Morphogenesis of Effective and Ineffective Root Nodules in Phaseolus vulgaris L.

Seedlings of Phaseolus vulgaris, inoculated with an effective strain of Rhizobium phaseoli (CIAT 73) or an ineffective strain (14c), were maintained on an N-free nutrient solution in sterilized sand. Acetylene reduction was measured periodically, and nodules were harvested for light and electron microscopy. In plant host cells infected with CIAT 73, there was an increase in cell volume, nuclear size, and mitochondrial length as bacteria multiplied and became bacteroidal. Golgi activity and rough endoplasmic reticulum (RER) were sparse at nodule maturity, but close associations of amyloplasts and elongated, energized mitochondria were observed. Nitrogen fixation peaked at 5 wk and dropped sharply coincident with flowering. Plant host cells infected with 14c typically contained numerous mitochondria; Golgi bodies and RER were conspicuous throughout nodule development. By the time the bacteria became bacteroidal, plant shoots were chlorotic, and under nitrogen stress the apex had made the transition to flowering. Data suggest that such ineffective nodulation may accelerate the process of normal nodule senescence.