Chloride reduces plant nitrate requirement and alleviates low nitrogen stress symptoms

Chloride (Cl^-) is traditionally categorized as an antagonist of nitrate (NO₃^-) because Cl^- hinders plant NO₃^- transport and accumulation. However, we have recently defined Cl^- as a beneficial macronutrient for higher plants, due to specific functions that lead to more efficient use of water, nitrogen (N) and CO₂ under optimal N and water supply. When accumulated in leaves at macronutrient levels, Cl^- promotes growth through osmotic, physiological, metabolic, anatomical and cellular changes that improve plant performance under optimal NO₃^- nutrition. Nitrate over-fertilization in agriculture can adversely affect crop yield and nature, while its deficiency limits plant growth. To study the relationship between Cl^- nutrition and NO₃^- availability, we have characterized different physiological responses such as growth and yield, N-use efficiency, water status, photosynthesis, leaf anatomy, pigments and antioxidants in tomato plants treated with or without 5 mM Cl^- salts and increasing NO₃^- treatments (3-15 mM). First, we have demonstrated that 5 mM Cl^- application can reduce the use of NO₃^- in the nutrient solution by up to half without detriment to plant growth and yield in tomato and other horticultural plants. Second, Cl^- application reduced stress symptoms and improved plant growth under low-NO₃^- conditions. The Cl^--dependent resistance to low-N stress resulted from: more efficient use of the available NO₃^-; improved plant osmotic and water status regulation; improved stomatal conductance and photosynthetic rate; and better antioxidant response. We proposed that beneficial Cl^- levels increase the crop ability to grow better with lower NO₃^- requirements and withstand N deficiency, promoting a more sustainable and resilient agriculture.