Chloride Improves Nitrate Utilization and NUE in Plants

Chloride (Cl(–)) has traditionally been considered harmful to agriculture because of its toxic effects in saline soils and its antagonistic interaction with nitrate (NO(3)(–)), which impairs NO(3)(–) nutrition. It has been largely believed that Cl(–) antagonizes NO(3)(–) uptake and accumulation in higher plants, reducing crop yield. However, we have recently uncovered that Cl(–) has new beneficial macronutrient, functions that improve plant growth, tissue water balance, plant water relations, photosynthetic performance, and water-use efficiency. The increased plant biomass indicates in turn that Cl(–) may also improve nitrogen use efficiency (NUE). Considering that N availability is a bottleneck for the plant growth, the excessive NO(3)(–) fertilization frequently used in agriculture becomes a major environmental concern worldwide, causing excessive leaf NO(3)(–) accumulation in crops like vegetables and, consequently, a potential risk to human health. New farming practices aimed to enhance plant NUE by reducing NO(3)(–) fertilization should promote a healthier and more sustainable agriculture. Given the strong interaction between Cl(–) and NO(3)(–) homeostasis in plants, we have verified if indeed Cl(–) affects NO(3)(–) accumulation and NUE in plants. For the first time to our knowledge, we provide a direct demonstration which shows that Cl(–), contrary to impairing of NO(3)(–) nutrition, facilitates NO(3)(–) utilization and improves NUE in plants. This is largely due to Cl(–) improvement of the N–NO(3)(–) utilization efficiency (NU(T)E), having little or moderate effect on N–NO(3)(–) uptake efficiency (NU(P)E) when NO(3)(–) is used as the sole N source. Clear positive correlations between leaf Cl(–) content vs. NUE/NU(T)E or plant growth have been established at both intra- and interspecies levels. Optimal NO(3)(–) vs. Cl(–) ratios become a useful tool to increase crop yield and quality, agricultural sustainability and reducing the negative ecological impact of NO(3)(–) on the environment and on human health.