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

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Abstract
Isolated roots of Phaseolus vulgaris were grown in aseptic culture using a modified Raggio technique in which the organic nutrients are fed to the cut (basal) end of the excised root while the root tip grows into a mineral-salt solution inoculated with Rhizobium. Treatments were applied to investigate the effects of varying the level of sucrose in the organic medium and of adding combined nitrogen as nitrate and urea to both media. There was a marked effect of sucrose on the growth and nodulation of the roots. Increasing the concentration increased the number of nodules both per root and per unit fresh weight of tissue. The growth of the roots was not affected by the level of combined nitrogen. Nitrate in the mineral-salt solution markedly reduced nodule numbers but in the agar medium it had only a slight delaying effect on nodulation. Urea in either the mineral-salt solution or the organic medium reduced nodule numbers though to a lesser extent than nitrate in the mineral-salt solution. The number of nodules formed was inversely correlated with the levels of soluble and inorganic nitrogen compounds in the tissues. It is suggested that the adverse effect of combined nitrogen on nodulation is due to the accumulation of unsequestered nitrogen compounds and a depletion of carbohydrates generally within the root tissues rather than the local effect of any particular nitrogen compound.