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Closed Greenhouse Systems in Arid Climate Conditions
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The closed greenhouse is an innovative crop system in the horticulture sector, integrating appropriate climate control equipment and optimized techniques to collect, store, and reuse solar energy for heating and/or cooling the greenhouse. This concept aims to improve the crop yield and quality with energy efficient and water-saving technologies. A specific focus on the opportunities of implementing closed greenhouses under arid climate conditions is detailed.

closed greenhouse arid regions crop production

1. Effects of Microclimate on Crop Growth in a Closed Greenhouse System

The greenhouse operating system should rely on techniques to regulate the inside environment parameters, including temperature, relative humidity, light, and carbon dioxide concentration, in order to maximize the production of the protected crops while preventing plant damage [1][2][3][4][5]. In closed greenhouse systems, it is expected that the average temperature, RH, and CO2 concentration are increased, compared to a conventional greenhouse [6].
Optimal conditions for crop growth depend on a great number of factors changing from moment to moment and year to year and growing with uncertainty. As a result of this dynamic behavior, the greenhouse microclimate is considered as a nonlinear multi-input, multi-output system. Several interactions between the outside conditions, the greenhouse climate and the greenhouse crop exist with strong variation of the adjusted parameters according to the development stage of the crops [7]. So many control parameters in the greenhouse still need to be dynamically identified for suitable plant growth [8][9][10].
Most protected crops’ product quality is greatly influenced by climatic variables. They have an impact not only on the physiological processes of the crops but also on the internal quality of the vegetables, both directly and indirectly. Sensory elements and ingredients, such as sugars, acids, and aromatic chemicals, which affect taste, as well as vitamins and secondary plant compounds, which are important for human nutrition and can be affected by changing climatic conditions in the greenhouse [11].

2. Air Temperature

Temperature is a major environmental factor that affects all stages of plant development, including germination, vegetative growth, flowering, and fruit ripening, as well as a number of physiological functions—such as transpiration—and biochemical functions—such as enzyme activity and photosynthesis [11][12][13].
The average temperatures, especially for the arid regions, are very high and generally exceed the optimum values with strong monthly, as well as daily, variation. For example, in the Saudi Arabian region, they can reach maximum values ranging between 39 and 46 °C, and for such values the relative humidity does not exceed 20%. The typical weather conditions of these regions require greenhouses equipped with cooling systems while maintaining optimal relative humidity for vegetable production [14]. Closed greenhouses can constitute, in this case, a suitable solution as climate control process can be less complicated and more precise, compared to conventional greenhouses.
Closed greenhouses are also characterized by vertical gradients of temperature (VGT) and humidity due to the location of the cooling ducts under the growing gutter. Temperature is increased by incident solar radiation at the top of the greenhouse, while it decreases at the lower part due to the cooled and dehumidified air [15].
Thermal amplitude also plays a relevant role in controlling the plant’s physiological and biochemical parameters. The height, internode length, petiole elongation, leaf orientation, shoot orientation, chlorophyll content, lateral branching, and floral stem length of plants are all affected by this differential [16]. Damage to cell membranes, proteins, and nucleic acids is a direct result of high temperatures. Indirect effects include pigment inhibition and degradation, which causes sunburn symptoms [11][17].
Low temperatures below 13 °C, that differ depending on the species, have a negative impact on the pollination and flowering of protected crops during the winter. To overcome these deficiencies, several techniques have been employed by the farmers; for example, for tomato they use mechanical vibration or hormonal treatment of flowers with gibberellic acid capable of causing the flowers to set and the fruits to grow even if the pollen quality is poor [18].
Gruda [11] reveals that night temperatures, below 14 °C, highly affect flowers, especially if combined with high humidity rates. This results in reducing the number of pollen grains, their release, and their germination capacity, as well as fruit set and shapes. Low temperature combined with low solar radiation causes swelling and blotchy ripening in tomato, as well as deterioration of taste due to lower sugar content [19].
Maintaining greenhouse temperatures at optimal values is only possible for closed greenhouses, since it has no ventilation windows and no air exchange with the outside [15].

