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.
