Investigating the interaction between photosynthetic photon flux density and far-red radiation in petunia seedlings under sole-source lighting

The ratio of red (R; 600 to 700 nm) to far-red (FR; 700 to 800 nm) radiation regulates phytochrome-mediated photomorphogenic responses to promote radiation capture and survival under shade conditions. The photosynthetic photon flux density (PPFD) is also reduced under shade, making it less clear which shade-avoidance responses are mediated by PPFD, R:FR, or their interaction. We grew seedlings of petunia (Petunia × hybrida ‘Wave Blue’) at 20°C under six sole-source light-emitting diode (LED) treatments with an 18-h photoperiod. All treatments included 32 µmol m^-2 s^-1 of blue radiation (LED emission peak = 447 nm) and different intensities (subscript in µmol m^-2 s^-1) of radiation from R (peak = 660 nm) and FR (peak = 731 nm) LEDs: R₆₄, R₆₄ + FR₃₂, R₆₄ + FR₆₄, R₂₅₆, R₂₅₆ + FR₁₂₈, and R₂₅₆ + FR₂₅₆. Stem length, individual leaf area, and shoot dry weight linearly decreased as the R:FR [or estimated phytochrome photoequilibria (PPE)] increased under PPFDs of 96 µmol m^-2 s^-1 (PPFD 96) or 288 µmol m^-2 s^-1 (PPFD 288). Independent of PPE, increasing PPFD decreased stem length and individual leaf area, while increasing shoot dry weight. In addition, inclusion of FR during seedling growth promoted flowering at both PPFDs, but to a greater extent under PPFD 96 than PPFD 288. We conclude that most shade-avoidance responses in petunia are mediated by a decrease in R:FR and PPFD individually, without interaction effects. An exception was for flowering, in which the promotive effect of FR was greater under the lower PPFD.