**8.4 Greenhouse supplemental lighting**

Supplemental lighting is necessary in seedling production, as well as in growing vegetable crops that have a longer vegetative period and high demands for light (tomatoes, peppers, cucumber). In greenhouses, it is usually needed during long winter months and periods of overcast. The supplemental lighting prolongs a day, compensates for a natural light limiting effect in winter, and enhances the amount of the available light. Supplemental lighting should not be confused with photoperiodic lighting, which is applied to create long days, thus controlling the plant growth and development processes. Supplemental lighting is high-intensity light (6000–7000 lx or more), whereas photoperiodic lighting is of a much lower intensity (70 lx–100 lx). For now, in the vegetable crop production practice, the high-intensity discharge (HID) lamps and LED lightning are in use, which provide wavelength light from 300 to 700 nm (which is needed for the photosynthesis process). The HID lamps, besides needed light wavelength, emit high heat (up to 50%), so they must be placed at about 02 m or more above the crops. Otherwise, such lamps could overheat the greenhouses (what change the climate in the greenhouses) and could damage the plants, too. The LED lightning are placed at about 40 cm or more above the crops or in between the crop's rows in a greenhouse, as they almost do not produce heat and could not damage the crops. There are studies about the ways how the plants use the incoming photosynthetic active radiation (PAR), which is based on a principle of an exponential increase of absorbed photosynthetic active radiation, with the increase of the leaf area index (LAI) [11–16] (**Figure 16**).

**Figure 16.** *Block-greenhouse with supplemental lighting.*

## **8.5 High-intensity discharge (HID) lamps**

The high-intensity discharge lamps include two types of lamps:

• Metal halide (MH)

Metal halides (MH) lights are commonly used during vegetative plant growth but are less popular than HPS lamps for flowering and fruiting. If MH lamps are used in the flowering or fruiting stages, they are often of a higher rated power to provide more red-light output.

• High-pressure sodium (HPS)

High-pressure sodium (HPS) lamps produce light mainly in the yellow and red end of the light spectrum, which makes these lighting systems a great fit for late-phase (flowering and fruiting) plant growth. They can also deliver greater light intensities than lamps used exclusively for vegetative growth (for instance, MH lamps).

#### **8.6 LED lightning and modern urban vegetable growing light-emitting diodes (LEDs)**

Light-emitting diodes (LEDs) represent a technology for the indoor vegetable production, which has technical advantages over traditional lighting sources, as well as a significantly positive impact on the plant photosynthesis process and on the crop yield. LED lightning has been tested for horticultural applications, both in greenhouses and in special chambers with a total control of climatic and other conditions necessary for the crop's growth and development. It is used in growing leafy vegetables and herbs, as well as in the production of tomato, pepper, cucumber, and some more vegetables in greenhouses. Depending on the vegetable varieties and their edible parts (vegetative part, fruit, immature flower heads), LEDs could be designed to emit light recipes for each phenophase of the crop's growth and development. One of the most important features of LEDs for vegetable growing is that the LED lightning almost does not produce the heat, which could damage the plants. There is a combination of additional lightning and its quality and supplemental carbon dioxide empowerment vegetable growing in the greenhouses. It is vital that both light and carbon dioxide are provided in sufficient amounts within the greenhouse, or otherwise, a lack of either may pose a limiting factor for the photosynthetic process and consequently for the crop's productivity. Controlling climate conditions and nutrition in protected area, without or with daylight, gives opportunity to produce vegetables, which is safe for humans and environment. It is called ""City Farming" or "Urban Agriculture." The most important factors are the LED lighting, carbon dioxide concentration, air humidity, and conditions that keep the leaf stomata open in order to uptake carbon dioxide [14–19] (**Figure 17**).

