Sustainable Protected Cultivation at a Mediterranean Climate.
Perspectives and Challenges
Stefania De Pascale and Albino Maggio
Department of Agricultural Engineering and Agronomy
University of Naples Federico II
Via Università, 100 – 80055 – Portici (Naples)
Italy
INTRODUCTION
Over recent decades, Europe has been progressing from a “sustained agriculture”
system to “sustainable agriculture”. The term “sustain”, from the Latin sustinere (sus-,
from below and tenere, to hold), means “to keep in existence” or to maintain and implies
long-term support and permanence. Sustainability is essentially seen as a criterion for
guaranteeing development based on ecological and social balance: “sustainable
development is development that meets the needs of the current generation without
undermining the ability of future generations to meet their own needs” (Hall, 2001). As it
pertains to agriculture, sustainable describes farming systems that are able to maintain
their productivity and usefulness to society indefinitely. These systems must be resourceconserving, socially supportive, commercially competitive and environmentally sound
(Weil, 1990). In terms of the European Union agricultural policy, this concept has been
translated into a move from a policy of subsidizing farmers based on their production to a
policy of incentives for achieving environmental targets. Achieving sustainability
generally implies a more efficient use of the available technology (Acutt at al., 1998).
However, it should be pointed out that, since agricultural sustainability requires
maximization of the net benefit to the society, the achievement of this objective must take
all costs and deriving benefits (economic, social, environmental, etc.) into consideration
(Bakkes at al., 2001). Recently, North Western European and Mediterranean protected
cultivation systems have been described in terms of resource use, efficiency of the
production process, environmental impact and related political and social-economic
issues (Baudoin, 1999; Baille, 2001; La Malfa and Leonardi, 2001; Castilla, 2002). The
differences that emerged enabled us to define a specific Mediterranean greenhouse
agrosystem (Castilla et al., 2004). We will confine the following review on some aspects
of resource use efficiency and sustainability in the greenhouse Mediterranean agrosystem.
GREENHOUSE AGROECOSYSTEM
In the last century, the birth and development of Ecology led to the definition of
the ecosystem concept (a self-organizing, self-maintaining and self-evolving structure)
(Odum, 1969). The simplest and most widespread ecosystem model is: Input-EcosystemOutput, where the system itself is treated like a “black box”. The ecosystemic approach
has also been applied to agriculture and, by analogy with the ecosystem, the term
agroecosystem (= ecosystem used for agricultural purposes) has been coined (Loucks,
1977).
The sustainability of an agroecosystem is represented by its ability to maintain a
given level of productivity over time and a given quantitative-qualitative level of
environmental resources (Loucks, 1977). The productivity assessment refers also to the
degree of input transformation into output (i.e. the transformation efficiency). The
greenhouse is also a form of agroecosystem where, unlike other agroecosystems, the
environment has been adapted to the crop in order to maximize its productivity. Other
significant differences include greater production stability (smaller productivity
fluctuations over time and smaller fluctuations in the quantity and quality of resources
used) and reduced autonomy (measured in terms of dependence on inputs from outside
the agroecosystem). Protected cultivation is becoming increasingly important to European
agriculture because of its capacity to produce all year round. The greenhouse system has
Proc. IC on Greensys
Eds.: G. van Straten et al.
Acta Hort. 691, ISHS 2005
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to take maximum advantage of the natural benefits offered by the location in certain
latitudes while minimizing the disadvantages (Castilla, 2002).
The greenhouse system components are represented by the following inputs:
natural resources (soil, water, air, organisms) and anthropogenic resources (chemicals,
materials, seedlings). The system output is represented by the useful product per covered
soil surface unit (dry matter, energy, proteins, income). The output concept should also
include modifications (positive or negative) of environmental resources (landscape, soil,
water, air, organisms) associated to the production system.
The greenhouse productivity is strictly dependent on its specific physical/
agronomic characteristics as indicated below:
1. environmental isolation level, direct (conditioning) and indirect (covering
materials, fertilizers and pesticides) energy inputs;
2. tendency towards maximizing production, which generates a consequent
acceleration in entropic processes (increased thermal and chemical energy consumption,
increased crop production cycles, unstable biological system, increased pest management
problems, increased waste production);
3. control level of the greenhouse climatic and agro/biological parameters
associated to the system’s complexity.
In terms of sustainability, the greenhouse agroecosystem is best when a) it is able
to produce high yields with low resource use intensity (low external energy input per
output unit); b) it is able to keep the production factors (e.g. soil fertility) and defense
mechanisms (e.g. biological phytophagous and plant disease control) intact (Brydges,
2001).
