Possible mechanism of the nitrate action regulating free-putrescine accumulation in ammonium fed plants

in order to investigate the possible involvement of nitrate in regulating free-putrescine accumulation induced by ammonium nutrition, we studied the plant concentration of free, no-covalently conjugated and covalently conjugated polyamines associated with nitrogen nutrition involving different nitrogen forms (ammonium and nitrate) in two plant species: wheat and pepper grown in saline and no saline conditions. Our results showed that the beneficial action of NO3- alleviating the negative effects of NH4+ nutrition on plant growth seems to be mediated by a complementary action decreasing the pool of total putrescine within the plant, and increasing putrescine conjugation, principally in the non-covalent forms. This effect is accompanied by a clear decrease in the value of the free putrescine: (free spermidine + free spermine) ratio, which is also associated with a better plant growth under saline stress. Salinity affected the concentration of total polyamines, principally putrescine, in the plant differently depending on the nitrogen form. However, salinity did not significantly affect the conjugation pattern of putrescine. (C) 2008 Elsevier Ireland Ltd. All rights reserved.