Abstract: Aims Approximately 19% of the photon flux density in sunlight is contributed by far-red light (FR, 700-800 nm), which plays a crucial role in plant photosynthesis. However, commonly used gas exchange systems are typically equipped with internal light sources that provide only blue (400-500 nm) and red (600-700 nm) wavelengths, with little or no emission in the far-red (700-800 nm) region. This omission may lead to inaccurate estimation of photosynthetic parameters and an underrepresentation of a plant’s true photosynthetic capacity under natural light conditions. To address this issue, the present study investigated the effects of supplementing far-red light to short-wavelength measuring light on photosynthetic parameters, and further examined whether such effects are influenced by growth light intensity. Methods One-year-old seedlings of the shade-tolerant tree species Quercus variabilis Bl. were used as the study material. A two-factor factorial design was applied to explore the combined effects of growth light intensity and measuring light quality on photosynthetic parameters. Two growth light environments with identical spectral characteristics (R/FR = 1.4 ± 0.01) were established: low light (photosynthetic photon flux density (PPFD) at 30% of full sunlight) and high light (PPFD at 70% of full sunlight). Seedlings were grown under these conditions for five weeks. Two types of measuring light were applied: (1) short-wavelength light (PPFD = 1100 μmol m⁻² s⁻¹); and (2) short-wavelength light supplemented with far-red radiation at a photon flux density of 300 μmol m⁻² s⁻¹ (PFDFR), maintaining an R/FR ratio of 1.4. For each measuring light condition, we measured steady-state photosynthetic and chlorophyll fluorescence parameters, as well as dark-light-dark induction curves of photosynthesis and fluorescence, and CO₂ response curves. Important findings Supplementing far-red light to the short-wavelength measuring light significantly affected most photosynthetic parameters of Quercus variabilis seedlings, and the extent of these effects varied with growth light intensity. For steady-state photosynthetic parameters, adding far-red light to the short-wavelength measuring light increased the maximum steady-state net photosynthetic rate (Amax) by 31% in the low light group and by 25% in the high light group. Maximum steady-state stomatal conductance (gmax) increased by 20% and 45% in the low and high light groups, respectively. The effective yield of PSⅡ photochemistry (ΦPSII) increased by 62% and 29%, while the photochemical quenching coefficient (qP) increased by 63% and 25%. The non-photochemical quenching (NPQ) also rose by 24% in the low light group and 7% in the high light group. However, there was no significant difference in the maximum carboxylation efficiency (Vcmax) among treatments. In the photosynthetic induction response, adding far-red light to the measuring light increased the maximum slope (SLmax) of the photosynthesis and stomatal conductance induction curves, and shortened the response time (τ) and lag time (λ) in leaves grown under low light. In contrast, leaves grown under high light showed little response. In addition, the addition of far-red light increased the maximum electron transport rate (Jmax) in the CO₂ response curves by 22% and 24% in the low and high-light groups, respectively. Measurements with added far-red light more closely reflect the actual photosynthetic performance of plants under natural light conditions, compared to conventional light sources without far-red light. Conclusion This study demonstrated that adding far-red light during photosynthetic measurements significantly altered most photosynthetic parameters in Quercus variabilis seedlings. The primary mechanism is that far-red light optimizes energy distribution between photosystem I and photosystem II, improves electron transport efficiency, and enhances light use efficiency. These effects collectively regulate the photosynthetic performance of the plants. The addition of far-red light to the measuring light significantly altered most photosynthetic parameters, highlighting the necessity of considering measuring light spectral quality when assessing photosynthesis in plants grown under natural light conditions. Accordingly, appropriately specifying the measurement light spectrum, especially by ensuring a sufficient far-red component, can improve the ecological relevance of derived parameters and enhance data comparability. This is important for plant physiological research and for field applications. However, this study was conducted under potted conditions with a single tree species and a fixed R/FR ratio during growth. Future research should extend to multiple species and a wider range of R/FR combinations to further verify the generality and applicability of far-red light regulatory effects.

Key words: Far-red light, Growth light intensity, Steady-state photosynthesis, Photosynthetic induction, Plant photosynthetic measurement, Photosynthetically active radiation, Excitation energy distribution