DETERMINANTS OF SUSTAINABILITY
The three main indicators that could enable us to assess whether a production
process is sustainable or not are:
- use of renewable resources
- use of non-renewable resources
- pollution levels.
In principles, the rate of renewable resource consumption should not exceed the
regeneration rate. Meanwhile, the rate of non-renewable resource consumption should not
exceed the rate of renewable replacement resource development. Finally, the pollutants
emission rates should not exceed the environment capacity to absorb and regenerate them
(York, 1991). Four major ecosystem processes are always at work in any given agroecosystem: energy flow, water and mineral cycles, and ecosystem dynamics. If functioning
properly, these processes will conserve the soil and water resources, eventually reducing
the overall operating costs (Louks, 1977). It is possible to increase surveillance and
management of these natural processes by increasing control technologies.
PROTECTED CULTIVATION IN THE MEDITERRANEAN REGION
According to the FAO, protected cultivation in the Mediterranean region has been
rapidly expanding from nil in 1950 to 120,000 ha of greenhouses and tunnels in 1985, up
to 200,000 ha in 1997 (Baudoin, 1999). There have been various recent works on the
current status of protected crops in the Mediterranean area (La Malfa and Leonardi, 2001;
Castilla, 2002; Castilla et al., 2004). Simple plastic greenhouses predominate in warmer
Mediterranean countries. The low-tech plastic covered greenhouses are largely developed
on the principle of minimum capital and technological input. In these regions, investing in
hi-tech is generally not justified due to an insufficient capital return (Baille, 2001). In
contrast, a closer look at protected cultivation in North Western Europe shows that
technologically advanced systems have gradually been installed, such as computerized
climatic control, hydroponics and inter-transportation (also known as agro-robot systems)
(Stanghellini et al., 2003). The Mediterranean greenhouse system is characterized by
some fundamental differences compared to North Western Europe in terms of quantity
and temporal distribution of natural resources (e.g. radiant energy). These climatic
30
differences have fostered the development of a greenhouse system based on simple
structures and unexpensive climatic control devices in the warmer Mediterranean regions.
On the other hand, these simplified systems present some limitations such as poor
ventilation and humidity control and reduced light transmission of plastic coverage
(Baille, 1999), which do not consent an efficient use of the available natural resources
(solar radiation, CO2).
SUSTAINABLE USE OF NATURAL RESOURCES IN GREENHOUSE
AGRICULTURE
Water
Since water is one of the most important resources in Mediterranean regions we
will particularly focus on some aspects of water use and management in the greenhouse
environment. Assessing sustainability for water management in greenhouse production
involves defining what is sustainable, in terms of water needs, for both the society and the
plant. Eventually these two aspects will merge to delineate what is sustainable for the
farmer and, ultimately, to draw possible strategies to improve the economic benefits of a
sustainable use of water in greenhouse agriculture. Although the amount of water utilized
in agriculture has decreased of approximately 20% from the 1960 to date, several
problems associated to a diminished availability of water still exist (Tognoni et al., 2002).
These include the decrease of drinking water in some countries, the increase in poor
quality usable water in agriculture, and the problematic allocation of available water
among diverse sectors (agriculture, industry, others). The increasing water shortage,
which are facing the Mediterranean regions, is calling the EU Institutions to reallocate the
available water resource according to criteria based on social and economic priorities. In
this scenario, the greenhouse industry is rapidly expanding since the economy of water
use in controlled environment agriculture better responds to the “cost recovery” principle,
recently promoted by EU regulations, compared to open field agriculture (Stanghellini et
al., 2003).
Achieving high water use efficiency in greenhouse production is possible by
exerting optimal control of both environmental parameters and cultural practices, which
will both eventually lead to obtain higher yield while reducing water waste. Since in
controlled environments it is possible to efficiently modulate environmental and cultural
parameters according to the plant needs, research on greenhouse water management has
mainly focused on identifying the best technology to fulfil plant water requirements.
Results in this area of research have led to the development of specific techniques such as
hydroponics, reuse of drain water and use of closed-loop growing systems (Bailey and
Day 1999; Albright, 2002). Despite the development of such precise technologies,
greenhouse grown plants are often exposed to diverse stresses, including drought and high
temperature stress [especially in those Mediterranean regions where greenhouse
production is performed using plastic shelters with rudimentary equipments (Baille,
1994)] and saline stress [often associated to use of poor quality water and re-use of drain
water]. In this respect, little attention has been put on understanding how plants respond
and adapt to the Mediterranean greenhouse environment. Recent advancement in the
molecular genetics of stress adaptation is offering a unique opportunity to elucidate the
physiology and functional biology of plant water requirements in this specific
environment and, ultimately, to understand how plant performances can be improved in
terms of water use. The plant environment and the plant physiology are therefore two
critical aspects that must be considered to design strategies aimed at improving the
economic water use efficiency in greenhouse production.
