Examining Turgor Maintenance Mechanisms in Rhododendron arboreum and Quercus lanata Within Shivapuri Nagarjun National Park, Nepal

This study aims to analyze the maintenance of turgor and phenological activities in two dominant evergreen trees, Q. lanata, and R. arboreum, as well as a shrub, all of which are found within Shivapuri Nagarjun National Park. Several parameters, including pressure–volume analysis, xylem conductance, twig starch content, specific leaf mass (SLM), and phenomorphological cycles, were measured every month from February 2019 to January 2020. Noteworthy fluctuations were observed in the water relations of the tissues, indicating variations in turgor maintenance for both species. Osmotic potential at zero turgor (Ψsz) and osmotic potential at full turgor (Ψsf) decreased during dry months, suggesting that Q. lanata and R. arboreum exhibited elastic adjustment to preserve turgor and displayed a considerable level of drought tolerance. Xylem conductance strongly correlated with phenological activities, as indicated by increased Hv value during leaf senescence for both species. Starch content (starch0 and starch1) displayed significant seasonal variations and showed a notable association with phenological activities. SLM decreased during senescence and increased during leaf expansion and maturity. Moisture content (%) at a depth of 15 cm showed the lowest and highest values in August and from March to September, respectively. However, Q. lanata maintained similar moisture content to that at 15 cm at a depth of 30 cm, while R. arboreum displayed the lowest value in February. Soil moisture content exhibited significant variations across different months but not between the sampled species.