#### **8.7 Light quality in supplemental lighting**

The suitable light quality in the greenhouse per vegetable species refers to the wavelengths (colors) that are efficient in triggering photosynthesis and other growing processes in vegetable crops. Successful growing vegetables in greenhouses depend on the visible spectrum wavelengths of about 390 nm to 760 nm, which is only a small portion of the sunlight (radiation) spectrum. In general, the visible light consists of the following: violet (380 nm to 430 nm), blue (430 nm to

*Advantages of Growing Vegetable Crops in Modern Greenhouses DOI: http://dx.doi.org/10.5772/intechopen.101469*

#### **Figure 17.**

*LED lighting (The Netherlands), Growth chambers without natural daylight, providing optimal conditions for vegetable crops growing under LED lighting (the Netherlands).*

500 nm), green (500 nm to 570 nm), yellow (570 nm to 590 nm), orange (590 nm to 630 nm), and red light (630 nm to 760 nm). The most important visible light range mainly corresponds to the PAR from about 400 nm to 700 nm. Those wavelengths have the right amount of energy for the biochemical processes, and their participation in the available light is important for the quality of light during vegetable growing. About half of the incoming sunlight energy participates in the photosynthetic processes. The rest of the energy is from the sunlight short wavelength spectrum (UV—ultraviolet radiation) and sunlight long wavelength spectrum (IR—infrared radiation).

**Blue section of the spectrum** is known as a cool light, which encourages vegetative and leaf growth through strong root growth and higher intensity of photosynthesis. **Red section of the spectrum** induces stem growth, tuber and bulb formation, flowering, and fruit production, and chlorophyll formation. **Far-red light** may cause plants to stretch (elongate) and may trigger flowering in some long-day plants.

The plants are exposed more to the far-red than to the red light, which could be a challenge with the greenhouse vegetable crop production due to possible shading, or due to the reduced plant vegetative space. **Green and yellow sections of the spectrum** that reach the plants are reflected. Most of the absorbed sunlight belongs to the blue and red part of the spectrum. However, the recent studies have shown that plants do also absorb some green and yellow light, using it in the process of photosynthesis [8]. For the time being, in the greenhouse vegetable crop growing practice, the high-pressure sodium (HPS) lamps are used, but also the LED lightning, which gains an increasing significance in the plastic and glass greenhouses as well as in the special chambers for vegetable production without daylight. Also, in The Netherlands, the latest studies at the research centers of Wageningen and Maastricht universities have their guidelines for greenhouse lighting with little or no natural daylight for special feature vegetable crops growing—increased vitamin C content, reduced nitrates content, increased sugar content, higher yield (**Figure 18**).

## **8.8 Supplemental carbon dioxide in the greenhouse**

The carbon dioxide (CO2) gas is the essential component for the process of photosynthesis. Plants uptake it through their stomata on the leaves. Photosynthesis is the biochemical processes where the light energy is used to convert carbon dioxide and water into complex sugar compounds (carbohydrates) and oxygen (O2) gas. The sugars in plants, formatted in the process of photosynthesis, are then used for the plant development and growth. In the air (outside the greenhouse), there is about 400 ppm of carbon dioxide. The CO2 concentration in greenhouses could be increased specific installation in the greenhouses, for example, from boiler room, when energy sources are burnt. However, inside the greenhouse, the amount of CO2 may be significantly depleted as plants use it intensively in the process of photosynthesis, or through the greenhouse windows, and it may lead to a decreased crop productivity and yield. For that reason, "CO2 fertilization" or "CO2 enrichment" is a standard practice in modern greenhouses and should be controlled in order not to form too high concentration (over 1500 ppm), which could be toxic for the vegetable crops (**Figure 19**).

**Figure 18.** *LED lighting between the rows of cucumber plants growing on rockwool in a modern glass greenhouse (the Netherlands).*

*Advantages of Growing Vegetable Crops in Modern Greenhouses DOI: http://dx.doi.org/10.5772/intechopen.101469*

**Figure 19.** *Photosynthetic process equation.*

Since there is about 500 times more oxygen in the air than carbon dioxide [17], it makes sense to increase the CO2 concentrations in the greenhouse (particularly in highly equipped glasshouses). It has a positive effect on the oxygen-carbon dioxide ratio. The photosynthesis is higher by 30–50% at CO2 concentrations of about 1000 ppm, regardless of the amount of light.

