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 (R, 600-700 nm) wavelengths, with little or no emission in the far-red region. This omission may lead to an 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 into 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) were used as the study material. A two-factor factorial design was applied to investigate 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 (LL, photosynthetic photon flux density (PPFD) at 30% of full sunlight (100% = 1 500 μmol·m^-2·s^-1)) and high light (HL, PPFD at 70% of full sunlight). Seedlings were grown and acclimated under these conditions for five weeks. Then two measuring light treatments were applied: (1) short-wavelength light (SWL, 1 100 μmol·m^-2·s^-1 PPFD); and (2) short-wavelength light supplemented with far-red radiation (SWL + FR, 1 100 μmol·m^-2·s^-1 PPFD + 300 μmol·m^-2·s^-1 far-red, PFD_FR), maintaining an R/FR ratio of 1.4. For each measuring light condition, steady-state photosynthetic parameters, and chlorophyll fluorescence kinetics, dark-light-dark induction curves of photosynthesis and fluorescence, and CO₂ response curves were measured.

Important findings The addition of far-red light into the short-wavelength measuring light significantly enhanced most photosynthetic parameters of Q. variabilis seedlings compared to SWL alone, and the extent of these effects varied with growth light intensity. For steady-state photosynthetic parameters, FR supplementation increased the maximum steady-state net photosynthetic rate by 31% under low light and by 25% under high light, while maximum steady-state stomatal conductance rose by 20% (LL) and 45% (HL) relative to SWL alone. Similarly, the effective quantum yield of photosystem II photochemistry increased by 62% (LL) and 29% (HL); and the photochemical quenching coefficient rose by 63% (LL) and 25% (HL) compared to measurements without FR. Non-photochemical quenching also showed increases of 24% (LL) and 7% (HL) respectively, versus non-FR conditions. However, there was no significant difference in the maximum carboxylation efficiency between FR-supplemented and control treatments. In photosynthetic induction responses, the addition of far-red light into the measuring light significantly enhanced the maximum slope of both photosynthesis and stomatal conductance induction curves, while simultaneously shortened the response time and lag time in leaves grown under low light (LL) compared to SWL alone. By contrast, leaves grown under high light exhibited little response to FR supplementation. Furthermore, the addition of FR into the measuring light markedly increased the maximum electron transport rate in CO₂ response curves by 22% and 24% under low and high light conditions, respectively, relative to non-FR conditions. This study demonstrated that adding far-red light during photosynthetic measurements significantly altered most photosynthetic parameters in Q. variabilis seedlings. The primary mechanism is that far-red light could optimize energy distribution between photosystem I and photosystem II, enhance electron transport efficiency, and improve overall light use efficiency, which collectively regulate photosynthetic performance. Incorporating far-red light into measuring systems provides more accurate representations of plant photosynthetic performance under natural light spectra than traditional far-red-deficient measurement systems. Consequently, standardizing the measurement light spectrum, especially by maintaining an adequate far-red component, can significantly improve the ecological validity of derived parameters and data comparability. These findings are important for plant physiological research and field applications. However, the current study was limited to potted seedlings of a single tree species under a fixed R/FR ratio. Future investigations should therefore be extended to multiple plant species and a broader range of R/FR combinations to further validate the universality and applicability of far-red light mediated photosynthetic regulation.

Key words: far-red light, growth light intensity, steady-state photosynthesis, photosynthetic induction, plant photosynthetic measurement, photosynthetically active radiation, excitation energy distribution