The balanced supply of Na⁺ and Cl^– promotes dry biomass accumulation and nutritional quality improvement of pakchoi

Objectives The optimum concentration of Na⁺ or Cl^– is beneficial to plant growth. Yet, it is unclear which one plays a dominant role and how the balanced relationship between the elements presents better beneficial effects. Therefore, this study was conducted to provide a reference for applying a low concentration of sodium chloride in leafy vegetable production.

Methods The seeds of pakchoi (Brassica campestris L. ssp. chinensis var. communis Tsen et Lee) were planted in a pot filled with a soilless matrix. Foliar application of Na⁺ and Cl^– was consecutively carried out at the 3-true-leaves stage. The treatments were composed of three groups and three levels of Na⁺ and Cl^–. The different groups were Na⁺= Cl^– (B), with Na⁺ and Cl^– at 6 mmol/L (B₁), 12 mmol/L (B₂), and 18 mmol/L (B₃). The second group had Na⁺< Cl^– (A), Na⁺∶ Cl^– at 0∶6 mmol/L (A₁), 6∶12 mmol/L (A₂), and 12∶18 mmol/L (A₃). The third group had Na⁺> Cl^– (C), with Na⁺∶ Cl^– at 6∶0 (C₁), 12∶ 6 mmol/L (C₂), and 18∶12 mmol/L (C₃) , respectively. We sprayed deionized water simultaneously as control (CK). The leafy tissue structure was observed. The fresh and dry biomass, leaf photosynthetic activity, and free amino acid content were measured immediately after spraying.

Results Compared with CK, all the Na⁺ and Cl^– treatments (P<0.05) increased the fresh and dry biomass and leaf photosynthetic activity of pakchoi. The Na⁺= Cl^– group also increased the free amino acid content and leaf area, especially the middle and upper leaves. The highest dry and fresh biomass and leaf area were recorded in B₂, which were increased by 35.1%, 43.7%, and 33.4%, respectively. Cl^– favored water accumulation in the absence of Na⁺, while Na⁺ favored dry biomass accumulation in the absence of Cl^–. Both elements (Na⁺ and Cl^–) favored water accumulation. Na⁺ was more conducive to the accumulation of free amino acids than Cl^– (i.e. Na⁺>Cl^– , 52.6% increase), Cl^– was more conducive to the accumulation of dry biomass and photosynthetic rate (i.e. Na⁺<Cl^–, 17.3% increase) and stomatal conductance (i.e. Na⁺<Cl^–, 145.3% increase) than Na⁺. A 2-factor intersubjective effect test showed both Na⁺ and Cl^– (P<0.05) affect the dry biomass, fresh weight, nutrients content, leaf area, specific leaf weight, and leaf gas exchange of pakchoi at the main and interaction levels.

Conclusions Foliar spraying of Na⁺ and Cl^– could promote the growth, dry biomass accumulation, and leaf photosynthetic activity of pakchoi. Supplying Na⁺ and Cl^– in equal concentration had a higher effect than unequal supply, and the optimum spraying concentration was NaCl 12 mmol/L.