Effects of Exposure to Ammonium and Transplant Shock upon the Induction of Nitrate Absorption

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Abstract
In barley (Hordeum vulgare L. cv Steptoe) seedlings, the time course for induction of root nitrate absorption varied significantly with pretreatment. Net nitrate uptake of nitrogen-deprived plants more than doubled during the 12 hours after first exposure to nitrate. For these plants, gentle physical disturbance of the roots inhibited net nitrate absorption for more than 6 hours and potassium absorption for 2 hours. Pretreatment with ammonium appeared sufficient to induce nitrate absorption; plants either grown for 2 weeks on or exposed for only 10 hours to a medium containing ammonium as a sole nitrogen source showed high rates of net nitrate uptake when first shifted to a medium containing nitrate. Gentle physical manipulation of these plants inhibited nitrate absorption for 2 hours and potassium absorption for more than 12 hours. These results indicate (a) that experimental protocols should avoid physical manipulation of the roots when-ever possible and (b) that ammonium or a product of ammonium assimilation can induce nitrate absorption.