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The long-term effect of the concentration and duration of application of nitrate from the lower part of soybean roots on the nodulation and nitrogen fixation in the upper part of roots was investigated using a two-layered pot system separating the upper roots growing in a vermiculite medium and the lower roots growing in a nutrient solution. Continuous absence of nitrate (hereafter referred to as "0-0 treatment"), and continuous 1 m~ (1-1 treatment) and 6 -(6-6 treatment) nitrate treatments were imposed in the lower pot from transplanting to the beginning of the maturity stage. In addition, 6 mM nitrate was supplied partially from the beginning of the pod stage till the beginning of the maturity stage (0-6 treatment) or from transplanting till the beginning of the pod stage (6-0 treatment). The values of the total plant dry weight and seed dry weight were highest in the 6-6 treatment, intermediate in the 1-1, 6-0, 0-6 treatments, and lowest in the 0-0 treatment. The values of the nodule dry weight and nitrogen fixation activity (acetylene reduction activity) were lowest in the 6-6 treatment. The value of the nodule dry weight in the upper roots was highest in the plants subjected to the 1-1 treatment and exceeded that in the 0-0 treatment. Total nitrogen fixation activity of the upper nodules per plant at the beginning of the pod stage was also highest in the 1-1 treatment. These results indicated that longterm supply of a low level of nitrate from the lower roots could promote nodulation and nitrogen fixation in the upper part of roots. Withdrawal of 6 m M nitrate after the beginning of the pod stage (6-0 treatment) markedly enhanced nodule growth and ARA per plant in the upper roots at the beginning of the maturity stage when the values of both parameters decreased in the other treatments. The nitrate concentration in the nodules attached to the upper roots was low, including the 6-6 treatment regardless of the stages of growth. This indicated that the inhibitory effect of 6 mM nitrate or promotive effect of 1 mM nitrate supplied from the lower roots was not directly controlled by nitrate itself, but was mediated by some systemic regulation, possibly by the C or/and N requirement of the whole plant.

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