Growth and nitrate assimilation in tomato (Solanum lycopersicon) grown with different nitrogen source and treated with cadmium

In tomato seedlings, the effects of Cd on growth and activity of enzymes involved in nitrate assimilation were studied on different nitrogen sources. After one week on basal growth medium, seedlings were transferred and grown for 14 days on a medium containing 5 mM nitrate, 5 mM ammonium or varying ratios of nitrate to ammonium. In each nitrogen medium, plants were divided into control and Cd treated. In nitrate-rich medium, plant dry weigh was improved while growth was more inhibited by Cd stress than in ammonium-rich medium. Regardless of nitrogen source, Cd treatments decreased nitrate contents in leaves and roots. Nitrate reductase and nitrite reductase activities were decreased by presence of ammonium in culture medium. Cd treatments inhibited NR and NiR activities independently of nitrogen form and plant organ. It could be concluded that tomato plants preferred nitrate as a nitrogen source where as external ammonium was found to inhibit growth. The addition of ammonium in nutrient solution slows down Cd absorption. Endogenous nitrate accumulation may mirror an important role of this metabolite in nitrate and nitrite reductase induction.