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Options for Design and Management of Greenhouse Production Systems in the Tropics

Anne Elings and Silke Hemming

Wageningen University and Research

Business Unit of Greenhouse Horticulture

P.O. Box 644, 6700 AP Wageningen

Droevendaalsesteeg 1, 6708 PB Wageningen, The Netherlands

E-mail: anne.elings@wur.nl; www.greenhousehorticulture.wur.nl

 

ABSTRACT

Greenhouse production systems in tropical regions must be environmentally and economically sustainable. Environmental sustainability implies high efficiencies of the use of nutrients, water and energy, and economic sustainability implies high production levels and low investment and running costs. Greenhouse production systems have proven to substantially increase production levels; to increase the amount of produce per unit water and nutrients, which can be costly and/or scarce; to improve product quality; to reduce pest and disease pressure and reduce chemical crop protection while enabling biological crop protection. Greenhouse production systems fit in modern value chains that are aimed at increasing populations of quality-oriented consumers that predominantly live in urban areas.

The most important design criterion for a greenhouse suitable for a tropical environment is sufficient ventilation to maintain air temperatures that are suitable for good crop growth and development. If very high-value crops are cultivated, mechanical ventilation can be considered, however, in most instances natural ventilation is the only economically viable option. This leads to a number of design solutions that depend on the eco-physical characteristics of the location.

Other design criteria include good light transmission but also the option to protect against the sun, good insulation against pests, a high volume, wind proof, and the availability of clean water that is free of harmful microorganisms and has a low electrical conductivity. Choices have to be made with regards to the cultivation medium (soil or artificial substrate), the crop and variety, the level of automation, etc. The choice of construction materials plays an important role in the design process, as it can influence for instance the light transmission and energy balance in the greenhouse.

Greenhouse horticulture is difficult: financial risks are high, climate management is critical, pests and diseases easily get out of hand, a power cut can devastate the crop, and the management of the crop itself is very different from that of an outdoor crop. This is solved by modernization of company management, training of staff, close integration with the value chain, and high standards of greenhouse maintenance. Also the enabling environment needs to be conducive: e.g. legislation that enables biological crop protection, good infrastructure, and good education, research and extensions systems.

These issues are illustrated on the basis of the Wageningen UR design for a greenhouse suitable for the tropics, that was first developed for Indonesia, was further improved and commercialized in Malaysia, and is now adapted and spreading to other countries (e.g., Thailand). A second example for subtropical Taiwan is given.

INTRODUCTION

Greenhouse production systems must be environmentally and economically sustainable. Environmental sustainability means that the amount of inputs is minimized, however, without jeopardizing the yield. Some of the most important inputs are nutrients, water, labour, energy, and crop protection means. Nutrients, energy, labour and crop protection means cost money. Water is mostly free or very cheap, however, can be limited in quantity or quality; in any case it is a public good that in increasingly scarce globally (it would be good to attach a price to water, as this would stimulate its responsible use). This translates to a high resource use efficiency: the amount of produce per unit of input, which increases as the technology level increases. For instance, 43 kg of sweet pepper can be produced with 1 m3 of water in a modern greenhouse, while 13 kg can be produced in a plastic greenhouse and 3 kg in the open field (Figure 1). In addition, it is good to reduce the amount of inputs per unit area: for instance, chemical crop protection needs to be reduced in favour of biological crop protection from a viewpoint of consumer’s demand and other concerns.

Fig. 1. The water use efficiency increases as the technology level increases.

 

Economic sustainability in practice means that after about 3-5 years, investments have been earned back and that operational profits are made. This can imply that the design of a greenhouse production system is technically relatively less advanced if this reduces investment costs and realizes early profits.

The design process follows a custom-made modular approach. For instance, a choice for a plastic greenhouse can be combined with soil or substrate cultivation, and with various levels of automation. Nevertheless, modules must be in balance for maximum sustainability, as there is no point in high investments in computer-aided automation if fertilizers of the right qualities are not available.

Urbanizing societies often show increasing care for product quality and accept associated higher prices. Themes such as high quality, urban horticulture, pesticide-free organic horticulture, local sourcing go well with protected horticulture. Greenhouse production systems fit in modern value chains that are aimed at increasing populations of quality-oriented consumers that predominantly live in urban areas.

DESIGN CRITERIA FOR A GREENHOUSE PRODUCTION SYSTEM IN THE TROPICS

A greenhouse design is determined by a large number of factors, such as the ambient climate, the crop and amounts to be produced, the financial situation, land availability, water availability and quality.

