Does Potassium Modify the Response of Zinnia (Zinnia elegans Jacq.) to Long-Term Salinity?

Salinity is one of the major abiotic stress factors hindering crop production, including ornamental flowering plants. The present study examined the response to salt stress of Zinnia elegans ‘Lilliput’ supplemented with basic (150 mg·dm−3) and enhanced (300 mg·dm−3) potassium doses. Stress was imposed by adding 0.96 and 1.98 g of NaCl per dm−3 of the substrate. The substrate’s electrical conductivity was 1.1 and 2.3 dS·m−1 for lower potassium levels and 1.2 and 2.4 dS·m−1 for higher potassium levels. Salt stress caused a significant and dose-dependent reduction in leaf RWC, increased foliar Na and Cl concentrations, and reduced K. About 15% and 25% of cell membrane injury at lower and higher NaCl doses, respectively, were accompanied by only slight chlorophyll reduction. Salt stress-induced proline increase was accompanied by increased P5CS activity and decreased PDH activity. More than a 25% reduction in most growth parameters at EC 1.1–1.2 dS·m−1 but only a slight decrease in chlorophyll and a 25% reduction in the decorative value (number of flowers produced, flower diameter) only at EC 2.3–2.4 dS·m−1 were found. Salt stress-induced leaf area reduction was accompanied by increased cell wall lignification. An enhanced potassium dose caused a reduction in leaf Na and Cl concentrations and a slight increase in K. It was also effective in membrane injury reduction and proline accumulation. Increasing the dose of potassium did not improve growth and flowering parameters but affected the lignification of the leaf cell walls, which may have resulted in growth retardation. Zinnia elegans ‘Lilliput’ may be considered sensitive to long-term salt stress.