Recent studies show that far-red photons (FR; 700–750 nm), when combined with photosynthetically active radiation (PAR; 400–700 nm), can drive canopy-level photosynthesis as effectively as PAR alone. This has prompted suggestions to redefine PAR as extended PAR (ePAR; 400–750 nm). However, few studies have evaluated whether FR and PAR photons produce equivalent photosynthetic rates at the leaf level. We investigated whether photosynthesis is maintained under increasing FR substitution at equal total photon flux (1000 µmol·m⁻²·s⁻¹). Five crop species (apple, blueberry, corn, strawberry, and tomato) were grown under natural field conditions (tomato in a hoop house) and sampled for leaf gas exchange using A/Ci curves under three light spectra: 0%, 15%, and 30% FR substitution. Leaf transmittance of FR photons was 3–11 times greater than PAR across species, indicating reduced FR absorption compared to PAR photons. Nonetheless, maximum photosynthetic rates were similar across treatments for all species. For most species, FR substitution did not affect Vc,max or Jmax, indicating that rubisco activity and electron transport capacity remained stable. However, blueberry showed declines in both parameters with increasing FR, while corn exhibited increased Jmax under FR substitution. Despite reduced FR absorption, photosynthetic performance was largely unchanged with up to 30% FR substitution. These results support the inclusion of FR photons in the PAR definition and reinforce the relevance of ePAR in both natural and controlled environments.