Climate

A tropical lowland climate is a non-arid climate in which all twelve months have mean temperatures of at least 18 °C. Tropical temperatures remain relatively constant throughout the year and seasonal variations are dominated by precipitation. Temperatures in the tropical highlands are lower than in the tropical lowlands. Still, they are fairly stable over the year, and relative air humidity is associated with the rainfall pattern. Because of the lower temperatures, this climate is more suitable for protected cultivation. Figure 2 gives an example of an annual temperature patter lowland and highland tropical Thailand.

Fig. 2. Temperature in Bangkok (lowland tropics) and Chiang Mai (highland tropics) in 2016.

Source: www.wunderground.com

 

Needs of the Crop

As a consequence of the high air temperatures and high humidity levels, one of the major design criteria for a tropical greenhouse is sufficient ventilation capacity to maintain temperature and humidity at levels that are low enough for good crop growth, production and development. The desired light levels and quality depend on the crop, and air CO2 levels should be as high as possible. Clean water and nutrients should be available in sufficient and well-balanced amounts.

Air temperature influences a number of crop physiological processes. Photosynthesis is stable over a fair range of temperatures, but decreases above approximately 34oC (Qian et al. 2012). More often than by high temperatures itself, photosynthesis is limited by reduced water availability or uptake leading to stomatal closure. Temperature influences crop development rate: a higher temperature implies a faster formation of leaves, fruits and/or flowers (for tomato: Heuvelink 1996). High temperatures have an effect on processes such as pollination, fertilization and fruit abortion (see Max et al., 2009 for a summary), however, little quantitative knowledge exists with regards to long-term effects on these processes. Personal observation in a number of fruit vegetables indicate that sensitivity of these processes to high temperatures is lower than often suspected. Kleinhenz et al. (2006) reported an unbalanced dry matter partitioning, which may point towards fruit set and development, but also towards improper management of the vegetative-generative growth habit.

Radiation is the energy source of photosynthesis, and higher radiation gives a higher photosynthesis rate, although increases at high radiation levels are smaller than at low levels. Therefore, transmission of the greenhouse construction and cover should be high, with the exception of e.g., shadow plants. Diffuse radiation penetrates better in the canopy than direct radiation, leading to a better horizontal and vertical light distribution and a higher total crop photosynthesis rate (Elings et al. 2012a). The haze value of a greenhouse cover indicates the fraction of direct radiation that is transformed into diffuse radiation (Li et al. 2014). The spectral properties of the light have an effect on photosynthesis (Elings et al. in press) and on crop architecture (Dieleman et al. 2015).

In tropical environments where light intensities are high, it is recommended to combine high light transmission with a high haze factor, which results in a high amount of diffuse radiation. Ultraviolet (UV) and near-infrared (NIR) lights must be blocked, as these radiation types carry energy and lead to a higher inside temperature. Furthermore, NIR and UV are not or only in small quantities useful to the crop (reports on negative effects exist, e.g. Kittas et al. (2009). UV is needed in small amounts for orientation of bumble bees that pollinate the crop.

It goes almost without saying that the CO2 concentration of the air should be as high as possible, but in an open greenhouse it will not be possible to create an atmosphere with elevated CO2 concentration. The best that can be achieved is an indoor concentration that is similar to the outdoor concentration through a large ventilation capacity for quick air refreshment. A completely closed greenhouse will only be economically viable if high-value crops are produced (e.g., pharmaceuticals).

This is only a brief summary of the needs of a crop, from which greenhouse characteristics are derived. Also, the greenhouse should be strong enough to withstand extreme weather conditions such as strong winds and hard rains. Next, decisions with regards to installations are taken.

Covering Materials

Covering materials of a tropical greenhouse should meet the following criteria:

  1. A high light transmission. Nowadays, a wide variety of covers with good light transmission are available.
  2. Good insulation properties to keep the heat outside.
  3. Spectral reflectance that block UV and NIR.
  4. Haze properties that transform direct radiation into diffuse radiation.
  5. The role of the cover can be completed by shading through screens. Shading reduces radiation, temperature and the transpiration rate, and to some extent the photosynthesis rate, although the resulting diffuse light partially mitigates this effect. The position of retractable screens can be adjusted on the basis of light intensity, which can be variable in the tropics.

