Introduction
Global food production and food safety will become increasingly challenging due to climate change and lack of farming lands as the world's population continues to grow (Kalantari et al., 2018). To provide a steady food supply and to cultivate high-quality food, the plant factory has recently attracted significant attention (Despommier, 2010).
The plant factory technology has been used to produce vegetables and fruits since the early 2000's (Goto, 2012). Recently, plant factories have been developed in different regions, especially in Asian regions, as a new kind of protected horticulture to produce food at a big scale (Nicole et al., 2016). Different types of growth chambers with a controlled environment in plant factories are closed facilities for growing plants and vegetables under artificially controlled conditions (Kozai, et al., 2015). Consistent and predictable plant growth and development, high harvest value per unit production area, as well as a short production time, are all possible advantages of growing plants in controlled environments (Benke et al., 2017). In comparison to growing plants and vegetables in a greenhouse or open field, a controlled environment offers numerous advantages, including the ability to optimize environmental conditions, avoid disease and insect invasion, increase production rate, etc. (Zhang et al., 2018). Although in a greenhouse, a controlled environment condition is maintained, there are influences from the outside environment. Sunlight condition and duration, an excessive amount of rain or snowing condition are also affected the greenhouse inside environment. As a result, the indoor ambient environment condition is poorly controlled. Furthermore, we have no control over the outside environment. These conditions do not exist in plant factories. It is completely unaffected by the outside environment, making the indoor environment of the plant factory more stable and controlled.
There are a number of environmental variables that must be kept in optimal condition at all times in the plant factory. Temperature, relative humidity, light, air velocity, and CO2 concentration are the key ambient environmental variables that impact on plant development in a controlled condition (Ahmed et al., 2020). Fig. 1 shows the ambient environment parameters in a plant factory. For plant growth, light is a critical environmental element, as it is the primary source of energy necessary for photosynthesis and many other physiological activities (Bayat et al., 2018). On the other hand, air velocity has an impact on the optimal management of environmental variables in a controlled environment (Kitaya et al., 2000). To manage the plant's internal conditions, the optimal air velocity is necessary, but it also offers a mechanism for heat exchange between plant leaves and surrounding air. CO2 and H2O diffusion through stomata are also significantly affected by air velocity (Ahmed et al., 2020). To manage the net CO2 uptake for the indoor environment, the appropriate temperature is key to maintain at all times. High level of CO2 increase transpiration rate and thicken the plant leaves. In a plant factory, relative humidity also influences the plant growth and vegetative development, such as leaf area, root and shoot development, number of buds, etc. (Mortensen, 1986). It also has an indirect effect on the stomatal conductance of plants (Kaiser et al., 2014).
Therefore, the primary focus of this review article is to provide an overview of the effects of ambient environmental variables on the growth and development of plants in the plant factory. As a result, it could provide an opportunity for automatic control and monitoring of the environment based on plant conditions. As well as, providing crucial information regarding plant development and productivity would be beneficial. The objective of this review is to focus on the effects of temperature, relative humidity, light, air velocity, and CO2 concentration on plants grown in a plant factory and monitoring technology for plant factory.
Ambient environmental variables affecting plant growth
Light
In plants, photosynthesis and many other morphological activities rely on light and light is the major energy source. (Bayat et al., 2018). Fig. 2 shows different light absorption bands for the plant photosynthesis process. PAR (photosynthetically active radiation) is the cause of plant photosynthesis with a wavelength of 400 to 700 nm. Aside from light between these wavelengths, photons for higher or lower wavelength also helps photosynthesis, different metabolic process and morphological changes (Thoma et al, 2020). Wavelength range between 300 and 800 nm are designated as less physically active radiation. Light properties such as light intensity, total amount of light, light timing, photoperiod, quality of light, and direction and distribution of light over the plant impact plant growth and development during the whole plant's life (Merrill et al., 2018). Increasing the amount of light available to plants would be a good option for enhancing growth rate, quality and yields (Wang et al., 2016).
