Dual-Isotope (δ2H, δ18O) and Bioelement (δ13C, δ15N) Fingerprints Reveal Atmospheric and Edaphic Drought Controls in Sauvignon Blanc (Orlești, Romania).

Grapevine water relations are increasingly influenced by drought under climate change, with significant implications for yield, fruit composition and wine quality. Stable isotopes of hydrogen, oxygen, carbon and nitrogen (δ2H, δ18O, δ13C and δ15N) provide sensitive tracers of plant water sources and physiological responses to stress. Here, we combined dual water isotopes (δ2H, δ18O), carbon and nitrogen isotopes (δ13C, δ15N), and high-resolution micrometeorological/soil observations to diagnose drought dynamics in Vitis vinifera cv. Sauvignon blanc (Orlești, Romania; 2023-2024). Dual-isotope relationships delineated progressive evaporative enrichment along the soil-plant-atmosphere continuum, with slopes LMWL ≈ 6.41 > stem ≈ 5.0 > leaf ≈ 2.2, consistent with kinetic fractionation during transpiration (leaf) superimposed on source-water signals (stem). Weekly leaf δ18O covaried strongly with relative humidity (RH; r = -0.69) and evapotranspiration (ET; r = +0.56), confirming atmospheric control of short-term enrichment, while stem isotopes showed buffered responses to soil water. We integrated Δ18O (leaf-stem), RH, ET, and soil matric potential at 60 cm (Soil60) into an Isotopic Drought Index (IDI), which captured the onset, intensity, and persistence of the July-August 2024 drought (IDI0-100 > 90; RH < 60%, ET > 40 mm wk-1, Soil60 > 100 cb). Carbon and nitrogen isotopes provided complementary, integrative diagnostics: δ13C increased (less negative) with drought (r = -0.52 with RH; +0.49 with IDI), reflecting higher intrinsic water-use efficiency, whereas δ15N rose with soil dryness and IDI (leaf: r ≈ +0.48 with Soil60; +0.42 with IDI), indicating constraints on N acquisition and enhanced internal remobilization. Together, multi-isotope and environmental data yield a mechanistic, field-validated framework linking atmospheric demand and edaphic limitation to vine physiological and biogeochemical responses and demonstrate the operational value of an isotope-informed drought index for precision viticulture.