A serious disadvantage of white-wash is that it can not be removed instantaneously.

  1. Covering material must be easy to clean, have a long durability, and be strong enough to withstand strong winds and rains.

Covering materials can be made of nets, screens and glass.

  1. A net is not waterproof but provides shade and protection against insects. The ventilation capacity depends on the mesh size, which also determines the permeability for insects. Larger openings result in a better ventilation but also in more insect species penetrating (Table 1). Greenhouses that are fully made of nets (net houses) are used for, for instance, seedlings, shade plants, ornamentals, etc.
  2. A plastic film protects against rains and insects. It must be combined with ventilation openings to prevent from high temperatures. The ventilation openings must be covered with insect nets. Depending on the quality of plastics, the material degrades in time under the influence of UV radiation, high temperatures, chemical or mechanical stresses. Plastic houses are the standard in the tropics.
  3. Glass, although it has a longer lifetime than plastic, is less appropriate for most settings in the tropics as it is not robust enough and fairly expensive.

 

Table 1. Overview of net characteristics and permeability for a number of insect species.

Mesh

Hole size (mm)

Hole length (mm)

Hole width (mm)

Thread diameter (mm)

Light trans-mission

Porosity (ɛ)

Source

Insects

 

 

 

 

 

 

 

 

 

32

 

 

 

0.285

 

 

 

 

 

0.74 x 1.17

 

 

 

 

0.63

4

Tuta absoluta

40

0.64

0.44

0.39

0.25

87

0.41

1,2,3

Leaf miner (Lyriomyza trifoli)

52

0.462

0.80

0.25

0.31

70

0.38

1,2,3

(Sweet potato) Whitefly (Bemisia tabaci)

78 / 81

0.34

0.29

0.18

0.19

86

0.30

1,2,3

(Melon) Aphid (Aphis gossipii)

Greenhouse whitefly (Trialeurodes vaporiorum)

Silverleaf whitefly (Bemisa argentifolii)

123

 

 

 

 

 

 

5

Silverleaf whitefly (Bemisa argentifolii)

132

0.192

 

 

 

 

 

3

Western flower thrips (Frankliniella occidentalis)

1 = Harmanto et al. 2006; 2 = von Zabeltitz 2011; 3 = Ghidiu and Roberts 2002; 4 = van Os et al. 2012; 5 = Harmanto 2006.

 

Construction

The construction of a tropical greenhouse must meet the following criteria:

  1. Good light transmission. Metal is the preferred material, as the low amount of material increases light transmission and therefore crop growth. Depending on availability, the investment costs are higher, but maintenance costs are normally lower than for wood. Wood is less durable and strong, and therefore, more material is required to realize a solid construction. Consequently, light transmission is relatively low. Next to that maintenance costs are considerably higher.
  2. Ventilation for temperature and humidity management. If this relies on natural ventilation then the sides must have sufficient net area, the sides need to be tilted, and top ventilation is necessary. The latter needs to be 2-sided if wind direction is variable. The climate will determine the width (number of spans) of the greenhouse (Hemming et al. 2006; Impron 2011). Air enters the greenhouse from the sides and heats once inside. It rises, which is stimulated by the air movement through a possible chimney in the top of the greenhouse. The faster the air rises, the narrower the greenhouse can be, as fast rising air will not reach the centre of the greenhouse. In practice, this means that the warmer the outside temperature and the lower the wind speed, the narrower the greenhouse can be.
  3. The greenhouse needs to have a high volume to enable good air circulation, realize a buffering capacity and to create a homogeneous indoor climate with gradual changes. In practice, this means that sufficient height; 5-6 meters gutter height is recommended.
  4. Winds can be strong, and the construction needs to be wind proof. This also leads to a choice for a metal construction with sufficient supporting elements.

Installation

The installation consists of the parts inside the greenhouse: computer, pumps, tubes, substrate, ventilators, etc.

