On the Lack of Morphological Response to Far-red at High and Low Photon Flux in Spinach

An increasing far-red (FR; 700–750 nm) photon fraction typically triggers shade-avoidant responses such as leaf expansion, but previous work in lettuce has indicated that this occurs only at a higher extended photosynthetic photon flux density (ePPFD; 400–750 nm). We report the interaction between the FR photon fraction and ePPFD in spinach. We grew spinach in growth chambers under ePPFDs of 100, 200, and 500 µmol·m –2 ·s –1 , each with an FR photon fraction of 0.03, 0.10, 0.17, or 0.33. Recent studies have demonstrated the photosynthetic value of FR photons, which makes it important to include these in the definition of photosynthetic photons. We thus substituted FR photons for shorter wavelength photons to keep photosynthetic photons (ePPFD) constant among treatments. As expected, leaf area and dry mass increased with increasing ePPFD; but, surprisingly, there was no effect of an increasing FR photon fraction on leaf area, regardless of ePPFD. These results show that FR photons are equivalent to shorter wavelength photons in spinach; but, unlike lettuce, an increasing FR fraction did not increase leaf area.