1. Water Use Efficiency. The ratio of plant biomass over the evapotranspired water
defines the water use efficiency (WUE) of a plant and it is generally considered as a
measure of its efficiency in utilizing water for biomass production. The WUE has
increased significantly on a world scale in the last fifty years. Considering that the annual
global evapotranspiration rate (9.7 • 1012 • m3 of water) probably did not change
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significantly in the same time period, the observed increased efficiency is likely due to an
increased crop yield (Tognoni et al., 2002; Turner et al., 1978; Constable and Hearn,
1978). In a greenhouse, the water consumption rate can be reduced by increasing the
resource use efficiency. The degree of efficiency is generally calculated on the basis of the
transformation efficiency of the irrigation water in the cultivation system according to the
following equation:
Yield
Yield
=
I .V . Tr + Ev + LR + loss
Where, I.V.=Irrigation Volume; Tr=liters of water transpired by the crop; Ev=
liters of water evaporated; LR=Leaching Requirements; loss=liters of water lost by the
water delivery system.
Protected cultivation is normally characterized by greater water use efficiency for
three main reasons: reduced potential evaporation (reduced solar radiation, less wind and
greater air humidity); higher productivity (better control of climatic parameters and plant
diseases); application of more advanced irrigation technology (drip irrigation, reuse of
drainage water). In Northern European greenhouses these factors can increase water use
efficiency up to five times (Stanghellini et al., 2003). In contrast, in Mediterranean
greenhouses the absence of an efficient climatic control technology limits WUE
improvements. In unheated Mediterranean greenhouses a tomato crop uses 40 liters of
water per kg of product, only 1/3 below the amount of water necessary for open air
production (60 liters) (Stanghellini et al., 2003).
Improvements of WUE can be achieved by 1) modifying the plant environment
and 2) modifying the plant physiology.
2. Modifying the “Plant Environment”. We may consider as plant environment
anything that is outside the plant and that may directly or indirectly affect plant water
uptake. For instance, by increasing the humidity of the air in contact with the leaves, it is
possible to decrease the vapor pressure deficit with a retroactive effect on transpiration
(Stanghellini, 1993). Air temperature (Pearce et al., 1993; Ho, 1996), relative humidity,
light (Photosynthetic Photon Flux Density, PPFD) (Grange and Hand, 1997), CO2
concentration (Li et al., 1999) will all affect the plant water demand and can be easily
controlled in the greenhouse environment. An integrated control of all these variables is
therefore pivotal in greenhouse water management. For instance, low crop transpiration
can counteract the detrimental effects of high EC (Ho, 2002). Similarly, increasing the
root temperature may help plants to tolerate high Cl concentrations of the nutrient
solution. The interaction between EC, CO2 (Maggio et al., 2002a) and solar radiation
(Dalton et al., 2001) should be also considered to obtain high quality yield while
optimizing the use of the available water (Ho, 2002). Matching the water supply with the
light intensity during the day time is a strategy to avoid overfeeding and to control the
balance between reproductive vs. vegetative growth (Ho, 2002).
Irrigation. The fundamental difference between open field and greenhouse
production is that in the latter the grower may adjust the potential evaporative demand,
consequently greenhouse growers have an almost complete control on both the demand of
water and the water supply (Baille, 1994). Similarly to open field crops, in greenhouse
irrigation management the key questions that need to be addressed are: 1) how much
water is necessary to satisfy the crop requirements, and 2) when is irrigation needed.
To address the first question, we may consider the general dose-response curve of
crop yield to irrigation, based on which crop yield is minimal at “very low” and “very
high” water dosage. From a practical perspective, we may hypothesize three different
possibilities:
1. Maximizing the crop yield. This can be achieved by replenishing the maximal
evapotranspiration (ETm). This practice is feasible on small fields such as those generally
covered by greenhouses and with non-limiting water resource (in terms of quantity and/or
quality).
WUE =
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2. Maximizing the water-use efficiency (i.e. optimizing the yield per unit of water
applied). This is very important in Mediterranean environments where water is limited
and increasingly expensive.
3. Maximizing the farmer’s yield, which means maximizing the difference
between gross income and costs.
