Chloride reduces plant nitrate requirement and alleviates low nitrogen stress symptoms.

Chloride (Cl ^- ) is traditionally categorized as an antagonist of nitrate (NO ₃ ^- ) because Cl ^- hinders plant NO ₃ ^- 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 CO ₂ 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 NO ₃ ^- 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 NO ₃ ^- 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 NO ₃ ^- treatments (3-15 mM). First, we have demonstrated that 5 mM Cl ^- application can reduce the use of NO ₃ ^- 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-NO ₃ ^- conditions. The Cl ^- -dependent resistance to low-N stress resulted from: more efficient use of the available NO ₃ ^- ; 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 NO ₃ ^- requirements and withstand N deficiency, promoting a more sustainable and resilient agriculture.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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