# The Irrigation Water pH Has a Dominant Impact on the Growth and Stress Markers of Bigleaf Hydrangea
## 1 Introduction  
The genus _Hydrangeea_ (family Hydrangeaceae) comprises a diverse group of ornamental plants that are highly valued worldwide for their impressive and attractive flowers. As important horticultural plants, they are cultivated as shrubs, potted plants, and cut flowers [1], with key species including _Hydrangea macrophylula_ and _Hydrangea paniculata_[1,2]. The genus holds significant economic value; _H. macrophylula_ alone accounted for over $120 million in US sales in 2014 [1], highlighting the importance of research into optimizing their cultivation. Data on flower production in the Republic of Croatia is very scarce. According to the latest report from the Croatian Bureau of Statistics [3], in 2024, flowers were cultivated on 162 hectares (including open fields and greenhouses). The Croatian Chamber of Economy [4] states that Croatia annually imports 1376 t of flowers worth 7.44 million euros, while simultaneously exporting 152 t, valued at almost one million (960,000) euros. Furthermore, official data on the specific types of flowers cultivated in the Republic of Croatia are currently unavailable. Insights into the current market situation were gathered directly from growers, who plan their production and flower selection based on customer demand. Based on this information, two varieties of hydrangea, which are in highest demand, were chosen as the test plants for this research. Successful hydrangea cultivation depends on understanding environmental requirements such as semi-shade and moisture [2], as well as sensitivity to key substrate factors such as pH and electrical conductivity (EC) [1,5,6]. The existing diversity within the genus, including different ploidy levels [7], suggests the need for research into specific varieties' responses to these variables. pH fundamentally influences plant physiology by regulating nutrient availability [8,9], whereby an extreme pH (too high or too low) can lead to nutrient deficiencies or toxicities [8, 9, 10, 11]. In _H. macrophylula_, pH directly controls the color of flower sepals through aluminium (Al\({}^{3+}\)) availability: acidic soils (pH 4.5-5.5) result in blue flowers due to higher Al\({}^{3+}\) solubility and uptake, while neutral to alkaline soils (pH \(\geq\) 6.0) lead to pink or red flowers due to reduced Al\({}^{3+}\) availability [1,12]. Furthermore, managing pH in hydrangea cultivation is crucial for achieving desired aesthetic outcomes and ensuring overall plant health, balancing the need for low pH to produce blue flowers against the risk of acidity stress or other nutritional imbalances [1,13]. Electrical conductivity (EC) measures the total concentration of dissolved salts in the substrate or irrigation water, which is crucial for nutrient management [14, 15, 16, 17, 18]. Low EC indicates nutrient deficiency, leading to reduced growth [6,18], while high EC (salinity) stresses plants through osmotic stress (hindering water uptake) and ionic toxicity (accumulation of harmful ions like sodium (Na\({}^{+}\)) and chloride (Cl\({}^{-}\)), disrupting metabolism) [6,13,17]. Previous study results indicate that the optimal EC level and tolerance vary depending on the species and varieties [13,16,19]. Accordingly, both parameters of irrigation water quality have an important direct or indirect impact on the growth and quality of hydrangeas. However, due to the interaction of these parameters, sources in the literature emphasize the dominance of water pH. [PERSON] et al. [20] claim that knowledge of irrigation water pH is critical for crop production. Although the EC primarily affects osmotic potential and plant water uptake [21] by reflecting the total content of macro- and micro-elements available to plants [22], pH directly controls the chemical forms of these ions and their availability to plants, as well as influencing plant physiological functions [23, 24, 25].