Far-red radiation promotes growth of seedlings by increasing leaf expansion and whole-plant net assimilation

Abstract By definition, photosynthetically active radiation (PAR) includes wavelengths between 400 and 700 nm and thus, far-red radiation (FR, 700–800 nm) is excluded when the photosynthetic photon flux density (PPFD) is measured and reported. However, FR radiation [and the ratio of red (R; 600–700 nm) to FR] regulates phytochrome-mediated morphological and developmental plant responses to promote radiation capture and survival under shade. We postulated that the inclusion of FR in a radiation spectrum would have little effect on photosynthesis but would increase radiation capture and plant growth, while accelerating the subsequent flowering of shade-avoiding species. Geranium ( Pelargonium  ×  hortorum ), petunia ( Petunia  ×  hybrida ), snapdragon ( Antirrhinum majus ), and impatiens ( Impatiens walleriana ) were grown at 20 °C under an 18-h photoperiod provided by sole-source lighting from light-emitting diodes that included 32 μmol m −2  s −1 of blue and the following intensities of R and FR radiation: R 128 (128 μmol m −2  s −1 of R), R 128  + FR 16 , R 128  + FR 32 , R 128  + FR 64 , R 96  + FR 32 , and R 64  + FR 64 . Plant height in all species studied and total leaf area of geranium and snapdragon linearly decreased as the R:FR (or the estimated phytochrome photoequilibrium) of each treatment increased. In geranium and snapdragon, the increase in total leaf area (by 7%) with the addition of FR to the same PPFD subsequently increased shoot dry weight (DW) (by 28–50%) while the increase in total leaf area (by 30–40%) with the partial substitution of R with FR partly compensated for the reduction in PPFD (by 40%), producing a similar shoot DW. Whole-plant net assimilation of geranium, snapdragon, and impatiens increased with additional FR radiation, showing a linear relationship with the calculated yield photon flux density of each radiation treatment. In addition, inclusion of FR during seedling growth promoted flowering in the long-day plant snapdragon. We conclude that FR radiation increases plant growth indirectly through leaf expansion and directly through whole-plant net assimilation and in at least some species, promotes subsequent flowering.