Although for flower and vegetable crops objective #1 and #3 almost coincide (i.e.
yield decreases significantly in the absence of sufficient water to replenish the ETm). The
scenario #2 must also be considered in a sustainable context. To address the question
“when is irrigation needed”, three major approaches can be followed:
1. based on the estimation of the soil moisture (visual appreciation of the “soil
dryness” and/or use of tensiometers, resistance blocks, neutron probes,…..);
2. based on the evaluation of plant water stress (visual appreciation of incipient
wilting; measurements of stomatal conductance, leaf water potential, leaf temperature via
infrared thermometry,….);
3. based on micrometeorological parameters and mathematical models.
Computer programs designed to calculate plant water requirements based on
continuous monitoring of micrometeorological parameters are available. This approach
allows an automated control of irrigation. Examples of integrated management, which
associates determination of climatic parameters with plant water status measurements are
also available and may result to be highly efficient systems (Baille, 1992).
Irrigation Efficiency. The irrigation efficiency may range between 25–50% in
furrow irrigation, between 50–70% using sprinkler systems and between 80–90% using
trickle (drip) irrigation. Consequently, for a specific volume of water, sprinkler and trickle
methods increase the irrigated land by 20–30% and 30–40%, respectively, compared to
furrow-irrigation. Drip irrigation is increasingly being utilized in Mediterranean regions
(La Malfa and Leonardi, 2001). However, we should point out that in saline environments
the fulfillment of leaching requirements significantly limits the irrigation efficiency.
Micro-irrigation is generally preferred in greenhouse cultivation because it allows an easy
delivery of nutrients (fertigation). Nevertheless, the simultaneous delivery of water and
nutrients may have some drawbacks in terms of fertilization efficiency (van Os, 1995).
Long-term irrigation also leads to environmental problems that need to be considered in a
sustainability context. These include: (1) build-up of sodium chloride and other salts in
the soil; (2) increasing concentration of nitrates and pesticides in surface and underground
water due to excessive use of these chemicals; (3) diminished soil fertility, caused by the
modification of soil structure and physicochemical properties; (4) waste of water
resources when inefficient water delivery systems and irrigation methods are used. These
problems can be controlled and minimized through efficient irrigation management,
which idealistically should synchronize irrigation with crop water demand. This could be
accomplished by: (1) timing and controlling the amount of water applied to the root zone
and reducing the amount of water loss in deep percolation; (2) maximizing the plant’s
ability to absorb nutrients in the root zone (i.e. minimizing nutrient loss in groundwater);
(3) implementing soil salinity control in the irrigation schedule.
Other practices include those targeted at reducing evaporation from the soil such
as mulching and enhancing a rapid and uniform crop soil coverage (transplant, choice of
suitable plant density and architecture). Good quality water can also be obtained from
recovering rain water in underground or aboveground tanks (Castilla et al., 2004).
Salinity. Salinity is a critical issue in greenhouse water management of
Mediterranean regions. The EC of the irrigation water in some coastal zones of Southern
Italy is 1.6-3.2 dS/m, whereas 40% of the soils has an EC over 4 dS/m. Salinization may
seriously compromise the soil physical-chemical properties (De Pascale et al., 2003).
Proper management of saline water has been thoroughly discussed by other authors and
we refer to their reviews for further details (Maas and Grattan, 1999). Utilization of saline
water for vegetable crop production is an important area of research. Data on vegetable
crop tolerance to salinity are available in the literature; nevertheless, analyses of the longterm effect of the use of saline and/or wastewater on the soil physical-chemical properties
33
are missing. Adequate utilization of low-quality water may substantially increase the
availability of usable water for agriculture purpose (Hamdy, 2002).
Soilless Systems. Water use efficiency in greenhouse production has considerably
improved upon introduction of soilless systems where water can be recovered and reused
(closed system). From an environmental perspective, soilless agriculture is a valuable
alternative to the lack of soil fumigants for greenhouse use and to the costly steam
sterilization (Jensen, 2002). In addition to a more efficient use of water and fertilizers,
soilless agriculture involves a minimal use of land area, efficient disease control and
suitability for mechanization. Moreover, it greatly reduces problems associated to soil
diseases, salinization and modification of its physical-chemical properties that may
overall affect the final yield. Despite the undisputed advantages, soilless systems still
have high capital costs and require specialized labor and good quality water, features that
confine this technology to high economic value cash crops (Jensen, 2002). For these crops
and limited surface areas (such as cut flowers cultivation) the use of deionization or
inverse osmosis systems may be recommended to overcome water quality problems.
