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Using far-red light to promote leaf expansion for young plant production




Percival, Anthony Christophe, author
Craver, Joshua, advisor
Newman, Steven, committee member
Peers, Graham, committee member

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At northern latitudes, a reduction in the natural light quantity during the winter production of young annual bedding plants (plugs) often necessitates the use of supplemental lighting to reach a target daily light integral (DLI) to ensure high plug quality. However, the low leaf area index (LAI) of plugs during the initial stages of production suggests that a portion of applied light is not intercepted by leaves. Because electric lighting represents a significant percentage of total production costs for greenhouses utilizing supplemental lighting, minimizing wasted light (photons not absorbed by the plant) is critical. Some species have shown an increase in leaf area in response to growth under light with a low ratio of red to far-red light (R:FR); this is generally considered as a shade avoidance response to improve light capture, but there is considerable variation across species. An early increase in leaf area would allow for more effective light capture by seedlings and a reduction in wasted light, but other shade avoidance responses such as elongation of stems and petioles are undesirable for plug production and could outweigh benefits of leaf expansion. Far-red mediated shade avoidance responses may also depend on background photosynthetic photon flux density, DLI, and temperature. The objective of this research was to investigate the effects of far-red radiation on leaf expansion and other shade avoidance responses for the popular annual bedding plant, Petunia ×hybrida (petunia), and to examine potential influences of other environmental variables. Reducing the R:FR in a greenhouse environment with supplemental lighting is challenging due to the relatively high proportion of natural light, so an end-of-day far-red (EOD-FR) lighting strategy was utilized to investigate the promotion of leaf expansion by far-red light for seedings of petunia 'Wave Purple', and 'Dreams Midnight'. Seedlings were grown in 128-cell trays in a common greenhouse environment under an ambient DLI of 5.26 mols·m-2·d-1 to simulate a winter light environment. Seedlings received no EOD-FR, supplemental lighting for the duration of the experiment, or one of the following EOD-FR treatments that varied in far-red intensity, R:FR ratio, and treatment duration: 10 μmol·m-2·s-1 of far-red light (R:FR ~0.8) for 30 minutes, 10 or 20 μmol·m-2·s-1 of far-red light (R:FR ~0.15) for 30 minutes, or 20 μmol·m-2·s-1 of far-red light (R:FR ~0.15) for 240 minutes. In addition to end-of-day (EOD) treatments, some seedlings under EOD-FR were moved under supplemental lighting after 2 or 3 weeks of EOD lighting. Destructive data was collected 2, 3, and 4 weeks after treatment initiation. Seedlings that received EOD-FR lighting showed stem elongation responses, and seedlings under the lower R:FR or longer EOD duration resulted in greater elongation, but no EOD treatment resulted in an increase in leaf area compared to control (no supplemental lighting or EOD lighting) or supplemental lighting treatments. Results of this study indicate that under low DLIs, EOD-FR light applied in the first three weeks of seedling production does not promote early leaf area expansion and reduces seedling quality under these experimental conditions. To further examine leaf expansion as a response to far-red radiation, seedlings of petunia 'Dreams Midnight' were grown for 28 days under the recommended target DLI of ~10 mols·m-2·d-1 using a 17.25-h photoperiod with either a high (~10.8) or low R:FR (~0.50). The effects of EOD-FR were also examined by subjecting seedlings grown under the high R:FR to a 1-hour low intensity (target total photon flux density of 46 μmol·m2·s-1) EOD lighting application with a very low R:FR (0.15). Lastly, the influence of temperature on the effects of far-red radiation were examined by growing seedlings at either 16 or 21 ℃ for the duration of the experiment, and by moving plants from a high R:FR in the 21 ℃ chamber during the day to the 16 ℃ chamber for the EOD-FR treatment and subsequent dark period. Overall, seedlings grown at a constant air temperature of 16 ℃ displayed stunted growth (lower leaf area, number of leaves, and total biomass) compared to those grown at 21 ℃ regardless of lighting treatment. At 21 ℃, the use of EOD-FR did not promote an increase in leaf area. Seedlings grown under a constant low R:FR (~0.50) at 21 ℃ did display increased leaf area, but lower stem dry mass per unit length (mg·mm-1), leaf mass per unit area (g·m2), and root dry mass indicated poor seedling quality. These results further show that morphological responses to far-red light are species-specific, and that plant responses to far-red light may differ based on a variety of environmental factors. Future research regarding leaf expansion in response to far-red light that incorporates other environmental factors (e.g., temperature, TPFD, photoperiod length) may lead to a more complete understanding of species-specific shade avoidance responses, and further work in this area may assist growers with the development of far-red lighting strategies to improve light capture and seedling quality for young plant production.


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