3. Relative Humidity

The relative humidity of the air is the ratio of the partial pressure of water vapor contained in the air to the saturation vapor pressure at the same temperature. This parameter affects the water status of plants, which, in turn, influences all processes related to transpiration and water balance, since it is responsible for the functioning of stomata [11][20][21]. Vapor pressure deficit (VPD) also influences the photosynthetic rate through leaf stomatal conductance, which decreased with increasing VPD level [22].
Plant transpiration is driven by the VPD, which varies exponentially with ambient temperature [23]. The potential to increase plant growth and productivity through VPD control has long been recognized [24]. Grange and Hand [25], claim that the growth and development of horticultural crops are generally unaffected by VPD values between 0.2 and 1.0 kPa (at 20 °C). Most plants can grow at a VPD that ranges from 0.5 to 0.8 kPa, according to J. C. Bakker [26]. The recommended ranges for VPD for plants are between 0.5 and 1.2 kPa, as stated in the advice provided by Zhang et al. [27]. The optimal VPD levels, though, change depending on the development stages of the plant. For instance, research recommends an ideal VPD of 0.8 kPa for clones, roughly 1.0 kPa for the vegetative stage, and around 1.2 to 1.5 kPa for the flower stage [28].
Plant transpiration is enhanced by high VPD (more than 1.0 kPa), in addition to low humidity and high temperature, whereas low VPD in combination with high humidity and low temperature results in dehydration, wilting, and necrosis [29]. Compared to conventional greenhouses, closed greenhouses typically have lower VPD and, as a result, lower transpiration rates [30].
Humidity levels above 90% (VPD more than 0.32 kPa at 25 °C) have an adverse effect on plant development and fruit quality, as well as stimulating disease attacks [12][31].
In addition, excessive humidity causes stomatal dysfunction in a variety of cropping systems and plant species. According to Arve et al. [32], stomatal pore opening and length were greater in plants cultivated under high RH (90%) than in plants grown under moderate RH (60%) conditions, which make it difficult to regulate the process of stomatal opening and closure for gas exchange and drought or darkness stress control.
Closed greenhouses have a higher relative humidity estimated to be more than 20%, compared to conventional greenhouses [6]. Excessive humidity can also cause condensation on crop plants, which can stimulate the spread of fungal infections [32][33][34]. Heuvelink et al. [35] reported that Botrytis infestations on tomato and cucumber crops resulted in 10% and 40% production losses, respectively, at the start of cultivation, as a result of excessively high humidity in closed greenhouses. The recommended VPD for greenhouse crops ranges between 0.32 and 1.58 kPa (at 25 °C). The excess of humidity inside closed greenhouses can be reduced through water recovery monitoring techniques that apply closed or semi-closed air cycles in the greenhouse design. Humid air condenses at its contact with a surface at a temperature that is lower than its dew point; the condensate is then collected and used depending on whether water is suitable for irrigation. This controlled condensation is either ensured by heat pumps, heat exchangers, or finned pipes  [12][36][37]. Thus, water recovery techniques offer an interesting solution in terms of taking advantage of high humidity inside closed greenhouses and reducing irrigation water consumption, especially in hot and arid regions that suffer from water scarcity.

4. Light Intensity

Light is the most significant primary environmental component that controls plant growth and the development of plants [39]. The plant uses only about 1% to 5% of the transmitted radiation; the remainder is absorbed and re-emitted as thermal radiation (heat) [40].
Excessive lighting influences both the external and internal quality of crop production. This excess of light can be beneficial to the development of certain plants, for instance, it can increase the content of essential oils in medicinal plants [31][41]. However, excessive illumination can also be harmful to a wide range of crops, such as tomatoes and peppers, as it causes organoleptic quality problems, such as pigmentation loss, tissue collapse, and cellular death [17][42].
Lack of radiation causes plant quality issues, including stunting and vegetative development at the expense of fruiting organs; malformed organs, such as high-oval tubers in kohlrabi or radish; tomato flower abortion; or radish tuber production failure [11].
Arid and semi-arid regions are characterized by abundant solar radiation, which seems to offer these regions great potential for agricultural operations and especially for closed greenhouses constructed with adequate covering materials. These greenhouses guarantee a better exploitation of this abundance, either for improvement of the productivity by increasing the rate of photosynthesis or for reducing energy use by storing extra heat to be used for heating the greenhouse in cold periods.
Arid regions have the disadvantage of having a dusty environment that causes dust accumulation on the greenhouse’s roof and reducing light transmission through the glazing. In this situation, providing cleaning devices in these areas is critical to resolve this problem [43][44]. Several improvements have been made to the covering materials of closed greenhouses in arid environments in order to have an adequate microclimate for the development of the crops. Baeza et al. [45] investigated the possibilities of using near infrared (NIR) reflective filters to improve the optical properties of closed greenhouse in Morocco, Malaysia, and the Netherlands. They demonstrated that using these filters results in a 36 % energy savings, a 40% reduction in maximum cooling power, and a 15% increase in potential tomato production when compared to using standard glass as a covering material.

