Abstract
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.