# The Irrigation Water pH Has a Dominant Impact on the Growth and Stress Markers of Bigleaf Hydrangea
## 1 Introduction
This is also confirmed by [PERSON] and [PERSON] [26] who claim that pH is considered to be the \"master variable\" in soil chemistry due to its profound impact on countless chemical reactions involving essential plant nutrients, phytotoxic elements, and pollutants. These research findings indicate that irrigation water with a high pH, in contrast to EC (electrical conductivity), is detrimental to the stem length and flower diameter of _Ranniculus asiaticus_[27], reduces the dry flower mass of margold [28], and results in a significant loss of margold's aesthetic value [29]. Furthermore, as stated byLoganathachetti and [PERSON] [30], bacterial diversity was lower in root and community patterns where irrigation water pH was a key structuring factor compared to EC. It is also important to note the moderate tolerance of hydrangea to the low salinity levels applied in this study. For instance, [PERSON] et al. [31] demonstrated that _H. macrophyllulla_ exhibited moderate tolerance to soil salinity at an EC range from 2.0 to 4.0 dS\(\cdot\)m\({}^{-1}\). However, a further, even smaller increase in EC values to 5.65 dS\(\cdot\)m\({}^{-1}\) resulted in a smaller plant size and reduced dry weight of aerial organs, including leaves, infloresances, and flowers [32]. Abiotic stresses, such as unfavorable pH and EC, negatively impact plant growth and productivity [18; 33], whereby plants respond through complex signaling pathways involving stress perception, signal transduction (including calcium--Ca\({}^{2+}\), reactive oxygen species--ROS, hormones), and reprogramming of gene expression, leading to metabolic and physiological adaptations [34; 35]. A common result of many abiotic stresses is oxidative stress, caused by excessive ROS accumulation, and plants defend themselves with a sophisticated antioxidant system (enzymatic and non-enzymatic) [36; 37; 38]. Oxidative stress is key to understanding plant responses to pH and EC variations and potential differences among varieties, while biochemical markers quantify the cellular stress response, i.e., malondialdehydedehyde (MDA), a product of lipid peroxidation, is a reliable indicator of the level of oxidative membrane damage caused by ROS [39]. Furthermore, proline, an amino acid, accumulates in response to osmotic stress (high EC, drought) and acts as a compatible osmolyte (helping to maintain water balance), osmoprotectant (protecting cellular structures), and potentially as an antioxidant [40; 41; 42]. Measuring MDA (an indicator of damage) and proline (an indicator of adaptive response) provides complementary information about varieties' coping strategies under pH- and EC-induced stress. In alkaline conditions, micronutrients like iron (Fe) and manganese (Mn) become insoluble and unavailable for root absorption, causing iron deficiency chlorosis. This condition inhibits chlorophyll synthesis and photosynthesis, ultimately suppressing plant growth and biomass. For hydrangeas, pH-regulated iron availability is crucial for healthy development [11]. Elevated substrate salinity creates osmotic stress by lowering water potential and hindering water absorption by the roots. To counteract this physiological drought, plants accumulate compatible osmolytes such as proline for cellular protection and osmotic adjustment [43]. Thus, an increased proline concentration serves as a reliable biochemical marker of osmotic stress. However, studies systematically evaluating the combined effects of pH and EC and the specific responses of MDA and proline in different hydrangea varieties are lacking [32]. We hypothesize that differences in the pH and EC of irrigation water will affect the morphological properties and biochemical responses, i.e., accumulation of stress markers (MDA, proline) in two distinct hydrangea varieties, reflecting their specific tolerance strategies.