The effect of low pH on different phases of nodule formation in pea plants

Summary A number of experiments was carried out with pea plants growing in a nutrient solution of low pH, inhibitory to nodule formation but not to plant growth when combined nitrogen was available. It has been found that on transferring plants from a neutral culture solution to an acid solution of pH 4.5 at different periods of time after inoculation, nodulation is reduced when the exposure to low pH takes place during the first days after inoculation, and particularly on the second or third day. Once nodulation has been initiated, nodule growth and nitrogen fixation proceed normally in an acid solution. Failure of nodulation at low pH, under the conditions of these experiments, was not due to the inhibition of bacterial growth. From these results an acid-sensitive period, occurring during the second or third day after inoculation, was demonstrated.