The Irrigation Water pH Has a Dominant Impact on the Growth and Stress Markers of Bigleaf Hydrangea

Hydrangeas are economically important ornamental plants whose growth and flower characteristics depend on irrigation water quality (IWC), i.e., hydrogenionic potential (pH) and electrical conductivity (EC). Unfavorable IWC causes plant stress, reduced growth and tissue damage, leading to physiological responses such as proline accumulation (for stress protection) and increased malondialdehyde (MDA, an indicator of damage). A greenhouse pot experiment was conducted as a three-factor study in three replicates. The study examined the impact of different pH levels (4, 5, and 6, compared to a control treatment of pH 7), and EC levels (2, 3, and 4 dS m−1, compared to a control treatment of 1 dS m−1) on biomass, i.e., plant height (cm), stem number (n), flower number (n), leaf number (n) and weight (g), flower weight (g) and diameter (cm), growth index (GI) and the proline and MDA concentrations in two hydrangea varieties (Early Blue and Bianco). Study results showed a significant impact of (p = 0.0001) pH on all tested morphological properties, except flower diameter. Notably, pH 6 maximized biomass accumulation, i.e., plant height (56.6 cm), leaf number (n = 97) and weight, flower weight (156.8 g), and GI (36 cm), while pH 4 promoted the highest number of flowers (n = 10) and stems (n = 10), which are both crucial for aesthetic and market value. EC significantly (p = 0.001) affected plant height (EC 2 = 56.3 cm), flower (EC 2 = 181.9 g) and leaf weight (EC 3 = 148.2 g), and growth index (EC 2 = 27.2 cm). The lowest stress indicators (proline and MDA concentrations) were recorded at pH 6 (MDA = 0.215 µmol g−1 FW, proline = 659.5 µmol g−1 FW) and EC 2 (MDA = 0.551 µmol g−1 FW, proline = 4068.2 µmol g−1 FW). The highest MDA and proline concentrations were observed under extreme conditions of pH 4 (MDA = 1.257 µmol g−1 FW, proline = 12,811.7 µmol g−1 FW) and EC 4 (MDA = 0.692 µmol g−1 FW, proline = 4880.8 µmol g−1 FW). No significant effect of variety on proline and MDA concentrations was observed, while the highest GI was recorded for the Early Blue variety (24.3 cm). This research identifies pH 6 and EC 2 as key conditions for optimizing hydrangea biomass and reducing stress, offering practical guidelines for growers. The findings provide a foundation for developing precise irrigation water quality strategies in the commercial production of ornamental plants.