The Effect of Combined Nitrogen on the Growth and Nodulation of Excised Roots of Phaseolus vulgaris L.

Isolated roots of Phaseolus vulgaris were grown in aseptic cultureusing a modified Raggio technique in which the organic nutrientsare fed to the cut (basal) end of the excised root while theroot tip grows into a mineral-salt solution inoculated withRhizobium. Treatments were applied to investigate the effectsof varying the level of sucrose in the organic medium and ofadding combined nitrogen as nitrate and urea to both media. There was a marked effect of sucrose on the growth and nodulationof the roots. Increasing the concentration increased the numberof nodules both per root and per unit fresh weight of tissue. The growth of the roots was not affected by the level of combinednitrogen. Nitrate in the mineral-salt solution markedly reducednodule numbers but in the agar medium it had only a slight delayingeffect on nodulation. Urea in either the mineral-salt solutionor the organic medium reduced nodule numbers though to a lesserextent than nitrate in the mineral-salt solution. The number of nodules formed was inversely correlated with thelevels of soluble and inorganic nitrogen compounds in the tissues. It is suggested that the adverse effect of combined nitrogenon nodulation is due to the accumulation of unsequestered nitrogencompounds and a depletion of carbohydrates generally withinthe root tissues rather than the local effect of any particularnitrogen compound.