Sunlight is the most common source of light for the plants, yet in plant factories, artificial light sources have taken its place. High-pressure sodium lamps, metal halide lamps, fluorescent light (FL), and light-emitting diodes (LEDs) are the most common artificial light sources used in controlled environments in plant factories. Fluorescent lights, the most prevalent type of light used in plant factories, have been increasingly replaced by LEDs. With LEDs, it is possible to simply adjust the light intensity and quality of the light which is different from crop to crop (Cioc et al, 2021). As a key factor in controlling biosynthesis processes in plants, light intensity has a profound impact on a wide range of physiological processes related to plant growth and photochemical reactions (Singh and Singh, 2015). Therefore, when it comes to plant development, choosing the correct light intensity and quality depending on the plant’s essentials are crucial. Currently, LEDs come in a wide range of light quality options that can be easily tailored to match the demands of any plant species. LEDs with a mix of red and blue light are the most effective for growing and developing leafy crops such as lettuce (Ahmed et al., 2020).
LED illumination was used to study the influence of light intensity on hydroponic strawberry root formation and growth by Zheng et al. (2019). The results revealed that at the rooting stage, 90 µmol m−2 s−1 was optimal at the light intensity range of 90-270 µmol m−2 s−1 with the increase of the stomatal conductance of newly formed leaves. Zhang et al (2018) studied the impact of different light intensities, photoperiods, and light quality combinations on the development of a lettuce plant in a closed plant factory. The result indicated that the photosynthetic photon flux density (PPFD) of 250 µmol m−2 s−1 with a photoperiod of 16 hrday-1 were shown to be optimal for the lettuce plant growth development. Lettuce growth and development were better when it was exposed to red and blue light. High and low lighting intensities on plant growth rate and light usage efficiency were studied by Urrestarazu et al. (2016). They came to the conclusion that plant growth rate is more closely connected to the photosynthetic response to light intensity than the other way around. Lee et al. (2016) studied the effects of artificial light on hydroponically grown Peucedanum japonicum growth and yield in plant factory. To test plant physiological characteristics, several combinations of red, blue, and white LED lights were investigated. The tests found that plant height and weight varied significantly over the growing period. Further research on LED pulses and duty rates is required in order to improve LED performances. Liu et al. (2019) evaluated the effect of different LEDs combinations on growth of pepper seedlings in plant factory. Pepper seedlings biomass and photosynthetic characteristics were found better under LEDS than fluorescent light. The red bad blue light ratio of 1.5 was best suited for the pepper seedling growth in the plant factory.
Photosynthetic rate and efficiency highly vary with the wavelength of the light absorbed by the plant. Photosynthetic efficiency in plant factories was studied by Goto et al. (2013) by examining the influence of light quality on plant growth. Photosynthetic rate increased incrementally by 320 µmolm−2s−1 as opposed to 160 µmolm−2s−1 for different types of vegetables with the maximum wavelength of 660 nm and minimum wavelength of 465 nm. The findings showed that the optimal wavelength for increasing photosynthesis varies with light quality, light intensity, and crop type. Pérez-López et al. (2018) found that strong light intensities enhanced chemical compounds in two distinct lettuce types. Short-term high light intensities increased leaf area, resulting in improved interception and encouraged biomass production. From germination until flowering, light duration or photoperiod has an impact on plants. These findings suggest that optimum light intensities for plant development and increased production range from 150 to 250 µmol m−2 s−1. A greater understanding of plant development and quality may be gained by using the alternating light sources.
Air velocity
In a controlled environment, air velocity is one of the ambient environmental variables that impact plant development and growth (Chowdhury, 2021). Plant canopy air temperature, relative humidity, and CO2 concentration are hugely managed by air circulation inside the plant factory (Shibuya et al, 2006). Plant development and yield are affected by air velocity both in direct and indirect ways. Plant's interaction with the environment is based on energy exchange, and the air velocity has a direct impact on this energy exchange and mass balance. CO2 and H2O, as well as sensible and latent heat, are transported through the leaf boundary layer by airflow (Shibuya et al, 2006).
