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Control systems include those for lighting, heating, cooling, relative humidity, and carbon dioxide enrichment. Whilst it is helpful to have a fully controlled environment, aquaponic cultivation can also thrive without it, or with only some of the parameters being controlled.
Maximum light transmission, of the appropriate quantity and quality (PAR, 400-700 nm), is crucial for optimal photosynthesis, growth and yield. If there is too much light in the summer, shade paint or white wash can be sprayed on the outside of the greenhouse. This will either wear off by the end of the growing season, or it can be washed off. External fabric shade cloths made of varying degrees of mesh size to exclude specific amounts of light (e.g. 30%, 40%, 50% shade) can be placed on the outside of the greenhouse or hung inside it. If there is too little light during the winter, white reflective ground covers can significantly increase light levels to the plant canopy (Rorabaugh 2015).
Artificial lights can be used to extend the winter growing season. Various different light technologies are used in greenhouses, but the most common type is light emitting diodes (LEDs). Unlike all other artificial lighting systems, LEDs contain no glass or gaseous components: all the components are solid state. They are therefore less fragile than other types of lamp, and can be located in places where other lamps may become damaged and pose a health and safety risk. However, one potential negative impact of using LED lighting in greenhouses is the lack of radiative heat that they produce, which reduces the overall energy saving as there is greater heating demand (Davis 2015).
LEDs are now available with almost any wavelength between 200 and 4000 nm. The advantages of LEDs are (i) their high efficiency (light energy output/electrical energy) compared to other lighting sources; (ii) that the light emitted is directional, which reduces the amount of stray light and ensures that the maximum amount of light reaches the crop; and (iii) that the overall spectrum can be modified for different applications by changing the number and colours of LEDs installed in a lighting unit. LEDs thus provide the potential for optimisation of light treatments that allow the enhancement of specific plant qualities or control over plant morphology and flowering time. To produce healthy plants, both red and blue light are required. Red light is most effectively used to drive photosynthesis, but plants are generally found to grow more effectively when some blue light is contained within the light spectrum, because it helps promote the stomata-promoting CO2 uptake. Stomatal responses to light do, however, differ between species, so not all species will benefit equally following the addition of blue light. In lettuce, for example, growth rates have been found to decrease as blue light was increased (Davis 2015).
There are instances where additional colours of light may provide additional benefits. The inclusion of green light has been shown to increase fresh and dry weight biomass accumulation in lettuce plants when the green light replaces some of the blue or red light in the mixture. Green light can also penetrate deeper into the plant canopy, and therefore drive more photosynthesis. Far-red light is important for plant development and performance throughout the life of a crop. While it can inhibit the germination of lettuce seeds, it can nevertheless increase leaf area, potentially allowing greater light capture and growth rates. During the later stages of crop development, on the other hand, it will cause stretching and bolting. The area where far-red light can perhaps be used to greatest effect is for controlling flowering time (Davis 2015).
LEDs also provide the opportunity to light crops in non-traditional ways. LEDs are cool light sources and, as such, can be placed close to crops or within a canopy to light leaves that would normally receive little natural or supplemental light. By adding light to leaves normally in the shaded region of the canopy, plants are able to use the light more efficiently. This means that ‘interlighting’ has the potential to increase yields more than the same amount of light added on top of the canopy. Interlighting blue light has been found to have mixed results in yields of cucumber plants and tomatoes (Davis 2015).
Spectral manipulation can also be used to improve pigmentation. Blue light is important for driving the synthesis of anthocyanin, which is one of the types of compounds that causes red pigmentation. Light is also important in regulating the biosynthesis of many of the compounds that function to directly alter the flavour and aroma of leaves, fruits and flowers. UVB light exposure has been linked to increased oil and volatile contents in a range of herb species, including lemon balm and basil (Davis 2015).
In the majority of research, the influence of light quality on crop quality is considered during the period of crop growth, but more recently the effect of post-harvest light treatments has also been considered. Post-harvest crop treatments provide the potential to enhance crop qualities during transport to delay the onset of senescence, thus extending shelf life. Exposure to two hours of low intensity red light was found to delay senescence of basil leaves for two days during storage at 20 ⁰C in the dark (Davis 2015).
The reaction of plants to various colours of the light spectrum can therefore be used to manipulate plants to satisfy different needs, including the following:
Ultraviolet light can be used to shorten the internodes
Blue and ultraviolet light can be used to increase plant stress tolerance before transplanting
Blue light can be used to stimulate vegetative growth and prevent shorter-day plants from flowering during their propagation stages
Red light can be used to induce flowering and lengthen the internodes to produce plants with longer stems and bigger flowers
Far-red light can be used to control the photoperiodism of plants
Lux meters are widely used in horticulture to measure the intensity of high-pressure sodium lamps (HPS). Lux meters have been designed to have the same sensitivity to different regions of the electromagnetic spectrum as the human eye, which is most sensitive to green light. However, for many of the horticultural LED lamps, especially those with predominantly red and blue LEDs, the emission spectra fall in regions where the lux meters are relatively insensitive, and provide very low estimates even when the actual intensity of these spectra is high. The most suitable light measurement for use with plants is PAR photon irradiance (also called photosynthetic photon flux density, PDFD). PAR photo irradiance indicates the number of photons that are incident on a surface measured in micromoles per metre squared per second (µmol m-2 s-1). Because photosynthesis is measured in similar units (µmol [CO2] m-2 s-1), use of PAR photon irradiance allows direct comparisons between the amount of light and the amount of photosynthesis to be made (Davis 2015).
