Nodulation and growth ofPhaseolus vulgaris in solution culture

Summary Conditions and techniques for achieving good nodulation ofPhaseolus vulgaris L. in continuously aerated solution were developed from greenhouse experiments.If nodules had been established, their growth and activity and the growth of the plant were at least as good in solution culture as in gravel culture. Nodule formation was observed within 10 days of inoculation in small volumes of solution culture (1 liter). In large volumes (19 liters), similarly prompt nodulation occurred only if the plants were inoculated before or immediately after the seedlings were transferred to the solution from gravel or vermiculite; and the nodules were restricted to the roots that had been present at the time of transfer. Delayed inoculation, 2 days after transfer to large volume solutions, led to sparse nodulation observed only after 3 weeks. Delay or inhibition of nodulation in large volumes of solution could not be explained by failute of bacteria to colonize roots or by sparsity of root hairs.Nodule initiation in solution culture was severely inhibited at pH below 5.4. An additional problem in growing N₂-dependent bean in solution culture was the buildup of Cl^– to toxic levels in the plant in nitrate-free media, even at solution concentrations as low as 0.4 mM Cl^–. Daily addition of 0.5 to 1.0 mg N per plant delayed nodule growth and activity slightly, but increased plant growth and alleviated the severe N-deficiency that otherwise developed before the onset of N₂-fixation.