Nitrate Reduction in Roots as Affected by the Presence of Potassium and by Flux of Nitrate through the Roots

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Abstract
Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed to treatment solutions containing Ca(NO3)2, NaNO3, or KNO3; KNO3 plus 50 or 100 millimolar sorbitol; and KNO3 at root temperatures of 30, 22, or 16 C. In all experiments, the accelerated phase of NO3− transport had previously been induced by prior exposure to NO3− for 10 hours. The experimental system allowed direct measurements of net NO3− uptake and translocation, and calculation of NO3− reduction in the root. The presence of K+ resulted in small increases in NO3− uptake, but appreciably stimulated NO3− translocation out of the root. Enhanced translocation was associated with a marked decrease in the proportion of absorbed NO3− that was reduced in the root. When translocation was slowed by osmoticum or by low root temperatures, a greater proportion of absorbed NO3− was reduced in the presence of K+. Results support the proposition that NO3− reduction in the root is reciprocally related to the rate of NO3− transport through the root symplasm.