Far-red radiation and photosynthetic photon flux density independently regulate seedling growth but interactively regulate flowering

Shade-avoidance responses can be triggered by a decrease in the red (R, 600–700 nm) to far-red (FR, 700–800 nm) radiation ratio, by a decrease in photosynthetic photon flux density (PPFD), or both. The effects of decreased PPFD on plant responses are often confounded with the effects of reduced blue (B, 400–500 nm) photon flux density, which is another signaling factor for shade-avoidance responses. We postulated that PPFD would not influence R:FR-mediated shade-avoidance responses if B photon flux density was constant. We grew seedlings of petunia (Petunia ×hybrida), geranium (Pelargonium ×hortorum), and coleus (Solenostemon scutellariodes) under three R:FR (1:0, 2:1, and 1:1) at two PPFDs (96 and 288 μmol m^–2 s^–1), all with a B photon flux density of 32 μmol m^–2 s^–1. As R:FR decreased, stem length in all species increased. Decreasing R:FR increased individual leaf area of petunia, and shoot dry weight of petunia and coleus. Increasing PPFD decreased chlorophyll concentration and increased leaf mass per area, net CO₂ assimilation, whole-plant net assimilation, and dry weight in at least two species, independent of R:FR. In petunia, a long day plant, decreasing R:FR promoted subsequent flowering at both PPFDs, but to a greater extent under the lower PPFD. In day-neutral geranium, the addition of FR had no effect on flowering, irrespective of PPFD. We conclude that with a constant B photon flux density, decreases in R:FR promote stem elongation and leaf expansion, and subsequent dry mass accumulation, independent of PPFD. However, for flowering of long-day plant petunia, the promotive effect of low R:FR is greater under lower PPFD.