# The Irrigation Water pH Has a Dominant Impact on the Growth and Stress Markers of Bigleaf Hydrangea
## 4 Discussion
While the low, slightly acidic pH (4) might seem like a stressor, our results suggest it likely provided optimal nutrient availability and uptake and optimized the solubility and availability of iron and manganese [60; 61; 62], which are crucial for various physiological processes, including those underpinning robust vegetative growth and flower initiation. As stated by [PERSON] and [PERSON] [63], the effects of pH on nutrient availability are not solely due to the effects of reaction with soils but are an interaction between these effects and the effects on the rate of uptake by plants. Also, in this study, no visible changes such as necrosis or leaf color changes were observed. Despite statistical significance within minor changes in irrigation water pH and EC (narrow range) in most of the observed morphological characteristics, both varieties maintained their aesthetically attractive appearance, flower size and color until the end of the growing season. Therefore, the visual score analysis for all study treatments showed a score of 5, corresponding to 0% damage. Nevertheless, the principal component analysis (PCA) effectively visualized the distinct morphological strategies of the two varieties. A clear separation along the PC1 axis revealed a fundamental trade-off: Early Blue tended towards taller plants with larger flower diameters, while Bianco produced a higher number of stems and flowers, resulting in a bushier habit with greater leaf mass. This finding is of importance for commercial growers, as the choice of variety can be tailored to the desired aesthetic outcome. Early Blue produces prominent, large blooms and Bianco produces dense, multi-flowered plants. Despite these significant differences in morphology, it is crucial to note that there was no statistically significant difference in the proline or MDA responses between the two varieties. This suggests that while their genetic blueprints dictate different growth habits, their fundamental physiological tolerance to the tested pH and EC stresses was comparable. To mitigate abiotic stress, plants employ several methods of avoidance, adaptation, and acclimation [64]. As stated by [PERSON] et al. [65], plants under abiotic stress can adapt to changing environmental factors through phenotypic plasticity, whereby the plants' salt and pH tolerance vary among species, and even cultivars within a species [66; 67; 68; 69; 70]. However, limited information is available on the salt tolerance of hydrangea, particularly among different species and cultivars [71]. It seems that the hydrangea varieties used in this study have the same adaptive response to abiotic stress, yet different morphological responses to abiotic stress. Similar findings were reported for alfalfa [72], hydrangea [73], _Pennisetum glaucum_ L. [74], _Acer rubrum_ L. [75] in response to various abiotic stresses. Furthermore, the proline and MDA data provide a clear physiological explanation for the morphological results. Proline, an osmoprotectant, and MDA, an indicator of lipid peroxidation and cellular damage, were lowest at pH 6 and EC 2--the same treatments that generally yielded the best morphological parameters. Conversely, their concentrations were highest at the extremes of pH 4 and EC 4, confirming that these conditions were physiologically stressful for the hydrangea plants. The strong positive correlation between proline and MDA (r = 0.86 for both varieties) further reinforces that the plants were experiencing significant oxidative stress under these adverse conditions [43; 76]. The negative correlations observed, particularly between stress indicators (proline/MDA) and growth parameters (e.g., leaf weight and leaf number), are also very telling. For instance, in Early Blue, as proline increased, leaf weight and the number of leaves decreased. This directly supports the conclusion that the physiological stress experienced by the plants actively inhibited their ability to produce and maintain biomass. The extremely high levels of proline and MDA at pH 4 indicate a significant stress response.