum nitrogenase activity was found at Po2 levels close to the growth Po2 in all cases and plants showed no short-term adaptation to oxygen. Specific activity of maximum nitrogenase was similar for all treatments, being within a factor of two. Nitrogenase activity showed rapid transient responses to step shifts in Po2 during assay and irreversible decline in activity at Po2 levels above optimum. Morphological responses to changing Po2 include a dramatic increase in nodule-root growth, inversely proportional to ambient Po2, and a variety of internal structural changes reducing nodule ventilation with Po2 increase. No modification in vesicle envelope thickness was observed over the range of Po2 studied. We conclude that oxygen protection in M. gale nodules operates through a complex suite of morphological controls, but that modification of nodule-root surface area is probably the prime mechanism.