# The Irrigation Water pH Has a Dominant Impact on the Growth and Stress Markers of Bigleaf Hydrangea
## 4 Discussion
It is possible that the highly acidic conditions, while potentially increasing the number of flowering branches, were detrimental to the overall health and biomass of the plant, essentially forcing a stress-induced flowering response rather than promoting healthy growth [32; 50]. This highlights a critical trade-off between the number of flowers and the overall quality and size of the plant. While less impactful than pH, EC also significantly affected morphological parameters, including plant height and flower and leaf weight, with a non-linear response to EC observed. The highest stress markers (proline and MDA) were observed at the highest EC level (EC 4), which is an expected outcome as elevated salinity induces osmotic stress and ion toxicity [16; 77; 78] and stimulates the accumulation of proline [79]. The data show a direct and significant correlation between the increasing EC of the irrigation water and the concentration of proline in the hydrangea leaf tissue. Specifically, the highest proline concentration (4880.794 umol g\({}^{-1}\) FW) was measured in the EC 4 treatment, which represented the highest salinity level in this study. When subjected to high salinity (a high EC value), the plant experiences osmotic stress, making it difficult for the roots to absorb water from the substrate [80]. However, the optimal morphological results were often found in the moderate EC treatments (EC 2) rather than the lowest (EC 1). This suggests that the EC 1 treatment may have provided insufficient nutrient concentrations for maximal growth, while the EC 4 treatment began to cause detrimental salinity effects. Therefore, an optimal irrigation water EC range exists that supplies adequate nutrients without inducing stress, and our findings place this optimum level at around the EC 2 and EC 3 levels tested in this study. This result aligns well with the findings of a previous study by [PERSON] et al. [32], wherein watering with irrigation water with EC 5.65 dS m\({}^{-1}\) severely decreased the size of all hydrangea aerial organs (leaves, inflorescences, and flowers), reduced the dry weight of the aerial part by 70%, delayed flowering development, and produced a darker, pink-colored flower compared with freshwater treatment. The same authors claim that a high EC also caused tip and edge burn in the leaves, the overall effect being a non commercial plant. Similar findings are reported by [PERSON] et al. [71] who assessed the relative salt tolerance of 11 cultivars of hydrangea, including _H. macrophylla_. The authors noted that the leaf area and total shoot dry weight (DW) of all cultivars in EC 5 and EC 10 treatments were significantly reduced compared to the control treatment (EC 1) and that the high EC levels were negatively correlated with visual quality, leaf area and shoot DW. In this study, the highest EC level (4) induced clear salinity stress, evidenced by the peak concentrations of proline and malondialdehydehydrode (MDA). This was due to a combination of osmotic stress, which limits water uptake and triggers proline accumulation for cellular protection and direct ion toxicity, which causes the cellular membrane damage indicated by the rise in MDA [76]. The highest GI (Figure 3h) was recorded at pH 6 (36 cm). A further decrease in pH led to a reduction in GI by 16.7% at pH 5 and a substantial 72.8% at pH 4. Conversely, an increase to pH 7 resulted in a 40.3% reduction. These findings align with [PERSON] et al. [1], who report that hydrangeas thrive best in soil or media with a pH range of 5.5-6.5. Regarding EC, the highest GI value (Figure 4h) was observed in the EC 2 treatment, reaching 27.2 cm. Further increases in EC led to significant GI reductions: 30.2% at EC 3 and 44.9% at EC 4. Conversely, in the control treatment (EC 1), the GI value was 12.1% lower compared to EC 2. Therefore, concerning vegetative growth, this study indicates that the most favorable irrigation water pH and EC values are pH 6 and EC 2, respectively.