3. Modifying the “Plant Physiology”. Water Use Efficiency is a complex trait which
cannot be easily improved via traditional breeding and/or genetic engineering (Maggio et
al., 2002b). Nevertheless, it has been demonstrated that some plants can grow “more”
with less water. Unravelling the physiological basis of WUE among plants species has
been for many years the goal of many scientists. The introduction of model plants such as
Arabidopsis together with the development of sophisticated molecular genetic techniques
and mutation analysis has greatly advanced this area of research. Today many
mechanisms underlaying plant adaptation to drought stress have been linked to specific
gene functions. Some of these mechanisms, which will be described in the following
sections, can be the targets for improving water use efficiency in greenhouse production
(Maggio et al., 2002b).
Role of Stomata in Reducing Water Loss. Stomatal morphology and behaviour
contribute by far the greatest to overall control of water movement out of the plant.
However, a tight relationship between the amount of water transpired and the amount of
biomass produced by plants exists (Hanks, 1983). Therefore, restricting the amount of
water that is transpired in order to avoid drought stress cannot be proposed as a strategy to
improve WUE since this would reduce the biomass accumulation. Nevertheless, particular
behaviour of stomata such as midday closure to avoid the highest degree of evaporative
demand is a common feature of plants adapted to arid environments (Schulze and Hall,
1982). Mutants with altered stomatal response can be isolated in Arabidopsis or other
model systems by providing a toxic gas (e.g. SO2 or O3) only during the period when the
stomata are desired to be closed (or at least partially closed). Using infra-red photography
has also been proposed as a strategy to discriminate between closed vs. open stomata
(Raskin and Ladyman, 1988). Some mutations in the ABA metabolism and/or
responsiveness have also been isolated (Li et al., 2000). Under the control of appropriate
promoters, these genes might be used to allow the opening of stomata only under
favourable conditions. Mutations affecting the formation, distribution and density of
stomata in Arabidopsis have been also described (Yang and Sack, 1995; Larkin et al.,
1997). These mutants could be used in model experiments to examine the relationships
between transpiration, drought/avoidance and growth.
Leaf morphology. Reduction of the leaf area may also affect the use of the
available water. Specific leaf shape and morphology have been associated to adaptive
features of xerophytic plants (Fischer and Turner, 1978). Several mutants with altered leaf
structure and morphology have been identified (Berná et al., 1999). These include the
elongata mutants that produce spine-like leaves (Berná et al., 1999), mutants with
abnormally thicker leaves and combinations of these traits (Bohmert et al., 1998). Even
more extreme leaf shapes might be produced by combining these types of mutations in
double or triple mutants. Genes like KNOTTED-1 can induce the formation of succulentlike lobed leaves when overexpressed (Lincoln et al., 1994). The effect of such leaf
morphology changes on both drought avoidance and biomass productivity could easily be
34
examined in Arabidopsis. Genes controlling the growth of root vs. shoot have also been
identified (Torii and Deng, 1995) and may provide a useful model to understand the effect
of different root/shoot ratios on the final yield and biomass distribution in plants grown in
specific systems such as hydroponics. Water loss from plant surfaces also is significantly
affected by incident radiation. When transpiration is limited, high leaf temperatures
caused by excessive absorbed radiation can significantly decrease water use efficiency.
Plants possess a number of adaptive features such as variation of the leaf angle (Berná et
al., 1999), waxiness (Negruk et al., 1996) and hairiness (Szymanski et al., 1998) of the
leaf blade that can increase their reflectance properties.
Water uptake and root morphology. In addition to control the water loss, plants
may overcome drought stress by enhancing their ability to extract the soil water. Root
architecture greatly affects plant’s ability to uptake water (Passioura, 1996). The specific
relationship between root morphology/architecture and water extraction properties may be
studied using the available collection of Arabidopsis mutants or isolating root mutants in
other vegetable crops (Maggio et al., 2001). Arabidopsis mutants with altered root
characteristics have been described (Watson et al., 1998) and some of these may have
profound implications for drought avoidance.
Root hydraulic conductivity. The resistance of water flow through the root plays a
key role in drought stress. The morphology of xylem vessels has the greatest influence
over hydraulic conductivity of the plant. The biochemical composition of cell wall may
also affect the apoplastic water flux. Arabidopsis mutants with altered vasculature
(Maggio et al., 2002b) have been identified, yet the influence of these mutations on plant
water uptake has not been studied in sufficient detail. Function and metabolic regulation
of plant aquaporins (water channels) has also been recognized to play a pivotal role
during water shortage. Plant aquaporins, located both in the plasma membrane and
tonoplast, facilitate the passive transport of water down a water potential gradient.
Genetic manipulation of water channel gene expression has demonstrated the importance
of these genes in plant water relations (Maurel, 1997). However the specific contribution
of these proteins in water stress adaptation is currently unknown.
