Chloride (Cl⁻) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO₃⁻), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl⁻ has gone from being considered a harmful ion, accidentally absorbed through NO₃⁻ transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl⁻ determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO₂, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl⁻ and NO₃⁻ molecules, the NO₃⁻/Cl⁻ plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl⁻ influx (ZmNPF6.4 and ZmNPF6.6), Cl⁻ efflux (AtSLAH3 and AtSLAH1), and Cl⁻ compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl⁻, as well as for cell turgor regulation and stress tolerance in plants.

Chloride (Cl^–) is a micronutrient that accumulates to macronutrient levels since it is normally available in nature and actively taken up by higher plants. Besides a role as an unspecific cell osmoticum, no clear biological roles have been explicitly associated with Cl^– when accumulated to macronutrient concentrations. To address this question, the glycophyte tobacco (Nicotiana tabacum L. var. Habana) has been treated with a basal nutrient solution supplemented with one of three salt combinations containing the same cationic balance: Cl^–-based (CL), nitrate-based (N), and sulphate+phosphate-based (SP) treatments. Under non-saline conditions (up to 5mM Cl^–) and no water limitation, Cl^– specifically stimulated higher leaf cell size and led to a moderate increase of plant fresh and dry biomass mainly due to higher shoot expansion. When applied in the 1–5mM range, Cl^– played specific roles in regulating leaf osmotic potential and turgor, allowing plants to improve leaf water balance parameters. In addition, Cl^– also altered water relations at the whole-plant level through reduction of plant transpiration. This was a consequence of a lower stomatal conductance, which resulted in lower water loss and greater photosynthetic and integrated water-use efficiency. In contrast to Cl^–, these effects were not observed for essential anionic macronutrients such as nitrate, sulphate, and phosphate. We propose that the abundant uptake and accumulation of Cl^– responds to adaptive functions improving water homeostasis in higher plants.