Photosynthesis depends on light, temperature, air humidity, and carbon dioxide contents in the greenhouse. There is often a question of what is the optimal concentration, but it is hard to give a correct answer to it as the process of photosynthesis does not depend solely on CO2. Also, a point should be made that climatic factors affect the stomatal opening mechanism (through which the plants uptake CO2). Generally, a small increase in the plant photosynthesis process may be achieved at 1000 ppm to 1200 ppm, but then, there is also an increased possibility of damage to the crops. One experiment done on eggplant crops showed that the first damage to the plants occurred at a constant CO2 level of 800 ppm [17]. Quite often, the intensity of the photosynthesis may be higher at lower doses of carbon dioxide and higher intensity of light, and the other way around. Supplementing the greenhouse air with carbon dioxide may not be necessary at all as long as the processes of the crop development and growth are quite satisfactory for the vegetable grower. At the same time, in a case of intensive greenhouse ventilation, the carbon dioxide concentration may drop below a level that is necessary for the normal photosynthesis process, so increasing the CO2 concentration may not be an economical measure. If the crop quality and production are below the satisfactory level, carbon dioxide supplementing should be the next measure. The vegetable production in cloudy period of the year, or cloudy days, increases the potential need for CO2 supplementing the greenhouse air, which actually corresponds to a lower ventilation rate due to mainly low outdoor temperatures. According to Kamp and Timmerman [9] normal ventilation provides an amount of carbon dioxide that is similar to its levels in the outdoor air (350 ppm–400 ppm). But then, frequent ventilation in the greenhouse may not be economically desirable for the enrichment indoor air with CO2. The necessary greenhouse carbon dioxide concentration is determined upon the type of the crops grown in the greenhouse, the greenhouse total volume and ventilation, lighting, temperature, air humidity, and stomatal opening. Since carbon dioxide is one of the products of burning (e.g., fuel for greenhouse heating system), this segment of the heating process can be used for supplementing the greenhouse air. There are various ways of extracting carbon dioxide from other products of burning (fuel), so that the CO2 from the boiler room can be dosed and at certain times directed and distributed into the greenhouse.

Also, pure carbon dioxide can be used, which is delivered to growers in special tanks, in liquid form and then can be converted into gas and distributed in the greenhouse. That way of supplementing the CO2 has become increasingly popular as it eliminates any potential damage to the crops, allows control of other greenhouse climatic conditions that regulate the process of photosynthesis and crop productivity, provides easy control of the carbon dioxide levels, and is more flexible for supplementing the CO2 when necessary. Also, it would be advisable to install a

proper system that registers the CO2 concentration and then distributes it in the greenhouse. Such a system, like in other greenhouse installation operations, has corresponding sensors that are linked to special computer software that registers, monitors, and controls all the greenhouse environment parameters. In this way, it is possible to detect a cause of each change and correct it in a short period of time, and potential damages of the vegetable crops could be easily prevented (**Figure 20**).

The distribution of CO2 depends mainly on-air movement within the greenhouse, as CO2 does not travel very far through diffusion systems. One of the pure CO2 distribution ways is by a central pump that pushes it into a system of flexible perforated plastic pipes (made of polyethylene or other plastic material). The pipes for CO2 distribution are placed below the substrate special gutters with plants (if crops are grown in such gutters), or in the lower sections of the crops (if the plants are not grown in gutters). Then, through the pipe perforation, the carbon dioxide is distributed in the air around the plants (**Figure 21**).