Energy
Rational greenhouse energy management normally aims at maximizing the energy
supply while cutting down on lost energy. In Mediterranean greenhouses, the objectives
change depending on the time of the year. During fall/winter, the objective is to maximize
the radiation quantity (by using long-lasting covering materials with high transmittance
indexes) and to minimize energy loss (double layered plastic coverage, energy screens,
lightweight structures). Greenhouse heating is a relatively recent practice in
Mediterranean regions. Producers currently see heating as an additional resource to
guarantee early production and constant quantitative-qualitative yield (Baudoin, 1999). In
Italy, it has been calculated that direct energy consumption for conditioning is
approximately 140,000 PET (Petroleum Equivalent Tons), with an incidence of 20-30%
on the total production cost. Mediterranean greenhouses have much lower energy needs
than Northern European greenhouses. In Southern Italy, one hectare of cut roses requires
between 5,200 and 6,800 GJ vs. 16,000 GJ/ha required in the Netherlands for cut flower
production (Stanghellini et al., 2003). However, despite these lower heating requirements,
it should be pointed out that the majority of Mediterranean greenhouse systems depend on
non-renewable energy sources with a high impact on the environment (fossil fuels).
Sustainability can be improved by using alternative energy sources such as organic waste
or renewable energy sources (solar and wind) and through the reuse of energy (Bot, 2004;
Short, 2004). During the spring/summer the main objective in Mediterranean greenhouses
is reducing high temperatures. This can be obtained by using shading nets or
whitewashing that reduce the incoming radiation. Mobile shading systems based on the
photosynthesis light saturation may be highly efficient for this purpose (Hansen et al.,
1996). Excess radiant energy can be used to enhance evapotranspiration by increasing
ventilation or using misting systems. This criterion is subject to application limits at high
35
temperatures and with poor water supply (quantity and quality). For high value greenhouse productions, advanced technological solutions could be proposed (cooling
systems). The majority of the Mediterranean greenhouses currently under-uses the
potential energy in the fall/winter period and is strongly limited during summer/spring
(approximately 3-5 months of non-productive time) due to a costly control of high
temperatures.
Soil
Soil in protected Mediterranean environments must be considered a limited and
non-renewable resource. The poor quality of irrigation water, shallow water tables and
intensive irrigation all pose serious risks of soil salinization and contamination. In
addition, the repeated and specialized cultivation may reduce the soil biological fertility
(Leonardi and La Malfa, 2001). For a sustainable use of the soil, strategies aimed at
preserving its fertility and alternative renewable resources have been developed. Good
practices for preventing salinization should be observed (Maas and Grattan, 1999). In
addition, studying the dynamics of the main soil fertility indexes (chemical and
biological) and identifying threshold fertility values (e.g. EC) may be a valid approach to
monitor soil use. The possibility of using organic fertilization, solarization, biological pest
control, resistant cultivars and grafting should be considered, also. The use of substrates is
a valuable alternative to soil cultivation. Soilless greenhouse cultivation has significantly
increased in Mediterranean countries (5,000 ha in Almeria (Spain), 1,400 ha in France,
1,000 ha in Italy). Although soilless cultivation may increase 4 times the net income
compared to soil cultivation (Caballero and De Miguel, 2002), high costs and inadequate
water quality are limiting its further expansion. Closed systems are environmentally
friendly, because they optimize water use while preventing soil contamination. However
their economic sustainability in Mediterranean environments may be questioned.
Conversely, open systems (the most common in Mediterranean greenhouses) still may
have a significant environmental impact. A problem that should not be underestimated is
that soilless agriculture generates approximately 2 ton/ha*year of mineral (rock wool,
perlite) or organic (such as peat) waste that has to be disposed (Stanghellini et al., 2003).
In this respect, organic substrates may have some advantages in terms of sustainability
because they can be recycled (compost, energy production).
Carbon Dioxide
The atmospheric CO2 concentration limits the potential photosynthesis of most
vegetable species and their productivity. The optimal CO2 concentration for growth and
yield seems to be between 700 and 900 vpm. Inside greenhouses with a dense canopy
concentration, CO2 drops below the atmospheric level even when the vents are open. In
Almeria, maximum CO2 depletion of 20% (Lorenzo et al., 1990) and 37% (SanchezGuerrero et al., 2001) were measured in open and closed greenhouses, respectively.
