Figure 2.

Stimulation of tobacco plant growth by macronutrient Cl⁻ contents. The data, obtained from Franco-Navarro et al. [2] and from Colmenero-Flores et al (umpublished results) illustrates growth stimulation in terms of dry weight biomass in a range of leaf Cl⁻ contents within the beneficial macronutrient range, which clearly overlaps with the previously reported toxic contents for this species [5]. There is a widespread belief that the accumulation of Cl⁻ in plant tissues does not necessary reflect the quantity requested. Chloride applications in the 1–5 mM Cl⁻ range determine increasing values of leaf Cl⁻ accumulation in a linear fashion, which in turn determine positive responses in terms of dry biomass, strongly supporting that plants regulate the required amount of Cl⁻ within the beneficial macronutrient range. Although the response remains positive with the application of 5 mM Cl⁻, the response curve inflection indicates that the trend can be reversed at higher Cl⁻ concentrations. For example, in a similar assay, in comparison to the 5 mM Cl⁻ treatment, 15 mM Cl⁻ application resulted in lower dry weight, indicating that this concentration exceeds the beneficial range of Cl⁻ nutrition.