Dynamic physiological response of Mongolian pine ectomycorrhizal seedlings to drought and re-watering

Mongolian pine (<i>Pinus sylvestris</i> var. <i>mongholica</i>) is a key species for ecological restoration in northern China, frequently exposed to cyclical drought-rewatering stress in arid and semi-arid regions. While ectomycorrhizal fungi (EMF) are known to enhance plant drought tolerance, their mechanistic role in mediating Mongolian pine's response to drought-rewatering cycles remains unclear. In this study, two-year-old Mongolian pine seedlings inoculated with <i>Rhizopogon</i> sp. (Rh) and <i>Tomentella</i> sp. (To) fungi. Outdoor pot experiments were conducted under controlled soil moisture regimes, establishing five hydrological gradients: well-watered control (CK0), mild drought (D7), moderate drought (D14), severe drought (D21), and extreme drought (D35), following by rewatering 1 day after the end of each stress period. Additionally, we implemented four inoculation treatments: a non-inoculated control, Rh inoculation, To inoculation, and Rh+To co-inoculation. We measured seedling growth, photosynthesis parameters, water, osmotic regulatory substances, antioxidant enzyme activities, as well as drought resistance, drought recovery ability and drought adaptation ability, along with their intercorrelations. The results demonstrated that (1) drought stress significantly reduced seedling photosynthetic, fluorescence parameters, and water potential indicators (<i>P</i> < 0.05). Specifically, photosynthetic and fluorescence parameters, leaf relative water content (RWC), and plant water potential declined progressively with increasing stress intensity increased. (2) After re-watering, physiological indicators under different drought stress degrees exhibited varying degrees of recovery. Photosynthetic and fluorescence parameters exceeded those observed during drought stress treatments, while water use efficiency (WUE) and root water potential (ψ_r) showed complex recovery patterns, in some cases exceeding levels observed in the well-watered control group (<i>P</i> < 0.05). (3) EMF inoculation, especially Rh+To, effectively alleviated drought-induced physiological inhibition, enhancing seedling drought resistance, recovery, and adaptation. (4) Drought recovery capability was closely linked to drought adaptation was critical for overall resistance. Maintaining relatively high RWC during drought periods and preserving elevated the maximal photochemical efficiency of photosystem II (Fv/Fm) values during re-watering emerged as key factors for enhancing drought resistance in Mongolian pine seedlings.