Nitrite Uptake by Nitrogen-Depleted Wheat Seedlings

Intact, 14-day-old nitrogen-depleted wheat (Triticum vulgare cv. Blueboy) seedlings were exposed to solutions of 0.5 mM KNO₂, 0.05 mM CaSO₄ and 1 mM sodium 2-N-morpholino]-ethanesulfonate, pH 6.1. Nitrite uptake was determined from depletion of the ambient solution or from incorporation of ¹⁵N in the tissue. An initial nitrite uptake shoulder was followed by a relatively slow uptake rate which subsequently increased to a substantially greater rate. This accelerated phase was maintained through 24 h. Nitrite accumulated to a slight extent in the root tissues during the first few hours but declined to low values when the accelerated rate was fully developed, indicating an increase in nitrite reductase activity paralleling the increase in nitrite uptake capacity. About 50% of the nitrogen absorbed as nitrite was translocated to the shoots by 9–12 h. Development of the accelerated nitrite uptake rate was restricted in excised roots, in intact plants kept in darkness, by 400 μg puromycin ml^?1 and by 1 mM L-ethionine. When puromycin and L-ethionine were added after the accelerated phase had been initiated, their effects were not as detrimental as when they were added at first exposure to KNO₂. The two inhibitors restricted translocation more than uptake. The data indicate an involvement of protein synthesis and a requirement for movement of a substance from shoots to roots for maximal development of the accelerated nitrite uptake phase. A requirement for protein synthesis in the transport of soluble organic nitrogen from roots to shoots is also suggested.