Investigating the merit of including far-red radiation in the production of ornamental seedlings grown under sole-source lighting

A combination of red (R; 600 to 700 nm) and blue (B; 400 to 500 nm) lightemitting diodes (LEDs) is commonly used in sole-source lighting of plants grown in highly controlled environments. R and far-red radiation (FR; 700 to 800 nm) regulate photomorphogenesis, including stem elongation and leaf expansion, and to some extent, photosynthesis. However, little research has been published on how FR LEDs can be used to improve plant growth and quality attributes during seedling production. We grew seedlings of snapdragon (Antirrhinum majus) at 20°C under six sole-source LED treatments with an 18-h photoperiod. All treatments included 32 μmol m^-2 s^-1 of B radiation (peak = 451 nm) and different intensities (subscript in μmol m^-2 s^-1) from R (peak = 660 nm) and FR (peak = 729 nm) LEDs: R₁₂₈, R₁₂₈+FR₁₆, R₁₂₈+FR₃₂, R₁₂₈+FR₆₄, R₉₆+FR₃₂, and R₆₄+FR₆₄. Plant height and total leaf area linearly increased as the R:FR or the estimated phytochrome photoequilibrium of each treatment decreased. Shoot dry weight was similar under the same total photon flux (400 to 800 nm) even when R radiation was partially substituted with FR (considered minimally photosynthetic) and linearly increased when R was constant and FR increased. Photosynthetic efficiency (PE), which is calculated as shoot dry weight per unit leaf area, was correlated positively with the calculated yield photon flux of each +FR treatment. In addition, PE linearly decreased as the amount of R was increasingly substituted with FR radiation. We conclude that supplementation of FR to R and B radiation can increase PE and subsequent dry mass accumulation without excessive leaf and stem expansion.