### **8.9 Greenhouse screens**

Greenhouse screens provide enough light and may reduce oscillations in the indoor climatic conditions. Today, there is a vast variety of greenhouse screens made of different materials. For shading, polyethylene, polyester, acryl, and aluminium knitted cloth is used. They may be of various types of knit, physical characteristics, and colors. Besides regulating the greenhouse lighting, they may be great in controlling the indoor temperature and relative air humidity, thus saving on heating and electricity. Depending on a particular use and a usage period, they are made of cloth, plastic, or aluminium. The choice of material, fiber thickness, and the type of knitting/weaving should correspond to the screen purpose. There is also a possibility of automatic, semi-automatic, and manual screen operating system. The choice of the screen or curtain system depends on the purpose, production period, the necessary crop growing conditions, the amount of the necessary lighting, certain climatic parameters in a given geographical area, the greenhouse total volume and ultimately on the quality of the shade screens, as well. The shading screen is mostly used to prevent a negative effect of sunlight radiation during daytime and excessive cooling during night or cold periods. In regions of moderate climate and a great number of overcast days, the screen system actually saves

**Figure 20.** *Liquid carbon dioxide tank for supplementing it in the greenhouse (Serbia).*

*Advantages of Growing Vegetable Crops in Modern Greenhouses DOI: http://dx.doi.org/10.5772/intechopen.101469*

**Figure 21.** *Plastic perforated pipes distributing CO2 (The Netherlands).*

energy, transmitting light and controlling the air temperature and humidity. The screen system can be installed horizontally (under the roof) or vertically (along the side walls). Horizontal systems have an important role in controlling the air relative humidity and can be installed at different heights in the greenhouse. Also, vertical screens or curtains can be installed as a mobile boundary and/or a partition wall, separating individual climatic units within the greenhouse. This is very useful when crops of different climatic requirements are grown. The screen systems are classified and designed according to their use in controlling the greenhouse climatic conditions, transmission of light, air temperature, and humidity, so that the environment parameter optimal values for growing crops can be reached. This can be achieved with either open or closed screen systems. If, say, only temperature has to be controlled (preserving heat), transparent energy-saving screens reduce energy consumption and allow nearly complete light transmission. If the greenhouse vegetable crop production goes on during the best part of the year, including both sunny and cloudy, both warm and cold parts of the year, then all the greenhouse parameters should be taken into account for the crop production cycle. If this is the case, a double screen system should be installed: An upper-level screens, closer to the roof, and a lower-level screen, installed below the former. The upper-level screen material should be of various woven plastic or aluminiums fibres, while the lower-level positioned ones should be transparent to transmit light, but also to maintain balanced humidity and temperature, and to save the indoor heat, too. Certain screen system is used for various purposes: protection against ultraviolet radiation or just for shading (light is not transmitted), or for transmitting and maintaining a section of the sunlight spectrum (wavelengths from 400 nm to 700 nm), which is crucial for the process of organic matter synthesis, thus producing good-quality vegetable crops (**Figure 22**).

**Figure 22.** *Greenhouse screens (The Netherlands).*

Also, it is very useful to install the screens on the greenhouse windows/vents. When vents are open, the screens protect against insects, birds, dust, or litter that winds may blow into the indoor area. At the same time, the screens maintain the indoor climatic conditions. The basic screen material component is polyethylene or polyester, so in a case of a greenhouse fire, the screens pose a great threat. For that reason, it is important to provide enough spacing between the screens or curtains (follow the manufacturer's recommendations) or if that is not possible, the screens should be selected from fire-resistant materials, so that the fire could be localized to just one section of the greenhouse. Such fire-resistant screens have fibers that are coated with fire-resistant material. In the modern greenhouse that covers a vast area (several hectares or tens of hectares), the screen and curtain system is moved around with special motors that automatically respond to the indoor climatic changes. Screen are necessary in greenhouses, particularly in the glass ones, as they are a great help in providing good or even optimal climatic conditions necessary for the crop's development and growth (**Figure 23**).

The screens are usually installed under the roof, and inside the greenhouse, they prevent heat from escaping (during night or cold periods) or excess heat (during summer). For this purpose, knitted polyethylene, polyester, acrylic, and aluminum curtains are used, but also curtain strips, plastic and aluminum film, or roof are painted with lime or special paints.

**Figure 23.** *Curtains, Screens, guide wires, and plant-supporting strings (The Netherlands).*