Important for tropical greenhouses is the cooling system. The crop itself contributes to cooling through transpiration, provided this is not hampered by very high humidity (Max et al. 2009). Natural ventilation (see above) relies on natural air movement for cooling the greenhouse crop down to outside temperature. Usually, a greenhouse with natural ventilation has sides with nets, and one or two-sided ventilation openings in the top of the greenhouse. These top openings can be fixed or flexible. Circulation fans can be placed inside the greenhouse to stimulate air circulation. This creates a more homogeneous climate and increases crop transpiration and cooling, but also requires electricity. Ventilation fans can be placed in the side walls of the greenhouse, usually in combination with a pad & fan system (see further on). Misting or fogging is applied to cool and humidify the air. It is especially useful to avoid peak temperatures. In itself, it reduces crop transpiration but the total water use may even increase as the misting or fogging itself requires water (of high quality). The difference between misting and fogging is in water drop size and pressure needed and therefore in the quality of the system performance. Application should be in such a way that the crop stays dry, demanding more than 1 m air space above the top of the crop. Hosing is a cheap method (if labour costs are low) to maintain air humidity and if water quality is too low for a misting system. Effects on cooling, however, are low. Pad & fan is also applied to cool the air. It requires both energy and water, and causes a temperature and humidity gradient in the greenhouse.

The choice of substrate is an important one. It is the medium in which the plant is rooted, and where water and nutrients are taken up by the plants. As these processes are crucial, the choice of the substrate has direct consequences for the fertigation strategy. The soil is the most simple and cheapest option. The advantage of soil is its large water availability for the plant. A dysfunctional fertigation system is not directly disastrous to the crop. It, however, also introduces the risks of soil-borne diseases. Bacterial wilt, for example, is wide-spread and can completely destroy the crop. It is therefore better to grow in pots and combine this with a robust fertigation system (uninterrupted electricity supply!). The pots can be filled with a wide diversity of substrates that each have their own characteristics: soil, coir, pouzzolane, rice husks, pumice etc. Important are the absence of soil-borne diseases, a decent water holding capacity, and maximum chemical neutrality. Slabs are made of, for example, rockwool that is chemically inert. As in the case of pots, it requires an uninterrupted electricity supply, It is mostly used in combination with a computerized fertigation system.

Water and nutrients are applied with a fertigation system. Most simple is manual application of water and nutrients. The system is cheap, always works, but is not very precise in terms of amounts of water and nutrients applied. Gravitational fertigation makes use of a water tank that is placed above field level. The water tank can hold water and nutrients that are mixed in a specific combination, and the water can be applied in specific quantities. The required amount of water and nutrients can be translated to a time period of water flow. The valves of the system are manually operated. Stand-alone soil moisture sensors that are accessed with a hand reader are a cheap technology to determine the moment of irrigation. A computerized system makes use of sensors and a pre-set fertigation regime to apply water and nutrients. It is usually combined with A and B nutrient tanks, and a pH buffering tank. The system obviously requires a non-interrupted supply of electricity. Because of its capacity to provide optimal amounts of water and nutrients, it contributes to improved production and product quality. Recirculation is applied to re-use water and nutrients. As water resources shrink, this system becomes more appropriate. However, water recirculation introduces the risk of spreading soil-borne diseases and the system therefore requires a disinfection unit (UV, heat treatment, ozone), which is often expensive. Slow sand filtration may be a cheap option for small nurseries. An alternative is to re-use the drain water for outdoor vegetables.

OTHER ISSUES

Market

The true starting point is a market assessment, whether production is for the local, national, regional or export market, and forms the basis for the decision with regards to crop, variety and production period.

Inputs

One of the first things to verify when considering greenhouse production, is the availability and quality of water resources. Any water needs to be checked water for presence of diseases and nutrients, and in case of diseases, the water can not be used without disinfection. Rivers and lakes are in principle easy sources of water, however, quantities and quality are not always sufficient. Especially during the dry season, water levels in rivers can be very low. Also the quality of surface water is not always good. Rain, river and lake water can be collected in reservoirs and be used for irrigation purposes. Greenhouse design should be adapted to collect rainwater falling on the roof. Spreading the water use over time leads to an improved water use efficiency; however, precipitation is often not sufficient for a complete year. Therefore, additional water sources remain necessary. Bore holes provide water from deeper soil layers. Normally, its quality is better and its supply regular – until the below-ground water resources are depleted.

The crop must be supplied with the right amounts of nutrients, mixed to an appropriate EC and pH. These values vary per crop and growth phase, and are influenced by the substrate and transpiration rate. The higher the transpiration rate, the lower the concentration of nutrients and EC. The availability of individual nutrients can be problematic in countries where greenhouse horticulture is only starting, leading to sub-optimal nutrient solutions, growth and production.