During winter, in poorly ventilated Mediterranean greenhouses, CO2 concentration may
reach 100-200 vpm. CO2 enrichment is not a current practice in Mediterranean regions,
due to greenhouse aeration requirements. Nevertheless, in Southern Italy CO2 greenhouse
enrichment (2500 l/h*ha) allows to maintain the CO2 concentration at 600 vpm (with low
radiation and closed vents) and to prevent CO2 depletion below 350 vpm (with high
radiation and open vents). In Spain, cucumber yield was significantly increased by CO2
enrichment and the effect was enhanced by controlling the minimum temperature
(Sanchez-Guerrero et al., 2001). We should not overlook that the sustainability of
greenhouse CO2 enrichment is associated to the possibility of using low cost CO2 sources
(side product of heating systems).
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ENVIRONMENTAL IMPACT
Landscape
Agriculture’s first and foremost impact on the environment is obviously linked to
the conversion of natural ecosystems into agroecosystems. Greenhouse areas have an
enormous visual impact and increase the impermeable land surface area. Although this
subject would certainly require a more detailed analysis, it should be mentioned that
greenhouses have a negative landscape impact in certain Mediterranean areas such as
Almeria, Ragusa and Naples where the covered surface is particularly extended.
Moreover, in most Mediterranean Region the scenic, historical, natural, cultural and
archeological values of certain landscapes may be seriously compromised by the presence
of greenhouses. Public administrations have already imposed restrictions on greenhouse
expansion. Buffer strips surrounding greenhouses can be used to reduce their
environmental impact on the natural landscape.
Waste Production
1. Plastic. There is a constant increase in the use of plastics for greenhouse coverage and
other uses (mulching, irrigation, etc.). It is estimated that for protected crops in Italy,
approximately 80,000 tons of plastic are used every year. There are problems associated
to plastic waste disposal. About 30% of all the plastic used in the greenhouse industry is
currently recycled to produce granules (La Malfa and Leonardi, 2001). It has been
estimated that in Almeria the production of plastic waste after renewing the polyethylene
covers (each 2-3 years) is 1.1 ton/ha*year. In addition, even the plastic strings used as
tutoring lines generate 112 kg/ha*year of waste and polypropylene chromatic insect traps
are responsible for another 50 kg/ha*year. Plastic from irrigation systems, containers, etc.
generates 500 kg/ha*year of waste (Stanghellini et al., 2003). According to our data,
estimated plastic consumption in Campania greenhouses is around 3.7-4.5 ton/ha*year, of
which 2.2-3.0 tons from polyethylene covers and 1.5 tons from mulching, irrigation
systems, containers, etc. In Sicily, 2.9 ton/ha*year of plastic are produced (La Malfa and
Leonardi, 2001). Several approaches have been proposed to reduce plastic waste: use of
long-life plastic films as covering materials, use of biodegradable mulching and plastic
recycling. Collection and recycling of plastics is increasing, however the costs and the
size of the potential market for lower-grade recycled material still limits its expansion.
2. Chemicals. The greenhouse cultivation system requires higher input levels (water,
chemicals, plastic materials, heating, nutrients, etc.) compared to outdoor cultivation. The
main sources of chemical pollution linked to modern greenhouse agriculture include pest
and nutrients management.
Pest management. In general, greenhouse productions require an average of 10
chemical treatments per crop, with peaks that may exceed 20 treatments/crops for certain
flower species. It can be roughly calculated that the number of chemical treatments to
control mites and insects may vary from 4-12 for tomatoes, 2-5 for strawberries, 2-7 for
peppers, 2-10 for eggplants, 2-6 for cucumbers and 10 for floral species. In Italy it is
estimated that around 14 kg of insecticide+pesticide/ha are consumed for each cultivation
cycle. According to our data, 47 kg/ha (active component) per year are used in the most
intensive Italian greenhouses vs. 31 kg/ha used in the Netherlands. Use of these products
can create serious toxicological/environmental problems. van Os et al. (1994) calculated
that 30 to 50% of these chemicals leave the greenhouse through the air (with a greenhouse
ventilation rates of 0.5 h-1). This percentage would increase in more ventilated
Mediterranean greenhouses (Stanghellini et al., 2003). A strong reduction in pesticides
consumption could be achieved by using an integrated pest management, which should be
strongly encouraged for a sustainable greenhouse management (Castilla et al., 2004).
Nutrients management. It is acknowledged that growers often use more water and
fertilizers than plants actually need. Today, only 30–50% of applied nitrogen fertilizer and
~45% of phosphorus fertilizer is taken up by crops (La Malfa and Leonardi; 2001;
Stanghellini, 2003). A significant amount of the applied nitrogen and a smaller portion of
37
the applied phosphorus is lost from agricultural fields. Such nutrient losses harm off-site
ecosystems, water quality and aquatic ecosystems, and contribute to changes in
atmospheric composition (Tilman et al., 2002). Excessive fertilization leads to both N
accumulation into the underground water and soil salinization. Over 20 years of cut
flowers cultivation in Campania (Southern Italy) greenhouses, potassium fertilization has
caused an increase of the exchangeable K2O to 1,000 ppm, with peaks of 1,800 ppm in
some areas. In the same greenhouses, the phosphorus content is approximately 5 to 10
times higher compared to the average soil phosphorus content of surrounding areas. In
these situations, improving fertilization efficiency is essentially a technical problem.
