Terminalia arjuna Switches from Adaptive to Survival Strategy Under Severe Water Stress

Terminalia arjuna (Arjun) is a tropical deciduous tree species significantly valued for its pharmaceutical properties for various heart diseases, as well as its economic role in the sericulture industry. However, the growth performance and physiological responses of T. arjuna under water stress conditions remain largely unexplored, particularly in the context of increasing climate variability and the growing challenges posed by climate change. Therefore, this study aimed to examine the morpho-physio-biochemical alterations, nutrient uptake changes, and adaptive strategies under different degrees of water stress with respect to field capacity (Fwc), maintained at 100% Fwc (control), 75% Fwc (mild), 50% Fwc (moderate), and 25% Fwc (severe). Key growth parameters, including shoot and root length, leaf traits and shoot dry biomass, were significantly (p < 0.05) reduced under the given water stresses. Root dry biomass showed a distinct response, increasing under mild to moderate water stress but failing to sustain its levels under severe stress. Increasing drought severity resulted in a substantial reduction in stomatal density (15–37%), while stomatal size increased (18–49%) under mild to moderate stress but decreased under severe stress. These responses were associated with significant reductions in gas exchange traits (45–75%), whereas water use efficiency increased by 59–99%, reflecting a survival-focused adaptive mechanism. Moderate water stress triggered the stress responses in T. arjuna through high proline accumulation and increased oxidative stress markers. The most critical impact was found under the severe stress with a substantial reduction in leaf relative water content and membrane stability index (MSI), although MSI was sustained above the critical threshold, reflecting cellular protection. Increased stress intensity also altered mineral uptake, decreased major nutrients, and increased potassium and calcium content, indicating an adaptive strategy. These findings suggest a threshold effect, where T. arjuna tolerates mild stress well and activates adaptive morpho-physiological mechanisms under moderate stress but shifts to survival-focused strategies under severe stress. The demonstrated tolerance of Terminalia arjuna to mild–moderate drought suggests that climate-resilient forestry policies and conservation programs should prioritize its cultivation and restoration in drought-prone landscapes while ensuring adequate water management to prevent severe stress and sustain its medicinal and economic benefits.