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

- Including far-red (FR) in a light spectrum increased plant size and photosynthesis.
- FR supplementation decreased phytochrome photoequilibrium and increased yield photon flux.
- Whole-plant photosynthesis positively correlated with yield photon flux.
- Inclusion of FR during seedling growth promoted flowering in one long-day species.

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: R128 (128 μmol m−2 s−1 of R), R128 + FR16, R128 + FR32, R128 + FR64, R96 + FR32, and R64 + FR64. 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.