Pests and diseases are a given. There is a variety of means to deal with them (use of clean water, sanitation, varietal tolerance/resistance), and the greenhouse with screens (!) itself protects against pests. But at some point, pests and diseases will most likely require the use of chemical agents. The problem with chemical agents are their effects on human health and the environment, and resistance build-up in the insect populations, leading to consumers and subsequently supermarket chains not to accept products with chemicals exceeding minimum residue levels. Growers respond to that by applying biological control agents. Unfortunately, not all governments have the rules and legislation in place that permit the trade in such products, which seriously hampers the development of the sector.

Skills and Competences

Growing a crop in a greenhouse is difficult. It is a misconception that the computer will take over and make life easier. On the contrary, a high level of understanding of the relevant biological and physical processes is needed, in addition to a management system that enables acquiring skills and utilizing competences, and allows the agronomist (plus colleagues) to take instant decisions whenever required. This must be backed by a system of education, research and extension that collaborates closely with the private sector in terms of research agenda, education of specialists and training capacity.

EXAMPLE 1: A GREENHOUSE FOR TROPICAL SOUTHEAST ASIS

A greenhouse was developed for a tropical lowland climate and evaluated in Indonesia (Impron 2011) and Malaysia (Elings et al. 2012b). The high temperatures demanded a high natural ventilation rate was required. Mechanical cooling was not an option because of the high energy costs and unavailability/uncertainty of availability of electric power. The first design in Indonesia had a single span and vertical walls, but the second design in Malaysia has a three-span cover and tilted side walls with insect nets to reduce air resistance and top vents allowing hot air to leave the greenhouse, especially in no-wind conditions that occur frequently around noon. The top vents have a ‘chimney’ construction that allows the passing of wind, which stimulates the air flow from the inside of the greenhouse. The covering of the greenhouse should reduce direct solar radiation by making the light penetrating the greenhouse diffuse, which reduces high temperatures in the top of the canopy. The greenhouse has insect nets at all openings and a double-door sluice to prevent insects from entering. The ground is covered with white plastic to prevent the growth of weeds and soil-borne diseases, and plants are grown in white polybags filled with locally available cocopeat. The greenhouse is equipped with a computer installation that can manage the application of water and nutrients (‘fertigation’) on the basis of the climate and the needs of the crop. All construction materials were obtained locally, which reduces costs. Only the computer has to be imported. It is tempting to reduce costs by using cheaper materials, to reduce the height of the greenhouse and therewith to reduce the amount of materials needed, or not to install a computer system. However, this will lead to a shorter life span, an adverse climate, and low yields. The consequence is that no profit is made. Reduced investments result in even greater losses of profit. In Thailand, in the hills near Chiang Mai where temperatures are lower, the design was further adapted by Dutch Agri-Tech and Trade (DATT): a substantial larger area, ventilators above the canopy and shade nets along the sides. High-quality tomatoes are now successfully grown.

 Fig. 3. Views of the tropical greenhouse at the premises of EastWest Seed Company at Purwakarta, Indonesia (topleft), those of the Department of Agriculture in Serdang, Malaysia (topright), and developed by Dutch Agri-Tech and Trade (DATT) near Chiangmai, Thailand (bottom left).

EXAMPLE 2: A GREENHOUSE FOR SUBTROPICAL TAIWAN

The subtropical climate in Taiwan is characterized by maximum temperatures between 25 and 35oC. This requires a high ventilation capacity to maintain greenhouse temperatures that do not exceed outdoor temperatures. A ventilation capacity of 30% considerably reduces the number of hours with temperatures above 35oC. If insect nets against white flies are placed, a ventilation capacity of 40% is required. Winter minimum temperatures can be as low as 5oC but normally are between 10 and 20oC. In summer, minimum temperatures are approximately 25oC, and night-time relative air humidity high. Day-time relative air humidity is a little lower, which makes adiabatic cooling through fogging possible. Computations show that with a fogging capacity of 300 g m-2, the maximum air temperature exceeds 32oC only for a few hours during day-time. Air circulation during night-time reduces the risk of condensation on leaves and stems. Installation of a small heating unit provides an additional option to reduce condensation in winter. Supplementary CO2 application is economically not useful, as most CO2 leaks away through the large ventilation openings.

