Growth, physiological and biochemical responses of Tribulus terrestris herb to water deficit stress

Tribulus terrestris is a valuable medicinal plant known for its drought tolerance, yet its physiological and biochemical responses to soil water deficit remain poorly understood. In this study, four-week-old seedlings were subjected to controlled soil moisture levels corresponding to 100% (control), 75% (mild drought), 50% (moderate drought), and 25% (severe drought) of field capacity for 14 days; from which changes in growth, leaf relative water content (RWC), proline accumulation, contents of photosynthetic pigments, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) of the plant under the drought treatments were determined to understand its responses. Results demonstrated a significant reduction in whole-plant fresh biomass (28.8–36.7%) and dry biomass (20.8–41.3%) under increasing drought severity, with shoots more affected than roots. Leaf RWC remained stable (88–92%), while proline content increased 2.2–6.2-fold under moderate and severe stress, indicating osmotic adjustment. Chlorophyll a, b, and total chlorophyll decreased only under severe drought (down to 60% of control), whereas carotenoids were unchanged until severe stress. H₂O₂ and MDA levels rose significantly under severe drought, reflecting oxidative stress. Our findings suggest that T. terrestris maintains water status and osmotic balance under mild to moderate drought, but severe water deficit impairs photosynthesis and induces oxidative damage. The plant might possess related mechanisms that adapt to drought stress, and further research on antioxidant enzyme activities and molecular mechanisms is warranted.