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Solid-state lighting based on the use of light-emitting diodes (LEDs) is potentially one of the biggest advancements in horticultural lighting in decades. LEDs can play a variety of roles in horticultural lighting, including use in controlled environment research, lighting for tissue culture, and supplemental and photoperiod lighting for greenhouses. LED lighting systems have several unique advantages over existing horticultural lighting, including the ability to control spectral composition, the ability to produce very high light levels with low radiant heat output when cooled properly, and the ability to maintain useful light output for years without replacement. LEDs are the first light source to have the capability of true spectral composition control, allowing wavelengths to be matched to plant photoreceptors to provide more optimal production and to influence plant morphology and composition. Because they are solid-state devices, LEDs are easily integrated into digital control systems, facilitating special lighting programs such as “daily light integral” lighting and sunrise and sunset simulations. LEDs are safer to operate than current lamps because they do not have glass envelopes or high touch temperatures, and they do not contain mercury. The first sustained work with LEDs as a source of plant lighting occurred in the mid-1980s to support the development of new lighting systems to be used in plant growth systems designed for research on the space shuttle and space station. These systems progressed from simple red-only LED arrays using the limited components available at the time to high-density, multicolor LED chip-on-board devices. As light output increases while device costs decrease, LEDs continue to move toward becoming economically feasible for even large-scale horticultural lighting applications.
Abstract
A system was developed for subjecting plants to elevated air ion levels. This system consisted of a rectangular Plexiglas chamber lined with a Faraday cage. Air ions were generated by corona discharge from frayed stainless steel fibers placed at one end of the chamber. This source was capable of producing varying levels of either positive or negative air ions. During plant exposures, environmental conditions were controlled by operating the unit in a growth chamber.
Abstract
A procedure for inducing intumescence injury (oedema) on leaf disks was developed using a species of wild tomato, Lycopersicon hirsutum PI LA 1625. This procedure used Plexiglas vessels with which various combinations of irradiance direction and leaf disk orientation could be studied. Disks (1.4 cm in diameter) were cut from leaves and floated on distilled water in these vessels. Irradiation was provided by cool-white fluorescent lamps and was filtered through ultraviolet- (UV) absorbing Plexiglas. Disks were scored for injury after a period of 72 hr. Intumescences that developed on leaf disks using this procedure appeared similar in size, shape, and coloration to those on intact plants. Disks taken from leaves that were almost fully expanded developed injury most consistently, and the adaxial disk surface was much more sensitive to this injury than the abaxial surface. Intumescences were generally greater on the disk surface in contact with water. Besides L. hirsutum, injury was induced successfully on disks cut from eucalyptus (Eucalyptus globulus), sweet potato (Ipomoea batatas), tomato (Lycopersicon esculentum), ivy geranium (Pelargonium peltatum), European aspen (Populus tremula), and white potato (Solanum tuberosum).
Abstract
Adjustment of the sprinkling application rate to existing atmospheric conditions to conserve water may be accomplished by turning systems on and off. The maximum off period that is tolerable is calculated. It is the sum of time required to freeze the applied water plus time during which the ice coated plant parts cool to the critical temperature. Values of the off period for typical frost conditions are proportional to wind speed and wet bulb temperature. Field test results indicate intermittent sprinkling provides a method to reduce water consumption in sprinkling for frost protection.
Light is one of the most important environmental stimuli impacting plant growth and development. Plants have evolved specialized pigment-protein complexes, commonly referred to as photoreceptors, to capture light energy to drive photosynthetic processes, as well as to respond to changes in light quality and quantity. Blue light can act as a powerful environmental signal regulating phototropisms, suppression of stem elongation, chloroplast movements, stomatal regulation, and cell membrane transport activity. An emerging application of light-emitting diode (LED) technology is for horticultural plant production in controlled environments. Work by our research group is measuring important plant responses to different wavelengths of light from LEDs. We have demonstrated positive impacts of blue wavelengths on primary and secondary metabolism in microgreen and baby leafy green brassica crops. Results show significant increases in shoot tissue pigments, glucosinolates, and essential mineral elements following exposure to higher percentages of blue wavelengths from LED lighting. The perception of energy-rich blue light by specialized plant photoreceptors appears to trigger a cascade of metabolic responses, which is supported by current research showing stimulation of primary and secondary metabolite biosynthesis following exposure to blue wavelengths. Management of the light environment may be a viable means to improve concentrations of nutritionally important primary and secondary metabolites in specialty vegetable crops.
Electrical cost, primarily for lighting, is one of the largest factors inhibiting the development of “warehouse-based” controlled environment agriculture (CEA). In a jointly sponsored collaboration, we have developed a reconfigurable LED lighting array aimed at reducing the electrical energy needed to grow crops in controlled environments. The lighting system uses LED “engines” that can operate at variable power and that emit radiation only at wavelengths with high photosynthetic activity. These light engines are mounted on supports that can be arranged either as individual intracanopy “lightsicles” or in an overhead plane of lights. Heat is removed from the light engines using air flow through the hollow LED strip mounts, allowing the strips to be placed in close proximity to leaves. Different lighting configurations depend on the growth habit of the crops of interest, with intracanopy lighting designed for planophile crops that close their canopy, and close overhead lighting intended for erectophile and rosette crops. Tests have been performed with cowpea, a planophile dry bean crop, growing with intracanopy LED lighting compared to overhead LED lighting. When crops are grown using intracanopy lighting, more biomass is produced, and a higher index of biomass per kW-h is obtained than when overhead LEDs are used. In addition, the oldest leaves on intracanopy-grown plants are retained throughout stand development, while plants lit from overhead drop inner-canopy leaves due to mutual shading after the leaf canopy closes. Research is underway to increase the energy efficiency and automation of this lighting system. This work was supported in part by NASA: NAG5-12686.
