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Yuyao Kong, Ajay Nemali, Cary Mitchell, and Krishna Nemali

, the spectral quality of LEDs can potentially influence EUE by affecting both SDW and EEC in opposite ways. However, studies simultaneously examining the effects of the spectral quality of LEDs on lettuce SDW, EEC, and EUE are limited in terms of the

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Andrew C. Schuerger and Christopher S. Brown

Plants were grown under light-emitting diode (LED) arrays with various spectra to determine the effects of light quality on the development of diseases caused by tomato mosaic virus (ToMV) on pepper (Capsicum annuum L.), powdery mildew [Sphaerotheca fuliginea (Schlectend:Fr.) Pollaci] on cucumber (Cucumis sativus L.), and bacterial wilt (Pseudomonas solanacearum Smith) on tomato (Lycopersicon esculentum Mill.). One LED (660) array supplied 99% red light at 660 nm (25 nm bandwidth at half-peak height) and 1% far-red light between 700 to 800 nm. A second LED (660/735) array supplied 83% red light at 660 nm and 17% far-red light at 735 nm (25 nm bandwidth at half-peak height). A third LED (660/BF) array supplied 98% red light at 660 nm, 1% blue light (BF) between 350 to 550 nm, and 1% far-red light between 700 to 800 nm. Control plants were grown under broad-spectrum metal halide (MH) lamps. Plants were grown at a mean photon flux (300 to 800 nm) of 330 μmol·m-2·s-1 under a 12-h day/night photoperiod. Spectral quality affected each pathosystem differently. In the ToMV/pepper pathosystem, disease symptoms developed slower and were less severe in plants grown under light sources that contained blue and UV-A wavelengths (MH and 660/BF treatments) compared to plants grown under light sources that lacked blue and UV-A wavelengths (660 and 660/735 LED arrays). In contrast, the number of colonies per leaf was highest and the mean colony diameters of S. fuliginea on cucumber plants were largest on leaves grown under the MH lamp (highest amount of blue and UV-A light) and least on leaves grown under the 660 LED array (no blue or UV-A light). The addition of far-red irradiation to the primary light source in the 660/735 LED array increased the colony counts per leaf in the S. fuliginea/ cucumber pathosystem compared to the red-only (660) LED array. In the P. solanacearum/ tomato pathosystem, disease symptoms were less severe in plants grown under the 660 LED array, but the effects of spectral quality on disease development when other wavelengths were included in the light source (MH-, 660/BF-, and 660/735-grown plants) were equivocal. These results demonstrate that spectral quality may be useful as a component of an integrated pest management program for future space-based controlled ecological life support systems.

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W.C. Lin and P.A. Jolliffe

The importance of light intensity and spectral quality on fruit color and shelf life of long English cucumber (Cucumis sativus L.) was studied in four greenhouse experiments. The intensity of cucumber greenness was measured nondestructively by video imaging, and shelf life was measured by visual observation of incipient yellowing. In the summer, filters were used to cover individual fruit to reduce light intensity reaching the fruit surface. The lower the light intensity incident on a cucumber, the shorter its shelf life. The average shelf life was 8, 5, or 1 days for cucumbers receiving 100%, 66%, or 31% of natural daylight, respectively. The fruit that were covered with a filter transmitting red (R) light were greener (low grey level via video imaging) than those with a far-red (FR) filter. In the fall, fruit receiving spectral R lighting from fluorescence tubes were greener and had a longer shelf life than those receiving FR lighting from incandescent bulbs. In the winter, high-pressure sodium (HPS) lighting was necessary to supplement natural daylight for crop growth and production. Under HPS, R and FR lighting produced the same fruit greenness and shelf life. In the spring, R-lighted fruit had longer shelf life than FR-lighted ones, although fruit color at harvest was similar. In these four experiments, postharvest shelf life of long English cucumber was generally related to fruit greenness upon harvest. The data suggest the importance of an open canopy in improving fruit greenness and shelf life of greenhouse-grown cucumbers.

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Cinthia Nájera and Miguel Urrestarazu

. 1986 Daily changes in uptake, reduction and storage of nitrate in spinach grown at low light intensity Physiol. Plant. 66 550 556 Urrestarazu, M. Nájera, C. Gea, M.M. 2016 Effect of the spectral quality and intensity of light-emitting diodes on several

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Fahrurrozi Aziz and Katrine A. Stewart

The types of plastic mulch used in horticultural production often fulfill only a few of the grower's needs. Black plastic mulch controls weeds, but can burn young plants. Clear mulch, while warming the soil and enhancing early crop growth, allows prolific weed growth and development. Accordingly, an experiment using a randomized complete-block design with-four replications was set up to compare the effects of black, microperforated black, silver, and wavelength-selective (IRT-76) green mulches, and bare soil on weed growth and development, and on soil temperature and moisture. Each mulch was evaluated for its optical properties. All mulches significantly reduced final stands of seeded weeds compared with bare soil. IRT-76 green had the warmest mean soil temperatures, followed by silver, black, and microperforated black mulches, and bare soil. Soil moisture content was generally higher under plastic mulches than bare soil.