Nishikawa et al., (2013) studied the effect of air flow for lettuce plant growth in the plant factory. The controlled and rotational air flow makes about 20% larger lettuce compared with the normal condition. The condition in wind velocity of 0.9 ms−1 has shown superior effect than the condition in wind velocity of 1.8 ms−1. This concluded that the air flow estimation is required depending on the conditions in the plant factory. A study by Lee et al. (2013) looked at the symptoms of tipburn in lettuce leaves grown in closed plant factory. Air velocity of 0.28 ms−1, 0.55 ms−1, and 1.04 ms−1 were applied as low, medium and high, respectively. According to the study, a horizontal air velocity of 0.28 ms−1 or higher significantly reduces the symptoms of tipburn, with not much differences in other tested air flow rates. A consistent horizontal air flow application along crop beds was shown to be more successful than regulating air temperature. To enhance growth and reduce tipburn symptoms, Ahmed et al. (2020) examined the influence of air flow rate utilizing a multi-fan system. There has been a significant rise in transpiration rate after increasing air velocity from 0.23 ms−1 to 0.75 ms−1 and diminished the tipburn effect of lettuce. Peiro et al. (2020) evaluated the effects of airflow distribution on lettuce plant growth uniformity and agronomical performance in a controlled environment. Results indicated that air speeds between 0.3 and 0.5 ms−1 maximized lettuce biomass production, whereas air speeds over 0.6 ms−1 inhibited lettuce plant development. Moreover, air movement is crucial not only for plant development, but also for proper water recycling through plant transpiration. A CFD simulation was conducted by Baek et al. (2016), which compared and assessed the effect of different operating modes of air flow devices for the growing environment in a vertical plant factory. Maximum air velocity is essential for gas exchange and plant development, as well as maintaining consistent environmental conditions inside the plant factory is a must, which is done by the air flow inside the plant factories (Kubota, 2016). To calculate the optimum air velocity, Chintakovid et al. (2002) suggested that the photosynthetic and transpiration rates should be taken into account. Besides, when calculating the optimal air velocity for a plant factory, other factors like plant species, plant size and shape, canopy length, and direction of air flow should also be considered.
Temperature
Plant growth and development are influenced by temperature, which is one of the major environmental variables. The rate of plant growth and development increases linearly as the temperature rises up to the maximum rate (Kubota, 2016). As a result, leaf temperature has a greater influence on plant growth and development than ambient air temperatures. However, air temperature has an impact on how quickly a leaf grows out. Hence, the ideal temperature and high level of organic matter are important for rapid plant development (Tian et al., 2014).
According to Hasegawa et al. (2014), the impacts of ambient temperature and light interruption duration on the evaluation of plant activities have been studied. The controlled growth chamber temperature was 15 to 35°C with a humidity of 60-70%. This study looked at how temperature affects the bioelectric potential responses at a variety of temperatures. Light irradiation and interruption caused the potential responses to alter instantly at all ambient temperatures. An experimental lighting system was employed in a plant factory where Hong et al. (2011) studied the effects of temperature changes to lettuce development on culture tables with LED as an artificial light source. The results showed that with the indoor temperature of 25ºC, the maximum growth can be obtained for the lettuce plant. Nevertheless, the plants required extra care in terms of temperature regulation because a lower or higher temperature can impact the lettuce plant's yields. Plant temperature was investigated in a controlled greenhouse by Nam et al. (2014). In fogging and airflow conditions, it was shown that plant temperature may be maintained at a temperature similar to or lower than the interior air temperature. Moreover, As a result, fogging and airflow may have a greater impact on plant temperature management, reducing high temperature stress and increasing photosynthetic rate.
Based on the effect of temperature in chemical compounds of plants, in a plant factory, Chowdhury et al. (2021) analyzed how temperature affected the development of kale plant and glucosinolate content. Kale plant’s optimum growth temperature was found to be 20 to 23°C and glucosinolate concentration was high in the early stages of growth with lower temperature at be 14 to 17°C . As a result, the amount of glucosinolate in kale dropped as the temperature levels rose. At the same time, a strong correlation was found between growth variables. A study by Fujiwara et al. (2004) found that an increase in air temperature substantially influenced the bud growth rate of sweet potato cultivated under fluorescent lights with a light intensity of 200 µmolm–2s−1 and a photoperiod of 16 hd−1. Sweet potato growth rates and development were influenced by air temperatures ranging from 23 to 35°C. Moreover, leaf layer resistance is also reduced by high air velocity, resulting in decreased leaf size in plants. It was also found that the overall polyphenol and anthocyanin levels, as well as the antioxidant activities, rose in lettuce plants during the low temperature in day and nighttime (Boo et al. 2012).
However, with the increased temperatures, physiologic processes and their integration are accelerated. In case of different plants and crops, high temperatures promote faster growth, development and larger yield, but they also remove essential components from leaves and fruits through high transpiration rates (Maibam et al., 2021). A range of chloroplast structural components are altered by higher temperatures, including thylakoids, granule stacking, and swelling with photosystem II reduction, all of which contribute to cellular disruption, cell disintegration and, ultimately, cell death (Chowdhury et al., 2021).
