Seasonal patterns of root nodule growth, endophyte morphology, nitrogenase activity, and shoot development in Myrica gale

Abstract

Myrica gale L. populations growing in central Massachusetts were observed throughout the ice-free season. Nitrogenase activity appeared in mid-May shortly after budbreak, was at its maximum between late June and mid-August, and disappeared in late October after all leaves had fallen. Growth of overwintering nodules began in early May and was largely complete by mid-July. Most nodules (88%) lived for 3 years or less and 61% of the nodule biomass present in autumn was produced during the current season.Colonizing hyphae of the Frankia sp. endophyte were seen throughout the year in partially expanded cortical cells near the nodule lobe apex. Vesicles first appeared in mature cortical cells coincident with the onset of nitrogenase activity in mid-May, occupied the bulk of the infected tissue during the summer, and disappeared as nitrogenase activity ceased in late October. Evidence is presented that the vesicles are the site of nitrogenase activity and are newly produced each season in freshly formed nodule lobe tissue. Sporangia frequently formed in mature infected cells in nodules at one site but were rare at another. The processes described here in M. gale are probably typical of winter-deciduous actinorhizal plants.