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Miguel Urrestarazu, Cinthia Nájera, and María del Mar Gea

Light-emitting diode (LED) lamps signify one of the most important advances in artificial lighting for horticulture over the last few decades. The objective of this study was to compare the cultivation of four horticultural plants using a conventional white LED tube (T0) light against one with a good spectral fit to the maximum photosynthetic response (T1) at two intensities. The experiment was carried out with two types of young lettuce, tomato, and bell pepper plants. In a controlled environment chamber, six and four lamps per square meter were used to achieve high (H) and low (L) intensity, respectively. We measured the lighting parameters illuminance (lux) and photosynthetic photon flux (PPF) intensity (µmol·m−2·s−1). The dry and fresh weight, leaf area (LA), and specific index were measured to gauge plant growth. The photosynthetic activity and energy efficiency (EE) were recorded for each species over 60 days of cultivation. The results clearly demonstrate that, compared with conventional LED lamps, the specific horticultural LED lamps with an improved light spectrum increased the EE of the evaluated vegetables by 26%. At both the studied light intensities, plant growth was clearly more closely linked to the spectral fit of the light to the maximum photosynthetic response recorded by McCree (1972) than to PPF or illuminance (lux). We therefore suggest that a specific, detailed spectral distribution study be conducted to predict the effect of the specific quantity and quality of light used in this study on a single parameter of plant growth.

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N.C. Yorio, C.L. Mackowiak, R.M. Wheeler, and J.C. Sager

Potato (Solanum tuberosum L. cvs. Norland and Denali) plants were grown under high-pressure sodium (HPS), metal halide (MH), and blue-light-enhanced SON-Agro high-pressure sodium (HPS-S) lamps to study the effects of lamp spectral quality on vegetative growth. All plants were initiated from in vitro nodal cultures and grown hydroponically for 35 days at 300 μmol·m–2·s–1 photosynthetic photon flux (PPF) with a 12-hour light/12-hour dark photoperiod and matching 20C/16C thermoperiod. `Denali' main stems and internodes were significantly longer under HPS compared to MH, while under HPS-S, lengths were intermediate relative to those under other lamp types, but not significantly different. `Norland' plants showed no significant differences in stem and internode length among lamp types. Total dry weight of `Denali' plants was unaffected by lamp type, but `Norland' plants grown with HPS had significantly higher dry weight than those under either HPS-S or MH. Spectroradiometer measurements from the various lamps verified the manufacturer's claims of a 30% increase in ultraviolet-blue (350 to 450 nm) output from the HPS-S relative to standard HPS lamps. However, the data from `Denali' suggest that at 300 μmol·m–2·s–1 total PPF, the increased blue from HPS-S lamps is still insufficient to consistently maintain short stem growth typical of blue-rich irradiance environments.

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Sandra B. Wilson, Keiko Iwabuchi, Nihal C. Rajapakse, and Roy E. Young

Broccoli (Brassica oleracea L. Botrytis group `Green Duke') seeds were cultured in vitro photoautotrophically (without sugar in the medium) or photomixotrophically (with sugar in the medium) for 3 weeks at 23 °C and 150 μmol·m-2·s-1 photosynthetic photon flux (PPF). Vessels were then stored at 5 °C under 1.6, 4.1, or 8.6 μmol·m-2·s-1 of white (400-800 nm), red (600-700 nm), or blue (400-500 nm) light. Concentrations of CO2 inside the vessels were monitored until equilibrium was reached. Light compensation point was reached at 3.5 μmol·m-2·s-1 for photoautotrophic seedlings and at 6.5 μmol·m-2·s-1 for photomixotrophic seedlings. Therefore, in the long-term storage experiment, seedlings were stored for 4, 8, or 12 weeks at 5 °C in darkness or under 5 μmol·m-2·s-1 (average light compensation point) of white, red, or blue light. Illumination during storage was necessary to maintain dry mass, leaf area, and regrowth potentials of in vitro seedlings. All seedlings stored in darkness were of poor quality and died when transferred to the greenhouse. Red light during storage increased seedling dry mass and chlorophyll content and improved overall appearance, whereas blue light decreased chlorophyll content and increased stem elongation. The addition of 2% sucrose to media increased dry mass and leaf area and maintained overall seedling quality during illuminated storage. However, plantlets stored for more than 4 weeks did not survive poststorage greenhouse conditions, regardless of light treatment.

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Sandra B. Wilson, Keiko Iwabuchi, Nihal C. Rajapakse, and Roy E. Young

Broccoli (Brassica oleracea L. Botrytis group `Green Duke') seeds were cultured photoautotrophically (without sugar) or photomixotrophically (with sugar) in vitro for 3 weeks at 23 °C and150 μmol·m-2·s-1 photosynthetic photon flux (PPF). In vitro seedlings were stored for 0, 4, 8, or 12 weeks at 5 °C in darkness or under 5 μmol·m-2·s-1 of white (400–800 nm), blue (400–500 nm), or red (600–700 nm) light. Photosynthetic ability and soluble sugar contents were determined after removal from storage. Photomixotrophic seedlings contained approximately five times more soluble sugars than did photoautotrophic seedlings. Dark storage reduced soluble sugars in both photoautotrophic and photomixotrophic plants, but photosynthetic ability was maintained for up to 8 weeks in the latter whereas it decreased in the former. Illumination in storage increased leaf soluble sgars in both photoautotrophic and photomixotrophic seedlings. Soluble sugars in stems decreased during storage regardless of illumination, but remained higher in illuminated seedlings. Red light was more effective in increasing or maintaining leaf and stem soluble sugars than was white or blue light. Regardless of media composition or illumination, storage for more tan 8 weeks resulted in dramatic losses in quality and recovery, as well as photosynthetic ability. Seedlings stored for 12 weeks comletely lost their photosynthetic ability regardless of media composition or illumination. The results suggest that carbohydrate, supplied in the media or through illumination, is essential for maintenance of photosynthetic ability during low-temperature storage for up to 4 or 8 weeks.