Production of young plants, such as seedlings and cuttings, under sole-source lighting is an alternative to greenhouse production where the natural daily light integral (DLI) in winter may be too low for the production of high-quality young plants (Randall and Lopez, 2015). Because the electrical cost associated with lighting are high, it is important that young plants use the provided light as efficiently as possible. Young seedlings intercept little of the light available to them in a production space because their leaves are small relative to the size of the production space. For example, ‘Goldsturm’ black-eyed susan (Rudbeckia fulgida var. sullivantii) seedlings sown in a 72-cell plug tray (20 × 10 inches) had a total canopy size of ≈15 cm2 after 10 d, which represented ≈1% of the surface area of the tray (Elkins, 2020). Plants this small may benefit from larger leaves to increase light interception and presumably canopy photosynthesis and growth. This has the potential to shorten cropping cycles of ornamental plants grown in controlled environments. One approach to enhancing growth is to include far-red light (λ = 700–800 nm) in the spectrum because 1) far-red light can trigger a shade-avoidance and/or acclimation response (Franklin, 2008; Keuskamp et al., 2010; Possart et al., 2014) and 2) it is photosynthetically active (Zhen and Bugbee, 2020; Zhen and van Iersel, 2017).
Shade-avoidance/acclimation responses are mediated by phytochrome, a pigment-protein photoreceptor that detects light intensity and quality. Phytochrome has two photo-interconvertible forms, the inactive red (Pr) and the active far-red (Pfr) forms. These allow plants to detect the red to far-red (R:FR) ratio of incident light that mediates a response in plants (Franklin and Whitelam, 2005; Possart et al., 2014; Ruberti et al., 2012). Red light is strongly absorbed, whereas far-red light is poorly absorbed and thus transmitted through leaves. Under-canopy plants are thus exposed to a low R:FR ratio, triggering shade responses (Casal, 2013; Franklin and Whitelam, 2005; Gommers et al., 2013). In response to shade, plants may respond morphologically, such as via elongated stems and petioles (shade-avoidance) (Franklin, 2008; Franklin and Whitelam, 2005) or may acclimate with traits such as increased specific leaf area (SLA) (shade-tolerance) (Evans and Poorter, 2001; Gommers et al., 2013; Gong et al., 2015). Excessive elongation is undesirable in seedling production and a potential drawback to providing high levels of far-red light. Compared with the PPFD, sunlight has about 19% far-red photons [701–750 nm, the range most responsible for phytochrome responses (Zhen and Bugbee, 2020)], so plants are accustomed to receiving a substantial amount of far-red light.
For decades, far-red light has been considered outside of the range of photosynthetically active radiation [PAR (400–700 nm)], as established by Hoover (1937), McCree (1971), and Inada (1976), even though they demonstrated that far-red light had at least some photosynthetic activity. Recently, Zhen and van Iersel (2017) found a synergistic interaction between far-red light and PAR, specifically an increase in net photosynthesis of lettuce (Lactuca sativa) in response to added far-red light. Zhen and Bugbee (2020) found that adding far-red light (up to 40% of PAR) increased canopy photosynthesis of 14 diverse agronomic and horticultural crop species as much as adding the same photon flux of PAR.
Previous studies examining the effect of far-red light on plant growth (Hurt et al., 2019; Li and Kubota, 2009; Meng and Runkle, 2019; Meng et al., 2019; Park and Runkle, 2016, 2017; Zou et al., 2019) and photosynthesis (Zhen and Bugbee, 2020; Zhen and van Iersel, 2017) have evaluated several intensities of far-red light, ranging from 0 to 160 µmol·m−2·s−1. Saturation levels have been determined for leaf and whole-plant photosynthesis (Zhen and Bugbee, 2020; Zhen and van Iersel, 2017), but not for growth responses. Our objective was to evaluate 18 intensities of supplemental far-red light, ranging from 4.0 to 68.8 µmol·m−2·s−1, on the growth and morphology of ‘Dalmatian Peach’ foxglove (Digitalis purpurea) seedlings. Foxglove was selected because it is a popular garden plant and one of the few perennials propagated from seed. It can grow well under a wide range of light conditions, and thus it appears to have the ability to acclimate to a range of light environments. We hypothesized that providing supplemental far-red light would result in leaf expansion, increased photosynthesis, and therefore more growth, but that above a certain photon flux, plant morphology and growth would no longer respond to further increases in far-red light.
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