Electric supplemental lighting can account for a significant proportion of total greenhouse energy costs. Thus, the objectives of this study were to compare high-wire tomato (Solanum lycopersicum) production with and without supplemental lighting and to evaluate two different lighting positions + light sources [traditional high-pressure sodium (HPS) overhead lighting (OHL) lamps vs. light-emitting diode (LED) intracanopy lighting (ICL) towers] on several production and energy-consumption parameters for two commercial tomato cultivars. Results indicated that regardless of the lighting position + source, supplemental lighting induced early fruit production and increased node number, fruit number (FN), and total fruit fresh weight (FW) for both cultivars compared with unsupplemented controls for a winter-to-summer production period. Furthermore, no productivity differences were measured between the two supplemental lighting treatments. The energy-consumption metrics indicated that the electrical conversion efficiency for light-emitting intracanopy lighting (LED-ICL) into fruit biomass was 75% higher than that for HPS-OHL. Thus, the lighting cost per average fruit grown under the HPS-OHL lamps was 403% more than that of using LED-ICL towers. Although no increase in yield was measured using LED-ICL, significant energy savings for lighting occurred without compromising fruit yield.
Abstract
Two related sprinkler application rate models used in frost protection, published in the mid-1960s, are shown to include an assumption leading to the erroneous conclusion that humidity does not affect the determination of the application rate. A third, 1981 model documents the effect humidity has on the application rate calculation. A distribution factor accounting for nonuniform application is described.
Although plants are envisioned to play a central role in life support systems for future long-duration space travel, plant growth in space has been problematic due to horticultural problems of nutrient delivery and gas resupply posed by the weightless environment. Iterative improvement in hardware designed for growth of plants on orbital platforms now provides confidence that plants can perform well in microgravity, enabling investigation of their nutritional characteristics. Plants of B. rapa (cv. Astroplants) were grown in the Biomass Production System on the International Space Station. Flowers were hand-pollinated and seeds were produced prior to harvest at 39 days after planting. The material was frozen or fixed while on orbit and subsequently analyzed in our laboratories. Gross measures of growth, leaf chlorophyll, starch and soluble carbohydrates confirmed comparable performance by the plants in spaceflight and ground control treatments. Analysis of glucosinolate production in the plant stems indicated that 3-butenylglucosinolate concentration was on average 75% greater in flight samples than in ground control samples. Similarly, the biochemical make-up of immature seeds produced during spaceflight and fixed or frozen while in orbit was significantly different from the ground controls. The immature seeds from the spaceflight treatment had higher concentrations of chlorophyll, starch, and soluble carbohydrates than the ground controls. Seed protein was significantly lower in the spaceflight material. Microscopy of immature seeds fixed in flight showed embryos to be at a range of developmental stages, while the ground control embryos had all reached the premature stage of development. Storage reserve deposition was more advanced in the ground control seeds. The spaceflight environment thus influences B. rapa metabolite production in ways that may affect flavor and nutritional quality of potential space produce.
Previous research in our group demonstrated that short-duration exposure to narrow-band blue wavelengths of light can improve the nutritional quality of sprouting broccoli (Brassica oleacea var. italica) microgreens. The objective of this study was to measure the impact of different percentages of blue light on the concentrations of nutritional quality parameters of sprouting broccoli microgreens and to compare incandescent/fluorescent light with light-emitting diodes (LEDs). Microgreen seeds were cultured hydroponically on growing pads under light treatments of: 1) fluorescent/incandescent light; 2) 5% blue (442 to 452 nm)/95% red (622 to 632 nm); 3) 5% blue/85% red/10% green (525 to 535 nm); 4) 20% blue/80% red; and 5) 20% blue/70% red/10% green in controlled environments. Microgreens were grown at an air temperature of 24 °C and a 16-hour photoperiod using a light intensity of 250 μmol·m−2·s−1 for all light treatments. On emergence of the first true leaf, a nutrient solution of 42 mg·L−1 nitrogen (N) (20% Hoagland’s #2 solution) was used to submerge the growing pads. Microgreens were harvested after 20 days under the light treatments and shoot tissues were processed and measured for nutritionally important shoot pigments, glucosinolates, and mineral nutrients. Microgreens under the fluorescent/incandescent light treatment had significantly lower shoot fresh mass than plants under the 5% blue/95% red, 5% blue/85% red/10% green, and the 20% blue/80% red LED light treatments. The highest concentrations of shoot tissue chlorophyll, β-carotene, lutein, total carotenoids, calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), glucoiberin, glucoraphanin, 4-methoxyglucobrassicin, and neoglucobrassicin were found in microgreens grown under the 20% blue/80% red light treatment. In general, the fluorescent/incandescent light treatment resulted in significantly lower concentrations of most metabolites measured in the sprouting broccoli tissue. Results from the current study clearly support data from many previous reports that describe stimulation of primary and secondary metabolite biosynthesis by exposure to blue light wavelengths from LEDs.