Control of Nitrate Uptake by Phloem‐Translocated Glutamine in Zea mays L. Seedlings

The putative role of glutamine, exported from leaves to roots, as a negative feedback signal for nitrate uptake was investigated in Zea mays L. seedlings. Glutamine (Gln) was supplied by immersion of the tip‐cut leaves in a concentrated solution. Nitrate (NO 3 − ) uptake was measured by its depletion in amino acid‐free medium. The treatment with Gln resulted in a strong inhibition of nitrate uptake rate, accompanied by a significant enrichment of amino compounds in root tissue. The effect of N‐availability on NO 3 − uptake was determined in split‐root cultures. The plants were subjected to complete or localized N supply. Inducible NO 3 − uptake systems were also induced in N‐deprived roots when the opposite side of the root system was supplied with KNO 3 . The inhibitory effect of Gln was unaffected by localized N supply on one side of the split‐root. The potential role of Gln in the shoot‐to‐root control of NO 3 − uptake is discussed.