The final greenhouse is design is of the Venlo type with a span width of 4.8 by 4 m, has a very diffuse cover of non-thermic foil with a high light transmission, good light scattering and low night temperatures. The natural ventilation is high to avoid high air temperatures during periods with high radiation and high outdoor temperatures. Two-sided roof ventilation in combination with side ventilation ensure greenhouse temperatures that do not exceed outdoor temperatures. All ventilation openings are covered with white fly insect nets. Additional cooling is supplied by a high-pressure fogging system to reduce greenhouse temperature through water evaporation (adiabatic cooling) and to avoid low air humidity during day-time. An air circulation system reduces condensation on stems and leaves during nights with high relative air humidity. A computer manages the greenhouse climate and fertigation (Hemming et al. 2013).

Fig. 4. Mid-tech greenhouse with cultivation of cherry tomato in the subtropical climate of Taiwan.

 

REFERENCES

Elings, A., T. Dueck, E. Meinen and F. Kempkes. 2012a. Analysis of the effects of diffuse light on photosynthesis and crop production. Acta Hort. 957:45-52.

Elings, A., I. Stijger, M. Sopov, J. Campen. 2012b. Greenhouse horticulture in Malaysia. A policy brief. Wageningen UR Greenhouse Horticulture, http://edepot.wur.nl/216874.

Elings, A., E. Meinen, J.A. Dieleman and P.H.B. de Visser. in press. The modelled photosynthetic effects of different light colours on tomato growth and production. Acta Hort. (in press).

Dieleman, J.A. and T.A. Dueck. 2015. LED lighting in greenhouse horticulture. http://www.hi-led.eu/wp-content/uploads/2015/01/HI-LED_Newsletter_January_2015.pdf

Ghidiu, G.M. and W.J. Roberts. 2002. Greenhouse screening for insect control. State Univ. of New Jersey, Fact Sheet.

Harmanto, H. 2006. Evaluation of net greenhouses for tomato production in the tropics. Ph.D. thesis, Univ. Hannover, Germany.

Harmanto, H., J. Tantau and V. M. Salokhe, 2006. Microclimate and air exchange rates in greenhouses covered with different nets in the humid tropics. Biosys. Eng. 94:239-253.

Hemming, S., S.L. Speetjens, D. Wang and J.R. Tsay. 2013. Greenhouse design for vegetable production in subtropical climate in Taiwan. Acta Hort. 1037:65-74.

Hemming, S., D. Waaijenberg, J.B. Campen, G.P.A. Bot, and Impron. 2006. Development of a greenhouse system for tropical lowlands in Indonesia. Acta Hort. 710:135-142.

Heuvelink, E. 1996. Tomato growth and yield: quantitative analysis and synthesis. Dissertation, Agricultural University Wageningen, The Netherlands, 326 pp.

Impron, 2011. A greenhouse crop production system for tropical lowland conditions. Ph.D. thesis, Wageningen Univ., http://edepot.wur.nl/176828.

Kittas, C., M. Tchamitchian, N. Katsoulas, P. Karaiskou, and Ch. Papaioannou. 2006. Effect of two UV-absorbing greenhouse-covering films on growth and yield of an eggplant soilless crop. Sci. Hort. 110:30–37

Kleinhenz, V., K. Katroschan, F. Schütt, and H. Stützel. 2006. Biomass accumulation and partitioning of tomato under protected cultivation in the humid tropics. Europ. J. Hort. Sci. 71:173-182.

Li, T., E. Heuvelink, T.A. Dueck, J. Janse, G. Gort, and L.F.M. Marcelis. 2014. Enhancement of crop photosynthesis by diffuse light: quantifying the contributing factors. Ann. Bot. 114:145-156.

Max, J.F.J., W.J. Horst, U.N. Mutwiwa and H.-J. Tantau. 2009. Effects of greenhouse cooling method on growth, fruit yield and quality of tomato (Solanum lycopersicon L.) in a tropical climate. Sci. Hort. 122:179-186.

Os, E. van, B. Speetjens, M. Ruijs, M. Bruins, and A. Sapounas. 2012. Modern, sustainable, protected greenhouse cultivation in Algeria. Wageningen UR Greenhouse Horticulture report GTB1263.

Qian, T., A. Elings, J.A. Dieleman, G. Gort, and L.F.M. Marcelis. 2012. Estimation of photosynthesis parameters for a modified Farquhar-von Caemmerer-Berry model using the simultaneous estimation method and the nonlinear mixed effects model. Environ. Exp. Bot. 82:66-73.

Zabeltitz, C. von. 2011. Integrated greenhouse systems for mild climates, DOI 10.1007/978-3-642-14582-7_10, # Springer-Verlag Berlin Heidelberg.


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