Chloride Improves Nitrate Utilization and NUE in Plants

Abstract

Chloride (Cl^-) has traditionally been considered harmful to agriculture because of its toxic effects in saline soils and its antagonistic interaction with nitrate (NO₃ ^-), which impairs NO₃ ^- nutrition. It has been largely believed that Cl^- antagonizes NO₃ ^- 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 NO₃ ^- fertilization frequently used in agriculture becomes a major environmental concern worldwide, causing excessive leaf NO₃ ^- accumulation in crops like vegetables and, consequently, a potential risk to human health. New farming practices aimed to enhance plant NUE by reducing NO₃ ^- fertilization should promote a healthier and more sustainable agriculture. Given the strong interaction between Cl^- and NO₃ ^- homeostasis in plants, we have verified if indeed Cl^- affects NO₃ ^- 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 NO₃ ^- nutrition, facilitates NO₃ ^- utilization and improves NUE in plants. This is largely due to Cl^- improvement of the N-NO₃ ^- utilization efficiency (NU_TE), having little or moderate effect on N-NO₃ ^- uptake efficiency (NU_PE) when NO₃ ^- is used as the sole N source. Clear positive correlations between leaf Cl^- content vs. NUE/NU_TE or plant growth have been established at both intra- and interspecies levels. Optimal NO₃ ^- vs. Cl^- ratios become a useful tool to increase crop yield and quality, agricultural sustainability and reducing the negative ecological impact of NO₃ ^- on the environment and on human health.