Relative humidity
As far as environment management is concerned in the horticulture sector, humidity is by far the most difficult element to manage with a control and monitoring systems to maintain a set point for moisture in the air (Amani, et al. 2020). As the air temperature changes, so does relative humidity, and plants continually release moisture into the air. In addition, humidity regulation is essential for the health of the crops and the avoidance of disease (Amani, et al. 2020). The relative humidity of the surrounding environment also has a direct impact on plant development by preventing water and nutrient intake by the plants itself. Transpiration causes relative humidity to become saturated. Consequently, a lack of air circulation at high relative humidity causes plants to stop transpiration and nutrient intake from the soil or growth medium, resulting in rotting (Han et al., 2019). According to several studies, photosynthesis rate is inversely related to relative humidity, since a higher range of relative humidity reduces water stress and increases stomatal conductance (Chowdhury et al., 2021, Avotine, et al., 2018).
Aung et al, (2014) studied the plant growth and fruit quality of blueberries in a controlled room under artificial light conditions and high humidity. The controlled room under artificial light had a temperature range of 15 to 25°C and a relative humidity range of 50 to 70%. High humid condition were selected to open up more stomata. There were no noticeable differences in plant development and leaf photosynthetic capabilities between cultivars grown under natural sunshine conditions and those cultivated in the glasshouse until the harvest period. Choi and Lee (2008) looked at the influence of relative humidity on growth variability, tipburn effect, and mineral nutrient distribution in morphologically diverse lettuce cultivars. It appears that the variations in internal absorption of nutrients across morphologically distinct cultivars may be related to the differences in tipburn symptoms under varying humidity levels. As a result of appropriate humidity levels during the day and night, tipburn symptoms can be reduced. According to Ryu et al. (2014), photosynthetic processes were reduced when the relative humidity fell below 40%. At high relative humidity, on the other hand, the evaporation rate decreases, causing the stomata to close. When it comes to pest and disease management, controlling the relative humidity for plant development is crucial. In humid circumstances, mold and bacteria thrive, causing plants to perish and crops to fail, along with root and crown rot (Velasquez et al., 2018). As a result, these pests and diseases become more prevalent.
CO2 concentration
For plants, CO2 serves as a carbon source, which is why it is so important for plant development. CO2 concentration in the air affects the rate of photosynthetic activity, metabolism, and physiological and chemical conditions of plants and CO2 deficiency would not only reduce plant biomass, but also weaken plants, making them more susceptible to disease (Chowdhury et al, 2021). The photosynthesis process requires CO2, which is a major component and taken directly by plants. In addition, CO2 has an effect on the plant's transpiration and the increase of CO2 might lower transpiration by as much as 22% in diverse plant species (Ainsworth et al., 2005). CO2 increases in the air have a positive influence on crop development and it was observed that crop yields increased by more than 22% in closed greenhouse conditions when CO2 levels in the air increased (Merrill, et al, 2018).
CO2 reduces the transpiration rate in plants, which allows it to deal with the required nutritional components as well as the water in plants. Chowdhury et al. (2021) showed that the glucosinolate content in kale plants increased with the increase of CO2 concentration. La et al. (2009) also showed that all physical growth factors in Chinese kale plants rose considerably with CO2 elevation at each nitrogen level, while total glucosinolate content only increased at low nitrogen levels and higher CO2 concentration.
At high CO2 concentration, different plant structural features such as leaf size and thickness, shot and root ratio, stomatal behaviour can be transformed (Terashima et al., 2014). The plant's ability to absorb CO2 is also influenced by other ambient environmental factors such as light, temperature, and air velocity as well by its growth stage. A study by Thongbai et al. (2010) demonstrated that CO2 and air circulation had an impact on tomato seedling photosynthesis and transpiration. With the increase of air circulation from 0.3 to 1.0 ms−1 or CO2 concentration from 273 to 545 µmol mol−1, the net photosynthetic rate was also increased by 62 to 76%. The net photosynthetic rate rose by 111% when CO2 concentration and air movement were both increased. Sgherri et al. (2017) found that increased CO2 in the presence of salt lowered the lettuce production, and phenolic chemicals were shown to be more abundant in the plant. According to Li et al, (2012), the absorption of CO2 in a controlled plant factory in lettuce plant varied with the leaf area index (LAI) and the hourly air exchange performance.
When growing plants in a closed controlled environment or plant factory, all these environmental variables are interconnected. The impacts of one can affect the consequences of another, and they are also interrelated. Table 1 lists the optimum ambient environmental conditions for most widely cultivated plants in plant factories for their optimal development.
