Plant spectral signatures can reveal specific metabolic responses through variations in reflectivity. However, the link between canopy reflectance and photosynthetic behaviour under water stress has been scarcely studied in potato, especially concerning xylem water potential. This study investigated the relationships between xylem water potential, gas exchange variables (net photosynthesis, stomatal conductance, and transpiration), and changes in the spectral signature of the visible region. Measurements were performed at the tuber differentiation and maximum tuberization phenological stages under three water stress intensities. There was a strong relationship between visible-range reflectance and gas exchange variables across all stress levels and growth stages. The Blue/Red Index (400R/690R) and the Blue/Green Index (450R/550R), along with their foundational spectral bands (550 nm and 710 nm), were powerful predictors of gas exchange dynamics and for the detection of moderate-to-severe water stress in potatoes. These results contribute to the monitoring of photosynthetic performance and the detection of water stress in potato plants, providing a direct pathway to integrate these visible-range indicators into precision agriculture platforms.