# The Irrigation Water pH Has a Dominant Impact on the Growth and Stress Markers of Bigleaf Hydrangea
## 4 Discussion  
The objective of this study was to analyze the effect of irrigation water pH and electrical conductivity (EC) on the morphological properties and biochemical responses of two hydrangea varieties (Early Blue and Bianco). Growing conditions during the study period were ideal for hydrangeas: the substrate water content (SWC) was in the range of readily available water (RAW, Figure 2), air temperature was moderate, and air humidity was high (Figure 1). Sources from the literature list different optimal SWC and MAD values for growing hydrangeas, such as 0.24 m\({}^{3}\) m\({}^{-3}\)[49] and 0.37 cm\({}^{3}\) cm\({}^{-3}\)[50] for pine bark substrate, suggesting that hydrangeas require relatively high water content in the growing substrate to maximize plant growth and avoid water stress [51]. Generally, hydrangeas require high air humidity, while a decrease in humidity leads to a decrease in height and in leaf growth [52]. As previously stated by [PERSON] et al. [53], temperatures between 15 and 18 \({}^{\circ}\)C promote generative growth, and temperatures above 24 \({}^{\circ}\)C enhance the vegetative growth of hydrangeas. The most important finding of this study was the effect of irrigation water pH on nearly all observed morphological properties (Table 3) as well as proline and MDA concentrations  
Figure 9: Biplot of principal components (left: 1 and 2, right: 1 and 3) from PC analysis of variation between two cultivars based on morphological properties, proline and MDA concentrations. Arrows represent the eigenvalues of each of the selected parameters. (Table 4). Our findings emphasize the importance of an optimal irrigation water pH range in hydrange cultivation, which is contrary to the findings of [PERSON] et al. [54] who claim that the pH of the irrigation water is not an acceptable criterion of water quality because the water pH tends to be buffered by soil, and most crops can tolerate a wide pH range. In this study, the highest values for most morphological parameters related to plant size and biomass, specifically plant height, leaf number, and the weight of both leaves and flowers, were observed at pH 6 (Figure 3). This result is consistent with established horticultural knowledge of acid-loving plants like hydrangeas [11]. Generally, a substrate pH in a slightly acidic range (5.5 to 6.5) is known to optimize the availability of essential micronutrients such as iron (Fe), manganese (Mn), aluminum (Al), and macronutrients, promoting robust vegetative and floral development [11; 47; 48], the micronutrients become less available at higher, alkaline pH levels [25]. This explains why the plants in our pH 6 treatment exhibited the greatest biomass (leaf and flower weight, (Figure 3f,g) and lowest stress indicators (Figure 6a,b) because plants were not limited by nutrient deficiency or toxicity and could perform metabolic functions like photosynthesis and protein synthesis efficiently. Conversely, limited growth (Figure 3) was recorded at pH 7, which likely induced nutrient deficiencies, particularly in iron, leading to reduced biomass accumulation [11; 47]. An interesting finding is that the highest stem (Figure 3b) and flower number (Figure 3c) were recorded at a low pH (4), usually associated with a change in hydrangea flower color [55; 56], which was not observed in this study. Although it is well-established that soil pH significantly influences anthocyanin pigment synthesis in _H. macrophulla_, leading to blue coloration at lower pH values and pink/red coloration at higher pH values, it should be noted that not all cultivars react with the same intensity or speed [57; 58]. Some varieties may require more extreme or prolonged low-pH conditions to exhibit a noticeable color shift, or the genetic background of the Early Blue variety tested might predispose it to less dramatic color changes. Nevertheless, as stated by [PERSON] [59], some varieties do not produce blue blooms, regardless of pH.