Combined light and transmission electron microscopy were used to examine the effect of nitrate on the development of root nodules in lucerne (alfalfa,Medicago sativaL.) following induction by the nitrogen-fixing symbiont,Rhizobium meliloti. The timing of NO3-addition was varied in order to study its effect on all of the recognized morphogenetic steps of nodule formation. Roots of plants inoculated in the presence of 18 mM NO3-had straight root hairs which were devoid of adherent rhizobia and infection threads, and developed no nodules. However, nodules were formed on roots if 18 mM NO3-was added 5 d after inoculation. At this time, the initiation of nodule primordia had already commenced in the root cortex. The histology and ultrastructure of young nodules which had developed for 5 d in the absence of NO3-and another 5 d in the presence of 18 mM NO3-resembled nodules developing under N-free conditions, except that in the infection threads within the infection zone of the nodule 1) some bacteria tended to loose their normal shape and gain more electron density, indicating premature degradation, and 2) the matrix of the infection threads was abnormally enlarged. In the presence of high NO3-levels in the medium, lysis and degeneration of the bacteria released from the infection threads were observed in the infection and bacteroid zones of developing nodules, indicative of premature senescence. On the other hand, the nodule meristems continued to proliferate even after 12 d of exposure of 18 mM NO3-. This was the only morphogenetic step of root nodulation which was insensitive to levels of combined nitrogen that completely prevented infection if present at the time of inoculation. These data indicate that all of the recognized steps of root nodule morphogenesis in which the bacteria play a key role are sensitive to the inhibitory effect of combined nitrogen.
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