Agriculture Reference
In-Depth Information
wide range of growth manipulations. Some examples are shading, cooling by wet
pad or fogging, heating and dehumidifying, and providing artificial illumination.
Most of the commercial plastic films and porous screens are made of low density
polyethylene with some additives. The latter are used for purposes like, avoiding
plastic film degradation due to UV radiation, preventing nighttime radiation cooling
by blocking infrared radiation (IR) transmittance to the sky, avoiding dripping of
condensed water vapor on the inner side of the film and decreasing dust accumula-
tion on the outside of the cover. Such additives may modify the crop radiation and
energy balances and hence greenhouse microclimate.
Microclimate of protected crops is a major factor in determining the internal
atmospheric water demand and hence potential crop water use. Shade and reduced
wind speed usually decrease the water demand in comparison to the open in tropi-
cal, subtropical, semi-arid or desert regions and hence may lead to increased
w
ater
u
se
e
fficiency (WUE) (see box 1). In tropical regions greenhouses, or the so called
rainshelters, are used to protect the crops from rain storms. Nearly 90 % of the en-
ergy costs in greenhouses in the northern European countries are for heating. Since
the first energy crisis at the end of the 1970's, the efforts for reducing the heating
costs in these countries have increased enormously. Not only do the growers benefit
from increased profitability due to higher yields and quality of produce grown in
greenhouses, but this fits very well with our current environmental concerns and the
objective to reduce carbon dioxide (CO
2
) emissions within protected cultivation.
Since each type of structure and cover induces a different microclimatic modi-
fication, it is outside the scope of this chapter to review in detail all these effects.
Rather, the chapter outlines the effects of most common types of structures on the
major environmental variables: radiation, temperature, humidity, air velocity, ven-
tilation, CO
2
concentration; and, in turn, how these modifications influence various
crop attributes like plant growth, productivity and product quality. For didactic and
practical reasons, the main approach of this chapter is to present the reactions of
protected crops to singular environmental variables. In order to view the entirety of
the concept, the reaction of crops to actual conditions within protected cultivations
is illustrated through the use of some examples.
Most crops in protected cultivation are vegetables, followed by cut flowers and
potted ornamentals and fruits. The reaction curves of plant growth and develop-
ment are optimum functions marked by a minimum, an optimum, saturation, and/
or a maximum of environmental conditions. However, the optimum points in curve
courses are not the same for different attributes or crops. In the past, enormous
investigation have been conducted concerning plant growth and productivity of pro-
tected crops, and different models have been developed; however the focus in this
chapter will be on product quality that in recent years, has become more and more
important due to consumer concerns.
Plant growth and productivity are very well defined; the first as a difference for
any given parameter in the course of time and the second as a source of production
for a given ground area of plant material. Product quality, on the other hand, is a
complex issue not only depending on different factors, but also on different per-
spectives. The different actors involved in the value chain, from breeders through
