Chloride Ions as a Beneficial and Essential Micronutrient Multifunctional, Role and Regulation in Plant Physiology: A Review

Chloride occurs predominantly as Cl in soil, plant, and considering as a micronutrient largely excluded by plants due to its ubiquity and abundance in nature. It is an essential micronutrient of higher plants and participates in several physiological metabolism processes. Including osmotic and stomatal regulation, evolution of oxygen in photosynthesis, disease resistance and tolerance. 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 (WUE). Increasing plant biomass indicates in turn that Cl may also improve nitrogen use efficiency (NUE). Structure of water around the sodium and potassium ions is a key test of the quality of interaction potentials, and are not completely aligned toward their electric fields, but rather tilted. This tilt is more defined for potassium than it is for sodium. The hydration number of sodium is restricted to either five or sex molecules, however for potassium has ranging from five to ten molecules. Most striking energetic difference between Na and K resides in the first shell. Water molecules have a very strong interact under such condition Na is more effect on the soil salinity than K. However, an increase in Na content is always accompanied by Cl accumulation and K loss in plants exposed to salt (NaCl) stress.Considering that N availability is a bottleneck for the growth of land plants excessive NO3 fertilization frequently used in agriculture becomes a major environmental concern worldwide, causing excessive accumulation leaf NO3 in crops particularly in vegetables, that poses a potential risk to human health. New farming practices aimed to enhance plant nitrogen use efficiency (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 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–NO − 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 intraand interspecies levels. Optimal NO3 versus Cl ratios become a useful tool for increasing crop yield and quality, sustainability of agricultural land and reducing negative ecological impact of NO3 on the environment and human health as well .