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potentially optimize seedlings by stimulating plant photoreceptors that regulate growth and morphology (photomorphogenesis) ( Gómez and Mitchell, 2015 ). The emerging high-intensity LED lamps are an alternative for supplemented photoperiods due to their long

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tropical crops is their sensitivity to photoperiod. Ginger and turmeric are SD plants that enter dormancy when exposed to natural SDs (<12 h of light), which decreases rhizome yield and increases fiber content ( Pandey et al., 1996 ). Therefore, field

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lights on stem elongation of microgreens, especially under different photoperiods, because photoperiod can also affect this plant trait ( Bergstrand, 2017 ). Previous studies indicated that under LED lighting at a photosynthetic photon flux density (PPFD

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The onion crop in Georgia is often damaged by suboptimal winter temperatures. Proper acclimation of seedlings is a way of limiting freeze damage. Because photoperiod is among the factors involved in plant acclimation, the effects of photoperiod on the acclimation of short-day Allium cepa seedlings was investigated. A single short-day cultivar, 'Granex 33', was greenhouse grown under an eleven hour photoperiod. After ten weeks of growth, four photoperiod treatments (8, 11, 14, and 24 hrs.) were administered during a two week hardening period at 3* C. Plants were then frozen in an ethylene glycol bath. Degree of acclimation was determined based on regrowth and visual observation. Acclimation of seedlings was completely inhibited by the 24 hour photoperiod. Varying degrees of acclimation were achieved with the other photoperiod treatments.

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of stevia is photoperiod-sensitive, and stevia has been categorized as a short-day plant ( Valio and Rocha, 1977 ). Stevia vegetative growth and morphological responses to irradiance and nutrient levels are only partially understood, and information

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flowers in response to shortening photoperiods. Hemp development occurs over a continuum that may be classified into juvenile (vegetative), photoperiod inductive (vegetative and flowering), and harvest maturity (anthesis) stages ( Amaducci et al. 2008a

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forcing). Shoot emergence, development, and flowering in clematis may be promoted by the cold treatment, photoperiod, and temperatures in the forcing environment; however, as noted for other herbaceous perennials, plants must be physiologically capable of

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regulated by ambient light, including PPFD ( Wei et al., 2019 ; Xu, 2015 ; Yan et al., 2019 ), photoperiod ( Ji et al., 2020 ; Wei et al., 2020 ), light direction ( Joshi et al., 2017 ), and light quality ( Hernández and Kubota, 2016 ; Wojciechowska et

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In 1996 and 1997, eight cultivars of cold-treated field-grown Astilbe were grown in a 20 °C green-house with short days (SDs = 9-h natural days) or long days (LDs = 9-h natural days with night interruption with incandescent lamps from 2200 to 0200 hr) to determine how photoperiod influences flowering. Cultivars studied were Astilbe × arendsii Arends `Bridal Veil', `Cattleya', `Fanal', and `Spinell'; A. chinensis Franch. `Superba'; A. japonica A. Gray `Deutschland' and `Peach Blossom'; and A. thunbergii Miq. `Ostrich Plume'. Flowering percentage was highest (≥90%) for `Cattleya', `Deutschland', `Fanal', `Ostrich Plume', and `Peach Blossom', regardless of photoperiod. Photoperiod did not affect the time to visible inflorescence or flower number for any cultivar studied. The time from visible inflorescence to first flower took 27 to 36 days, irrespective of photoperiod. Time to flower varied by cultivar; `Deutschland' was the earliest to flower (31 to 41 days) and `Superba' was the last to flower (51 to 70 days). `Fanal' and `Ostrich Plume' flowered slightly but significantly faster (by 1 to 6 days) under LDs than SDs. For five cultivars, the inflorescence was taller under LDs than SDs. All cultivars reached visible inflorescence and flower significantly faster (by 1 to 15 days) in 1997 than in 1996.

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Petunia `Red Flash' seedling were grown under HPS (175 μmolm-2 s-1) photoperiod treatments of 10, 12, 14 or 16 hr at 20C soil temperature in a shaded glasshouse where the maximum peak PPF was reduced to 150 μmolm-2s-1. Seedling dry weight and individual leaf area were determined daily. The photosynthetic rate was determined when seedlings reached the second true leaf stage.

The dry weight response to increasing photoperiod durations was cubic with a peak at 14 hr. Seedling dry weight increased slowly during days 5 through 10 then increased rapidly during the next 7 to 10 days. This increase coincided with the unfolding of leaves one through four. The total leaf area showed a cubic response to the photoperiod treatments. The leaf area increased slowly then began an exponential increase after day 10. The photosynthetic rate per gram dry weight was increased by the 10 hr photoperiod treatment when compared to the 16 hr treatment. The increased photosynthetic rate was not observed when the data was calculated on a fresh weight or leaf area basis.

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