When compared at a level of 1 ft-c for 16 hours at a night temperature of 20°C, light from 5 types of sources delayed flowering of short-day plants (Chrysanthemum, marigold, Rieger begonia), promoted vegetative growth of 2 species of Ulmus, 2 each of Acer, and 1 of Koelreuteria, Rhododendron, Rhus, and Zelkova, and promoted flowering of long-day plants (carnation, marguerite daisy, Petunia) in the order from most to least effective: incandescent (INC) > high-pressure sodium (HPS) > > metal halide (MH) = cool white fluorescent (F) > > clear mercury (Hg). Poinsettia, Betula, Catalpa, Platanus, and Tilia continued to grow vegetatively in response to all sources. Ilex and 2 species of Pinus did not respond. Foot candles of light from HPS lamps had to be increased at least 4- to 8-fold to regulate vegetative growth of long-day plants and delay flowering of short-day plants in comparison to INC lamps. High-pressure sodium lamps were ineffective in promoting early flowering of long-day plants, regardless of intensity or duration.
The relative efficiencies of high-pressure (HPS) and low-pressure sodium (LPS) lamps for plant growth were determined for 32 species of foliage and flowering plants in greenhouse under winter conditions at 37°N latitude. HPS with a relatively wide spectral emission peaking at 589 nm, and low-pressure sodium (LPS) with a monochromatic line at 589 nm were compared at 42 W/m2 irradiance in the 400–700 nm region for various lengths of time at various times of day. Although radiation in the far-red region (700–800 nm) differed, HPS and LPS equally accelerated rates of increase in fresh weights, and heights, and flowering of most herbaceous and tropical foliage plants evaluated. High-pressure sodium and LPS were ineffective, however, in promoting growth of deciduous trees and some woody plants and had no more effect than exposure of the plants to natural winter days with 0.9 W/m2 from incandescent lamps for 8 hrs (2000-0400) night interruption (long day controls). After 16 hours, about half the species showed photomorphogical differences between plants grown at intensities of 21 and plants grown at 42 W/m2 from LPS. All showed significantly better growth characteristics (fresh weight, height, early flowering) than the long day controls. Lighting during the day or night (42 W/m2 from 0800–1600 or from 2000–0400) was equally effective in promoting growth responses with 15 of the 32 species evaluated. Night lighting was more effective than day lighting with 10 of the 32 species tested. The majority of the species grew equally well when lighted 16 hours daily (0800-2400) with 21 W/m2 or 8 hours daily (2000–0400) with 42 W/m2. Effectiveness of the lighting was generally unrelated to the photoperiodic requirements of the plants. Many of the plants, which were previously classified as day-neutral (DN), flowered as if they were long day plants. Such day-neutral plants apparently required adequate light (intensity and duration) for photosynthesis. Increased daylength alone, without regard to intensity, was not sufficient to accelerate growth and early flowering of day-neutral plants. Since HPS and LPS were equally effective on most species tested, we concluded that light quality was less important than total irradiation (energy) for the growth and early flowering of many herbaceous plants.
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