Effects of NO2- or NO3- supply on polyamine accumulation and ethylene production of wheat roots at acidic and neutral pH:: implications for root growth

Exposure of plant tissues to nitrite ion or nitrite-derived NO at acidic pH results in the degradation of important macromolecules and may lead to the formation of reactive molecular species. Polyamines as free radical scavengers protect plant tissues against membrane and DNA damage during stress and may contribute to the acclimation processes caused by nitrite as an abiotic stressor at acidic pH. The putrescine content of wheat roots grown under low salt conditions increased only transiently at pH 7.0 when the nutrient solution was replaced by 1 mM KNO2, KNO3, NaNO2 or NaNO3, but the concentration of this diamine remained high after a 24-hour incubation at pH 4.0. The acid stress-induced putrescine accumulation was further enhanced by an external N source, especially by nitrite. The contents of spermine and spermidine in the 24-hour samples were also higher in N-supplied roots at acidic pH. Polyamine contents were not closely correlated with the ethylene production by the intact roots. Nitrite treatment, however, significantly decreased the ethylene release from the root apex, but not from the basal parts at pH 4.0. The peroxidative capacities of the tissues in the soluble fractions were also inhibited by nitrite in the apical zones, which might modify the H2O2-coupled oxidative processes. Nitrite ion at acidic pH may react directly with guaiacol-like phenolic compounds and in this way interfere with the lignification process. The low ethylene release by the apical zones in acidic environment may be a symptom of the nitrite-induced inhibition of root extension.