TIME‐LAPSE PHOTOGRAPHIC OBSERVATIONS OF MORPHOGENESIS IN ROOT NODULES OF COMPTONIA PEREGRINA (MYRICACEAE)

Abstract

Seedlings of the sweet fern Comptonia peregrina (L.) Coult. were grown aeroponically with their roots bathed in a nutrient mist lacking nitrogen except for 10 ppm N at the outset. The initiation and early development of root nodules capable of fixing atmospheric nitrogen were recorded with time‐lapse photography through early development to the establishment of highly branched, roughly spherical nodules. In Comptonia multiple primary nodule lobes are formed at or near the site of infection with as many as 10 primary lobes occurring together. On the shoulders of the swollen primary lobes new primordia develop, forming secondary nodule lobes, which may persist without nodule root elongation, giving a coralloid appearance. The tips of the lobes may elongate, forming nodule roots which grow vertically upward, or, if disturbed, in random orientation. Nodule roots occasionally form lateral roots. The root axis upon which the nodule forms undergoes secondary thickening on the proximal side of the nodule attachment; the distal portion of the root shows no secondary thickening and later atrophies. Thus, nodules are perennial structures on a woody root system. The endophyte infects and occupies the basal cortical tissues of the primary nodule lobes and successive nodule lobes as they are formed, being restricted to the swollen bases and not infecting the elongate nodule roots. Development of the nodule is interpreted in terms of complex host‐endophyte interactions involving the initiation of multiple primordia forming nodule lobes, the active inhibition of nodule lobes and finally nodule root elongation. Anatomical evidence for the endogenous origin of nodule primordium formation substantiates the view obtained from time‐lapse photomacrography.