Abstract: Abscisic acid (ABA) is a key endogenous hormone involved in the regulation of seed dormancy, stomatal movement, root architecture and plant responses to abiotic stresses. Rapid and reliable monitoring of ABA in living leaves is therefore important for evaluating crop stress status and supporting precision agricultural management. However, conventional methods such as high-performance liquid chromatography, liquid chromatography–mass spectrometry and enzyme-linked immunosorbent assay generally require co the type-II band alignment and the resulting suppression of electron-hole recombination. On this basis, an ABA-specific aptamer was immobilized on the BiOI/g-C₃N₄/ITO electrode to construct a signal-enhanced PEC sensing interface. The binding between ABA and the aptamer induced conformational rearrangement of the recognition layer, reduced the interfacial charge-transfer resistance and further promoted electron transport, leading to an increased cathodic photocurrent. Under optimized conditions, the fabricated aptasensor showed a good linear relationship between photocurrent intensity and the logarithm of mplex sample pretreatment, expensive instruments and laboratory-based operation, which limits their use for on-site and dynamic analysis. In this study, a photoelectrochemical (PEC) aptasensor based on a bismuth oxyiodide/graphitic carbon nitride (BiOI/g-C₃N₄) type-II heterojunction was developed for in situ detection of ABA in tomato leaves under salt stress. BiOI was selected as the main visible-light-responsive semiconductor, while g-C₃N₄ was introduced as both a carrier and a photosensitizer to improve the separation and transfer of photogenerated charge carriers. The BiOI/g-C₃N₄ composite was prepared by a simple physical mixing and stirring-assisted method and was systematically characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy. The results confirmed that g-C₃N₄ nanosheets were tightly coupled with flower-like BiOI microspheres, forming an effective heterojunction interface with enhanced visible-light absorption and improved interfacial charge transport. Compared with pure BiOI, the BiOI/g-C₃N₄-modified electrode exhibited an approximately 13-fold higher photocurrent response, which was attributed toABA concentration in the range of 1 pmol/L to 300 nmol/L, with a correlation coefficient of 0.974 and a detection limit of 0.33 pmol/L. The sensor also exhibited high selectivity against common coexisting substances, including inorganic ions, amino acids, sugars and other plant hormones, even when the interferents were present at 100-fold higher concentrations. In addition, stable responses were maintained within the temperature range of 15～35℃, and 91.8% of the initial signal was retained after 14 days of storage at 4℃. Recovery tests in tomato leaf extracts gave recoveries of 93%～106% with relative standard deviations of 1.1%～2.1%, indicating acceptable accuracy and precision in complex plant matrices. Finally, the paper-based PEC sensing system was applied to the in situ monitoring of ABA in tomato leaves during salt stress. The ABA level increased rapidly at the early stage of stress, reached 0.896 nmol/L at 24 h and then decreased to 0.417 nmol/L at 72 h. This trend was consistent with the physiological role of ABA in early osmotic stress adaptation and subsequent metabolic adjustment. These results demonstrate that the proposed BiOI/g-C₃N₄ PEC aptasensor provides a sensitive, selective and practical approach for dynamic ABA monitoring in plant leaves and offers a useful sensing strategy for crop stress diagnosis and precision agriculture.