Moderate salinity and high ammonium/nitrate ratio enhance early growth in "summer wonder" lettuce cultivar

Because its impact in plant development and growth and its interaction with Na⁺ and Cl^-, the supply of different N-forms to crops can be an easy-to-use tool with effective results on salinity tolerance. Here the effect of four N-NO₃^-/N-NH₄⁺ ratios (mM; 2/0, 1.6/0.4, 0.4/1.6, 0/2) on adaptation to salt conditions (15 mM NaCl in a first experiment and 40 mM NaCl in a second experiment) was studied in young lettuce (cv "Summer wonder") plants. The experiments were carried out in greenhouse and under hydroponics conditions. The results show that this cultivar tolerates and adapts to moderate salinity by deploying several structural and physiological mechanisms; (i) increasing allocation of biomass to the root, (ii) increasing root Na⁺ uptake and storing it in the shoot and root tissues, (iii) increasing intrinsic water use efficiency and (iv) increasing root N and P uptake. The beneficial effect of salt exposure on growth was greater when the predominant N-form was N-NO₃^-. These plants with higher tissue N-NO₃^- concentration, decreased Cl^- uptake and shoot and root Cl^- concentration. Regardless of salt conditions, plants with a high proportion of N-NH₄⁺ (1.6 mM) and a low proportion of N-NO₃^- (0.4 mM) had a greater growth and nitrogen use efficiency, that was associated with the improved uptake of nutrients, and the maintenance of water status.