Response of tomato plant to ammonium and nitrate nutrition using the relative addition rate technique

Different N sources (NO₃^-, NH₄⁺, or NH₄NO₃) at relative addition rate (RAR) of 0.25 day^‑1 were supplied to tomato (Lycopersicom esculentum Mill.), a species sensitive to NH₄⁺ toxicity. At RAR 0.25 day^‑1 growth reduction occurred by NH₄⁺ application, slightly. Tomato roots had a more efficient NH₄⁺ assimilation than shoots when N was supplied solely in the form of NH₄⁺, and dominant amino acids in tomato were glutamine and asparagine. Low tissue levels of calcium and magnesium in the NH₄⁺-fed plants constituted part of the NH₄⁺-toxicity syndrome. The supply of NO₃^- led to the accumulation of Fe in tomato roots, but leaves Fe concentration in NO₃^--fed plants was low in comparison to NH₄⁺ supply. It is concluded that due to the high sensitivity of tomato to NH₄⁺, even at RAR 0.25 day^‑1 the relative addition rate technique was not able to reduce the deleterious effects of NH₄⁺ in tomato plants.