Ambient environment monitoring for plant factory
In order to develop healthy plants in a plant factory, there must be enough light for 8 to10 hours per day. Monitoring of light is essential for proper plant growth and development. Traditional way of monitoring light conditions inside a plant factory is to observe the plant itself (Lakhiar et al., 2018). A better alternative would be a light sensor that monitors the amount of light in the plant factory. This light sensor transforms light energy into electrical signals as its output Light sensor for monitoring light inside the plant factory would be a better option. A light sensor is a passive device that converts light energy into electrical signals as output. Photoresistors, photodiodes, and phototransistors are the three main types of light sensors widely used. The working principle of each light sensor type is shown in Fig. 3. A photoresistor commonly known as light dependent resistor (LDR), is a device whose resistance decreases with the increase of light intensity. A photodiode is a type of P-N junction diode that uses light source to generate electric current. The behavior of a phototransistor is similar to that of a photodiode when combined with an amplifying transistor, which amplifies the little current that is induced by the photodiode. These light sensors distinguish the type of light in a growth chamber and adjust the brightness to a more comfortable level. With the use of light sensors, light conditions in the plant factory can be adjusted to provide more pleasant lighting conditions for plant growth.
Maintaining air flow within a plant factory is a big challenge. The air flow is not fluent in the plant factory, because of the physical structure in it. Stagnant air in the plant chamber and reduction of air velocity by the artificial light heat also hamper plant growth rate and quality (Baek et al, 2016). Ventilation system in the closed plant factory is also vital for favorable growing condition (Zhang et al., 2016). To improve air circulation in the plant factory, various air flow devices such as circulation fans, air conditioners, external fans, etc. and so on were used to increase growth rate and a better air circulation inside the plant factory (Baek et al, 2016; Nam et al., 2014).
The temperature in the plant factory must be regulated within a particular range in order to maintain a high level of metabolic activity. Unstable temperatures are detrimental to plant development. Due to the varying temperature requirements for different plants or crops for photosynthesis and growth, temperature monitoring is a need for better plant development and yield. Ideally, the temperature in the plant factory should not drop below 4°C or go over 30°C (Lakhiar et al., 2018). Temperature variations can affect root development, respiration and transpiration as well as blooming and dormancy. Therefore, the temperature sensors may be utilized to monitor the temperature variations in the plant factory. The temperature sensor senses the resistance changes to the flow of electricity, with the change of temperature of the metal or object. There are two basic principles used for the temperature sensors that are most commonly used as resistance temperature detector and negative temperature coefficient thermistor. The working principle of different temperature sensor types is shown in Fig. 4.
Relative humidity in plant factory growth chambers is another essential component that must be controlled for substantial plant development. Too much or too little moisture in the growing environment, the plant will suffer. A reliable and exact way to measure the moisture content in a growth chamber would help farmers monitor their crops and provide the plant with an appropriate environment for growth. Humidity sensor is one kind which is widely used in different sectors. Humidity sensors generally include two components: a humidity detecting element, and a thermistor to detect temperature. The sensors are two types: capacitive sensor and resistive sensor. The working principles are similar to the temperature sensors. Ceramics, polymers, and composites are all types of sensing materials utilized in humidity sensors (Zor and Cankurtaran, 2016).
CO2 sensors could be used to monitor the CO2 concentration and variation in the plant factory. Infrared (IR) radiation absorbed by CO2 molecules is used to monitor gaseous CO2 levels. In order to create infrared radiation, the sensor uses a heated metal filament as an IR source. The working principle of a typical CO2 sensor is shown in Fig. 5. The measuring range for the CO2 sensor is usually between 0 and 10,000 parts per million. Two types of CO2 sensors commonly used in plant factories: nondispersive infrared carbon dioxide sensors (NICDS) and chemical carbon dioxide sensors (CCDS). In a NDIR sensor, an infrared (IR) light source passes waves of light through a tube filled with sample air toward an optical filter before an IR light detector. The IR detector measures the sum of IR light source. As the IR light passes through the length of the tube, the CO2 gas atoms assimilate the particular band of IR light whereas letting other wavelengths of light pass through. Finally, the IR detector detects the remaining amount of light, which is not absorbed by the CO2. CCD sensors utilize oxidation-reduction responses to measure the gas concentration. The gas atoms to be recognized by an oxidative response at a sensing electrode, producing ions and electrons. An external circuit exchanges ions with the counter electrode, resulting in a reduction. Table 2 shows the different types of sensors to detect and monitor ambient environment factors in plant factories.