5. Carbon Dioxide Concentration

The increase in yield is mainly attributable to higher rates of photosynthesis in closed greenhouses, resulting from higher CO2 concentrations, compared to open and semi-closed greenhouses. CO2 concentrations constitute the main characteristic that distinguish closed greenhouses [33]. They are often maintained at up to 1000 ppm in the summer when solar radiation is high, while they are about 400 ppm in conventional greenhouses due to ventilation losses [46]. The optimal CO2 concentration is determined by solar radiation, as well as the rate of photosynthesis and rate of ventilation [47][48].
It has been shown that elevated CO2 concentrations result in an increase in the rate of photosynthesis even under low light conditions, as well as a reduction in the transpiration rate by decreasing stomatal conductance, which is valuable to the plant by protecting it from dehydration [49]. It may also improve energy efficiency by 5–10 percent without affecting photosynthesis or growth [8]. High CO2 concentrations have a short-term favorable effect on photosynthesis; however, a long-term positive effect was not proved, as maintaining a CO2 concentration of 1000 ppm for several weeks has not guaranteed a continuous rise in leaf photosynthetic rate [50].
Several studies have demonstrated the positive impact of CO2 enrichment on plants, concluding that raising the concentration in a greenhouse helps the plant to grow in height, weight, biomass, and lateral branches. Furthermore, high CO2 concentrations have an impact on the optimal temperature, humidity, and light levels [51]. Higher CO2 concentration also means a higher optimum growth temperature, which can lead to an increase in the plants overall growth rate, particularly in hot regions where high solar resources are available [51].
Dong et al. [52] studied the effect of carbon dioxide enrichment on fruit quality and found that fruit quality does not necessarily correlate with increased yield. They proved through a meta-analysis that rising CO2 concentration increased fructose, glucose, total soluble sugar, total antioxidant capacity, total phenols, total flavonoids, ascorbic acid, and calcium in the food part of vegetables, but they also decreased protein, nitrate, magnesium, iron, and zinc concentrations. They proposed several techniques to solve this problem based on selecting species or cultivars that respond better to high CO2 concentrations, maintaining optimal environmental conditions at the same time as high CO2 levels, harvesting late-stage vegetables and combining this factor with mild environmental stress (e.g. salinity or Ultraviolet-B radiation).

6. Combined Effects of Climatic Factors

The impact of these climatic factors on plants is sometimes described as a dynamic mixture of two or more factors, and many studies have documented their interaction [29][31].
The temperature affects VPD by varying water availability in the plant and its ability to regulate water absorption. In this case, exposure of plants to high relative humidity and high temperature reduces stomatal functions, thus, affecting plant growth, transpiration, and photosynthesis [29].
Low temperatures affect the absorption of solar radiation by interfering with the photosynthetic cycle in a greenhouse environment, and light levels influence both ambient and plant leaf temperatures [12]. The combined effects of light, CO2, and temperature affect the rate of photosynthesis. This rate is affected by the greenhouse’s temperature, and it reaches its optimum value when light intensity and CO2 concentration are both high [15]. The increase in CO2 concentration, according to Dannehl et al. [30], compensates for the reduction in photosynthesis caused by low light intensity. They found that a concentration of 1000 ppm in the photosynthetic flux density (PPFD) range of 303 to 653 mol m−2s−1 will compensate for a 40% loss in light, a 51% increase in net photosynthesis, and a 5–8% drop in transpiration. The reduction in transpiration caused by high CO2 levels can, under a high light intensity, be useful for plants by protecting them from dehydration or can be harmful by restricting the amount of latent heat the plant can dissipate through evaporation. Additionally, increased CO2 content reduces the effects of ethylene produced by plants [49].
The combination of all of these parameters, including optimal temperature, VPD, high light intensity, and a high CO2 concentration at about 1000 ppm, leads to enhanced crop productivity in closed greenhouses [33].

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