Efficient fertilization management should consider more carefully crop removal and
nutrients dynamics (Giacomelli, 2002; van Noordwijk, 1990). Generally speaking,
intensive horticulture cannot rely on the cultural residues, whose amount is rarely
consistent (e.g. tomatoes non-yield dry biomass varies between 2 and 3 ton/ha*year,
equivalent to 0.3 –0.4 ton/ha of stable humus). Moreover, it is generally preferable to
destroy these residues to improve pest control. In terms of environmental sustainability,
recycling of organic residues should be fostered. This would require developing efficient
collection and processing infrastructures.
The environmental benefits that can be achieved by using soilless cultures are
limited to closed cycle systems (van Os et al., 1991). In contrast, most soilless systems of
Mediterranean greenhouses are based on open cycles. In this case, a 20-35% leaching
fraction is required to assure a sufficient moisture level in the substrate while minimizing
salt accumulation. The excess nutrient solution (which may be much higher than 35%)
still contains a significant amount of nutrients, which will pollute the environment (De
Pascale et al., 2001). To reduce the waste solution it is essential that the EC of the water
does not exceed 1 dS/m (Brun and Settembrino, 1995; De Kreij and van den Berg, 1990).
PERSPECTIVES AND CHALLENGES
Sustainability is fundamentally based on the following five aspects: 1) Policy and
problem management on an economic, cultural and social level; 2) Energy and production
inputs: energy sources, fertilizers, crop protection from pathogenic attacks, organic
agriculture, research and technology; 3) Genetic resources: identification, evaluation and
use; 4) Climate: impact on production; 5) Soil and water: available resources and crop
requirements. However, while the definition of these elements seems to provide a clear
picture of the problem, strategies to improve sustainability are not easy to define.
Although many practices targeted at improving greenhouse sustainability are
agronomically sound, it is also necessary to assess their economic sustainability. In this
respect, the year-round production system may be not sustainable for the Mediterranean
environment. In North Western Europe, the proposed strategy for reducing the impact of
protected crops on the environment aims at transforming the greenhouse from an “open”
agricultural system to a “closed” system. This involves: 1) reduction/reuse of waste
material and toxic residues, 2) automation and computerization; 3) pest monitoring and
control, 4) “soilless” cultivation and nutrient solution recycling (Nichols and Christie,
2002; Kozai and Chun, 2002; Ting, 2002). The application of this strategy to
Mediterranean protected cultivation may be proposed only for some cash crops.
Alternatively, a strategy aimed at transforming the Mediterranean greenhouse into a
“semi-open” system is proposed. This would require: 1) reinforcing a specific
Mediterranean greenhouse industry for developing lightweight protection structures and
covering materials with suitable thermal and optical characteristics (e.g. materials that
may adjust their transmission properties on the basis of radiation intensity); 2) improving
technology management of the available natural resources (water recovery, organic and
integrated pest management, waste recycling); 3) fostering investments in marketing of
innovation (products variety and presentation); 4) promoting genetic selection of cultivars
suitable for the Mediterranean greenhouse environment; 5) advancing knowledge on the
eco-physiology of plant stress adaptation (high temperature, drought stress, high solar
radiation, etc); 6) developing prediction models for cost-benefits analyses; 7) training of
38
Mediterranean growers on the sustainable use of the available resources. In addition,
since the semi-open greenhouse system typical of the Mediterranean regions gives a
product that is closer to an open-field product and, as such, is perceived by the consumer
as a “more natural product”, efforts should be put in emphasizing this aspect of
Mediterranean productions. Marketing strategies should take advantage of the worldwide
recognized image of “high quality” and “better taste” (high dry matter percentage, low
nitrates content), certainly associated to the particularly favorable climatic conditions in
which these products are obtained. In this respect, it would be better to invest in specific
research programs, marketing and commercial infrastructures rather than in “imported”
un-sustainable technologies. In this context, the definition of quality brands represents a
real opportunity for Mediterranean greenhouse products. The “natural” aspect associated
to an “environmental-friendly” cultivation technique may represent a sound ground for
developing a Mediterranean eco-label that would certainly improve competitiveness of
this product.
ACKNOWLEDGMENTS
We would like to thank Giancarlo Barbieri for stimulating discussions.
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