[EN] The objectives were to verify the effects of the lack of cytokinins (CKs), comparing tomato cv. Micro-Tom (MT, wild type) to MT CKX2 (transgenic with less CKs) fed with nitrate (NO3-) and ammonium (NH4+), in the presence and absence of silicon (Si); verify if the attenuation of NH4+ toxicity by Si depends on the increase of CKs in MT; and verify if 6-benzyladenine (6-BA) attenuates NH4+ toxicity in MT. Three experiments were performed with treatments via nutrient solution. First, MT and MT CKX2 were grown with NO3- or NH4+ (5.9 mmol L- 1), in the absence and presence of Si (1.28 mmol L- 1). Second, MT was grown with NO3- or NH4+ (5.9 mmol L- 1), in the absence and presence of Si (1.28 mmol L- 1). Third, MT was grown with NO3- or NH4+ (5.9 mmol L- 1) and 6-BA (from 1e(-10) to 1e(-6) mol L-1) associated with NH4+. The MT and MT CKX2 had a decrease of 18% and 48% in the shoot dry weight, respectively, when fed with NH4+, compared to NO3-. Si attenuated NH4+ toxicity in MT, but not in MT CKX2. This attenuation in MT was accompanied by a decrease in trans-zeatin (tZ) content in the roots and increase in the shoots. 6-BA did not improve the shoot growth of MT fed with NH4+. In conclusion, the alleviation of NH4+ toxicity by Si was dependent on the increase in tZ content in shoots. In CK-deficient plants, Si did not alleviate NH4+ toxicity, and 6-BA did not alleviate NH4+ toxicity in MT shoots.

This study was financed in part by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior-Brasil (CAPES)-Finance Code 001.