ROLE OF NH₄⁺ NUTRITION ON SALT-INDUCED OXIDATIVE STRESS IN CARRIZO CITRANGE PLANTS

Salinity is amongst the most significant environmental factors responsible for substantial losses in agricultural production worldwide and this is a critical problem especially in citrus since they are one of the most globally important horticultural crops considered salt sensitive. Salt stress has two effects on plants, an initial osmotic shock followed by a toxic phase due to ion. Moreover, a high cellular NaCl concentration causes enhanced formation of reactive oxygen species (ROS). To avoid such salt stress-derived injuries, tight regulation of ROS homeostasis is necessary and is provided by a network through complex loops between oxidants and scavengers. NH₄⁺ is a paradoxical nutrient ion because it is a main N source, but high concentrations of this ion in soil may have damaging effects. In this study, we have analyzed influences of NH₄⁺ nutrition on Carrizo citrange plants undergoing 90 mM NaCl. Pants were grown with 1 mM NH₄NO₃ (control) and 5 mM de N-NH₄⁺. To clarify the relationship between the antioxidant mechanisms induced by NH₄⁺ nutrition and salt tolerance, we analyzed antioxidant activities (SOD, CAT, GR) and the role of ascorbate and glutathione pools.
Carrizo citrange plants showed optimal growth in both treatments and we confirmed that N-NH₄⁺ treatment enhanced resistance to salt stress. Based on this evidence, we hypothesized that N-NH₄⁺ treatment triggers mild chronic stress which could be the prime Carrizo citrange defenses by stress imprinting, thus conferring plants resistance. Moreover, sub-lethal concentration of NH₄⁺ could act as a mild oxidative stressor triggering antioxidant cellular machinery against subsequent salt stress.