Environment monitoring protocol with sensors
A typical monitoring platform of a plant factory is shown in Fig. 6. The plant factory monitoring mainly consists of the system, data acquisition, controlling the equipment, data transmission module, cloud data processing server, social communication platform, and mobile application (Lakhiar et al., 2018). The data acquisition system comprises three main units: sensors and its connection through circuit board, data receiver and saving unit, and programming and control unit. The sensors are placed in the plant growing area to collect the real-time data from each sensor and transmit the real-time data to the control and management unit through the wireless network system (WNS). The control and management unit consists of a central processing unit (CPU), which consists of control programming with several protocols to control each specific unit separately or in combination. The system processes accurately for monitoring and sends it to the web server as well as saves data for future use.
Future applications
Technological advancements such as Artificial Intelligence (AI) enable mass production without natural limits. Using artificial intelligence, plant factories may gather and process larger data for automation. Numerous artificial intelligence based monitoring systems for controlled plant factories have been studied, including neural networks, fuzzy logic controllers, and PIDs with nonlinear adaptive PID controls. Among some of the new findings, an automated method to monitor and record plant weight throughout plant development in a factory was developed by Chen et al. (2016). The plant weight is affected by different growing conditions, which is an important aspect for quantifying growth. Crop image acquisition system was developed by Hwang et al. (2014) to gather the crop information which would be suitable for crop growth such as crop shape, size, color, and length prediction. Image based data system can provide valuable information about the plant growth situations. A cooling air exchanger was introduced to plant production systems by Wang et al. (2016) in conjunction with an artificial light system. For the ambient environment control and optimization, Zheng et al. (2016) investigated the impacts of various density treatments on potatoes in different seasons. Plant density shows influences on plant height, photosynthesis processes, and chlorophyll synthesis. Although single density planting is preferable, it will not produce an effective yield.
Plant factories can be benefited from artificial intelligence technologies. AI has enormous potential for expanding the scope and efficiency of environmental inspections while also significantly improving regulatory effectiveness. As a result of the plant factories, artificial intelligence undergoes deep learning algorithms, which evaluates the ideal cultivation circumstances of plants and crops, and then automatically creates the optimal environment for each crop in the plant factory using artificial intelligence. For this data, it is possible to develop an integrated platform for plant factory operations. Massive amount of data collection from the sensors will support extensive ambient condition monitoring solutions that will give a substantial opportunity for the grower. AI techniques can draw out admissible features beyond human interference and this is a ground-breaking advantage, as AI can give insights that human investigators might not be capable to receive on their own claim.
Conclusion
This paper summarizes the effect of ambient environment factors and monitoring technology for plant factory, how they affect plant growth and development in a controlled closed condition. Among the most important environmental variables affecting plant development and yield are light intensity and type, air velocity, temperature, relative humidity, and CO2 concentration. Light is the main energy source and mostly all the morphological activities depends on it. Air movement in plant factory controls the plant canopy temperature, relative humidity, and CO2 concentration. There is a linear relationship between plant growth and development and temperature rises. Leaf temperature has a greater influence. Surrounding relative humidity has a direct impact on plant's water and nutrient intake. CO2 concentration in the air affects the rate of photosynthetic and other metabolic activities.
The plant factory is a novel way to cultivate plants or crops all year round in a controlled environment. It's a closed-in setting with a lot of possibilities for food production. In order to develop plants in the plant factory, the ambient environment must be closely monitored. To develop healthy plants in a plant factory, 8 to10 light hours per day is required which can be monitored using different light intensity sensors. Physical structure makes it challenging to maintain air flow within a plant factory. Different circulation fans, air conditioners, external fans were used to increase air flow. Temperature sensor senses the resistance changes to the flow of electricity, which is used to monitor optimum temperature inside the plant factory. Plant growth will be hampered if there is too much or too little moisture. CO2 concentration and variation in the plant factory are monitored using nondispersive infrared carbon dioxide sensors and chemical carbon dioxide sensors in common. To reduce the adverse effect of the environmental factors, advanced monitoring systems in plant factory systems might open up new possibilities for plant factory users. We are certain that our assessment of the effect of ambient environment factors will boost the usage of modern monitoring sensors and technologies. Plant scientists can use the approach to learn more about the relationship between ambient environmental factors and plant development in controlled plant factory.
Acknowledgement
This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through Smart Farm Innovation Technology Development Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (Project No. 421035-04), Republic of Korea.
