Abstract: Existing portable chlorophyll fluorescence instruments mainly provide basic physiological parameters, whereas obtaining an integrated indicator of electron transport across light levels often requires separate data processing. To support on-site evaluation of crop photosynthetic potential, this study developed a portable meter that combines modulated fluorescence detection, simultaneous measurement of photosynthetically active radiation (PAR), and embedded light-response analysis. The proposed photosynthetic potential index summarizes the response of apparent electron transport over a defined irradiance range; it is not a direct measure of carbon assimilation. The meter uses three independently driven light sources: measuring light (ML), actinic light (AL), and a saturating pulse (SP). A common controller coordinates their operation while fluorescence and PAR are measured at the leaf. The fluorescence channel combines narrow-band filtering, photodiode detection, transimpedance amplification, programmable gain adjustment, and synchronous phase-sensitive detection to process weak fluorescence signals. The built-in PAR channel records actual incident irradiance during each AL level. After a steady-state criterion is met, the system records steady-state fluorescence, applies an SP to obtain light-adapted maximum fluorescence, and pairs these measurements with the mean PAR reading from the same period.The system calculates the effective quantum yield of photosystem II (PSII) from the fluorescence measurements. Electron transport rate ( J ) is then estimated using PAR, a leaf absorptance coefficient of 0.84, and a PSII energy-partitioning factor of 0.5. Measurements at successive light levels form a J -PAR response curve. A modified rectangular hyperbola is fitted using the Levenberg-Marquardt algorithm to obtain initial quantum efficiency, maximum electron transport rate, and light saturation point. The integral of the fitted curve from zero irradiance to the light saturation point is defined as an integral-based photosynthetic potential index. The embedded program checks input validity and fitting results before reporting parameters. The fitted curve and derived parameters are displayed on the meter without separate offline processing.Broccoli seedlings and oilseed rape seedlings were used for preliminary validation. Under controlled illumination, a VSM-300AL-GH-V05 reference PAR sensor was used to assess the consistency of the built-in PAR channel. Monitoring Pen MP100 served as the fluorescence comparison instrument. Because the MP100 does not directly output J , its effective PSII quantum-yield readings were converted to J using the same PAR inputs and coefficients as the developed meter. The two instruments measured corresponding areas of the same leaves sequentially, and their J -PAR responses and derived parameters were compared.The built-in PAR channel showed a coefficient of determination of 0.9926 and a mean absolute relative error of 7.30% against the reference sensor. For both crops, J increased with PAR at low irradiance, approached a plateau, and declined slightly at high irradiance; the two instruments produced similar overall curve trends. Relative to the MP100-based results, the mean absolute relative errors across measured light levels were 9.18% for broccoli seedlings and 10.20% for oilseed rape seedlings. Across the two materials, the developed meter gave initial quantum efficiency, maximum electron transport rate, and integral-based index estimates that were higher by 9.42%-10.12%, 8.72%-9.19%, and 5.05%-5.62%, respectively, while light saturation point estimates were lower by 2.51%-3.38%.These findings provide preliminary evidence that synchronized fluorescence and PAR acquisition can be combined with embedded J -PAR modeling to output an integral-based photosynthetic potential index. The index complements conventional light-response parameters by incorporating information across the defined irradiance interval. Validation remains limited to two seedling materials, and sequential measurements may introduce differences in leaf position or light-adaptation state. Additional crops, measurement conditions, and tests of instrument stability and fitting sensitivity are needed to establish broader applicability.