Figure 13: Growing under UV light https://commons.wikimedia.org/wiki/Category:Aquaponics#/media/File:Light_on_Aquaponics.jpg
Heating devices will maintain the temperature within the optimal range during periods of cold weather. Insulating material (cloth or film curtains) can be positioned above the crop or near the roof to retain heat near the crop. The insulating material used during the night can be the same as the material used for shading during the day (Rorabaugh 2015).
High temperatures can be detrimental to plant growth, especially is there is low light availability. High temperatures can cause problems such as thin, weak stems, reduced flower size, delayed flowering and/or poor pollination/fertilization and fruit set, and flower and bud/fruit abortion. Passive ventilation systems include shade cloths or shade paint/white wash which, besides regulating the light intensity, can also help to cool the greenhouse. Ridge vents in the roof of a greenhouse allow hot, interior air to escape. The area of the vents should be 25% of the floor area. Roll-up side walls can be used in flexible glazing (polyethylene film) greenhouses to allow a natural horizontal flow of air over the plants. As with ridge vents, the area of the side wall vents should be 25% of the floor area. Water cooled pads at the top of cooling towers can be used to cool the surrounding air which then drops, thereby displacing warmer air below. Recent greenhouse designs can include a roof that retracts completely for natural ventilation. This allows greenhouse grown plants to adapt to outside conditions (Rorabaugh 2015).
Active cooling systems involve fan and pad ‘evaporative cooling’ where air from the outside is pulled through porous, wet pads (usually cellulose paper). Heat from the incoming air evaporates water from the pads, thereby cooling the air. Evaporative cooling will also help to increase the relative humidity in the greenhouse. Alternatively, fogging systems also use evaporative cooling, but incorporate a dispersion of water droplets that evaporate and extract heat from the air. This system gives better uniformity since the fogging is distributed throughout the greenhouse, and not just near one pad end as with the fan and pad system. The smaller the droplet size, the faster each droplet evaporates, and therefore the faster the rate of cooling. Relative humidity can be increased by running the cooling pads or by fogging, and can be decreased by running heaters or simply by venting (Rorabaugh 2015).
The rate of photosynthesis is dependent upon the availability of carbon dioxide. Ventilating may provide sufficient CO2 during the spring, summer and autumn, but in winter, or anytime in cold climates, it will result in cold air being brought into the greenhouse. Heating will then be needed to maintain the proper temperature, which may become uneconomical. CO2 generation is therefore an effective way to increase levels in the greenhouse during the winter or in cold climates. CO2 generators can burn various types of fuel, including natural gas (most economical) or propane. Open-flame generators also produce heat and water vapour as by-products. Therefore, hydroponic growers sometimes use CO2 generators in the winter, when the extra heat production is welcome, and bottled CO2 and dosers in the summer, since they produce no extra heat or humidity. Since CO2 is released by plants through respiration at night, it is not uncommon for levels to build up to between 0.045% and 0.070% in the growroom by morning. Setting the timer to begin dosing CO2 one hour after the lights come on, with the last dose one hour before the lights go off, is the most economical way to provide supplemental CO2. To keep CO2 at optimal levels, it is best to dose for short periods of time at higher volumes than to dose for longer periods of time at low volumes. (Rorabaugh 2015). In aquaponics, the fish tanks are often in the same room as the hydroponic component. The fish respiration raises the CO2 levels of the system water, and CO2 also enters the atmosphere. Therefore, additional CO2 inputs are either not required, or are very low (Körner et al. 2014).
One reason for having a greenhouse is to create a ‘controlled environment’ for all of the plants. However, especially during times when the heating and cooling systems are not in operation, pockets of high or low temperature, relative humidity or carbon dioxide may develop which can be less than optimal for plant growth or flower/fruit development. Horizontal air flow (HAF) fans can be placed in the rafters of the greenhouse to circulate air above the crop. This helps to minimize pockets of warm or cold air and high or low humidity or carbon dioxide. HAF fans can be used in conjunction with hot air heating systems to circulate warm air throughout the greenhouse (Rorabaugh 2015).
Environmental control systems can be very simple or very complex. The simplest systems involve manually rolling up a side vent, opening a roof vent or door, or turning on a heater or cooler. Simple controllers operate from a thermostat in the greenhouse and will automatically set day and night temperature ranges, open and close vents, and turn heaters and coolers on or off. Step controllers will also automatically control 1 or 2 heating stages, depending on the number of heaters, and control several cooling stages using cooling fans and pump(s) to wet the pads. The most complex environmental control systems use sophisticated computers which operate from a temperature sensor in the greenhouse and automatically set day and night temperature ranges, control heating equipment including boilers, root zone heating, heat retention curtains, etc., control other equipment including HAF fans, exhaust fans, vents, pad pumps, fogger systems, etc., control relative humidity, and control shade curtains and artificial lighting depending on light requirements. Sophisticated computers can also monitor an external weather station and use the data collected (light, temperature, relative humidity, rain, and wind) to control internal conditions in the greenhouse. They can also operate the fertigator system by automatically using light quantity (e.g. X ml of solution/Y amount of light), and controlling the timing of watering, duration of watering, nutrient solution pH and EC, and misting (Rorabaugh 2015).
Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).
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