Abstract
<title>Abstract</title> <p> Ultraweak luminescence (UWL) is a spontaneous photon emission derived from metabolic processes in living organisms. Despite its low intensity, UWL is closely linked to plant physiological metabolism, though its mechanisms remain poorly understood. In this study, <italic>Cabernet Sauvignon</italic> grapevines were used as plant material, and drought stress was simulated through controlled water-deficit treatments. We examined the associations between UWL intensity, reactive oxygen species (ROS), photosynthetic characteristics, and energy status. Drought stress caused a gradual decline in UWL intensity in grapevine leaves, alongside reduced soil and leaf water content and altered water distribution—decreased free water and increased bound water. Membrane damage was evident through elevated ROS indicators, including hydrogen peroxide (H₂O₂), superoxide anion radical (O₂⁻·) production rate, malondialdehyde (MDA) content, and relative electrolyte conductivity. Photosynthetic function deteriorated significantly, with decreased chlorophyll content, gas exchange parameters, including net photosynthetic rate (Pn), stomatal conductance (Gs), and chlorophyll fluorescence parameters, including Fv/Fm, effective quantum yield (Yield), and photochemical quenching (qP), while intercellular CO₂ concentration (Ci) and non-photochemical quenching (qN) increased. Energy metabolism was suppressed, with reduced ATP levels and energy charge, and increased ADP and AMP. Correlation analysis showed strong negative relationships between UWL intensity and ROS markers, and positive associations with photosynthetic traits, fluorescence metrics, ATP levels, and energy charge. These results suggest that UWL reflects drought-induced physiological stress, particularly involving photosynthesis and energy status. ROS likely influence UWL indirectly via membrane and metabolic disruption. </p>