Chloride nutrition provides a more efficient use of nitrogen (NUE) in tobacco and tomato plants

1. Background and Objectives: 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 supposedly unspecific role as cell osmoticum, no clear biological roles had been explicitly associated with Cl- when accumulated to macronutrient concentrations until our group published a novel perspective on the matter, in which we proposed Cl- to be both a micronutrient and a beneficial macronutrient for higher plants (Franco-Navarro et al. 2016). We showed that 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 1-5 mM range, Cl- played specific roles in regulating leaf osmotic potential and turgor, allowing plants to improve leaf water balance at both cellular and whole-plant level. Regarding the ion content Cl¿, which is not assimilated, was preferentially accumulated over other anionic macronutrients like NO3-, SO42- and PO43- that are assimilated in anabolic processes. Our data supported the NO3¿ diversion hypothesis by which macronutrient accumulation of Cl¿ prevents vacuolar compartmentalization of NO3¿ as an osmolyte or charge-balancing molecule, facilitating its assimilation and increasing plant biomass. This would represent a radical change in the perception of Cl¿ as an NO3¿ antagonist. Our working hypothesis and the objective of this work was to verify whether or not Cl- is a nutrient that promotes a more efficient use of nitrogen in two different species (tobacco and cherry tomato plants) of the Solanaceae family. 2. Material and Methods: Tobacco plants (Nicotiana tabacum L. var. Habana) and Cherry tomato plants (Solanum lycopersicum L. cv Zarina) were alternatively treated with a basal nutrient solution supplemented with three salt combinations containing the same cationic balance: 5 mM Cl--based (CL), 5 mM nitrate-based (N) and sulphate + phosphate-based (SP) treatments. Experimental design, greenhouse conditions, nutrient list of the irrigation treatments, harvesting procedures, anion determination and statistical analysis are fully described at Franco-Navarro et al. (2016). The equations that determine NUE parameters are described in Ríos et al. (2010). 3. Results: Plant biomass, anion content and NUE parameters will be presented. 4. Conclusions: The NO3¿ diversion hypothesis is clearly supported by a better NUE in Cl-treated plants coinciding a boost in biomass production. All parameters measured were quite similar in two different species from the same family. This gives light to a brand-new interpretation of Cl- properties as a beneficial macronutrient in higher plants, and therefore a potential resource in agricultural production of both crops. Franco-Navarro et al. (2016). J. Exp. Bot., 67. Pages 873-891 Ríos et al. (2010). J. plant growth regul., 29(2). Pages 164-170 This work was supported by the Spanish Ministry of Science and Innovation-FEDER grants AGL2009-08339/AGR and AGL2015-71386-R.