Chloride Improves Nitrate Utilization and NUE in Plants.

Miguel A Rosales,Rosario Álvarez,Juan D Franco-Navarro,José M Colmenero-Flores,Procopio Peinado-Torrubia,Pablo Díaz-Rueda

doi:10.3389/fpls.2020.00442

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 (NUTE), having little or moderate effect on N–NO3– uptake efficiency (NUPE) when NO3– is used as the sole N source. Clear positive correlations between leaf Cl– content vs. NUE/NUTE 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.

Highlights

Nitrogen (N) is the main limiting nutrient for land plants and, has been classified as an essential macronutrient
It is important to notice that the leaf Cl− content in tobacco plants treated with low Cl− levels (SP and N treatments) exceeded the critical deficiency threshold reported for Cl− in non-halophytic plants (
These results show that Cl− stimulates plant growth when it is supplied at macronutrient levels and ruled out the occurrence of Cl− deficiency in plants subjected to low Cl− treatments (SP and N treatments)

Summary

Introduction

Nitrogen (N) is the main limiting nutrient for land plants and, has been classified as an essential macronutrient. Nitrate (NO3−) represents the major N source and a signal molecule involved in the control of many physiological and developmental processes, strongly improving crop yield (Frink et al, 1999; Wang et al, 2012; Krapp et al, 2014; Guan, 2017). The decisive role of N in crop yield has led to excessive use of NO3− in agriculture over decades. When the application rate of NO3− exceeds the plant growth needs, overaccumulation of NO3− in leaves reduces the nutritional quality of crops (Prasad and Chetty, 2008; Xing et al, 2019). Many large-leaved plants such as beets, cabbage, celery, lettuce, or spinach tend to store huge amounts of NO3− (MAFF, 1998), posing a serious risk to human health. NO3− is rapidly converted to nitrite and N-nitrous compounds as nitrosamines or nitric oxide causing methemoglobinemia or “blue baby syndrome” in infants and gastric cancer among other pathological disorders (Comly, 1945; Santamaria et al, 1999; Mensinga et al, 2003)

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