Agriculture Reference
In-Depth Information
Photosynthesis allows plants to convert
different inorganic compounds into veget-
able organic matter (or biomass), using
energy from the Sun. Respiration supplies
the plants with the energy required for them
to function, consuming part of the biomass
generated in the photosynthesis. When pho-
tosynthesis is greater than respiration, the
surplus biomass is used by the plant to 'fuel'
its growth and development. The climate
control in the greenhouse is aimed at opti-
mizing this balance (photosynthesis/respi-
ration), to achieve the desired growth and
productivity.
Of the whole fresh weight of the green-
house plants, approximately 90% is usually
water and the remaining 10% is organic
matter (Levanon
et al
., 1986). In order for
the plant to grow, the difference between
photosynthesis (carbon absorption in the
form of CO
2
to be converted into biomass)
and respiration (energy and CO
2
release)
must be positive; in other words, the 'car-
bon balance' (or, otherwise, the net photo-
synthesis) must be positive.
The productive process is complex,
with short- and long-term responses. The
short-term responses (minutes, hours) are
the water and assimilate status, processes
that supply energy, construction materials
and water for tissue growth, whereas in the
long term the productive process may be
characterized by the accumulation of dry
matter and the development and distribu-
tion of such dry matter, not to forget the
quality of the product (Challa
et al
., 1995).
which they feed. Therefore, the primary
source of all the metabolized energy used
on our planet is the Sun and photosynthesis
is fundamental to the preservation of living
beings.
Fossil fuels (coal, oil) are decomposi-
tion products of animals and land and sea
plants and the energy that they store was
captured by living organisms millions of
years ago, coming initially from the Sun's
radiation.
In photosynthesis, the most important
step, chemically, is the conversion of carbon
dioxide (CO
2
) and water into carbohydrates
and oxygen. The reaction, schematically, is:
6CO
2
+ 6H
2
O → C
6
H
12
O
6
+ 6O
2
+ Energy
(6.1)
Photosynthesis may be described as the
process in which solar energy is converted
into chemical energy by plant tissues in the
presence of chlorophyll. This chemical
energy is stored in the form of different
compounds (carbohydrates, mainly, ATP
and NADPH). By means of this process car-
bon is fixed into carbohydrate molecules,
and oxygen (O
2
) along with highly energetic
compounds (ATP and NADPH) are released
to be later used by the plant in the synthesis
of amino acids, organic acids and other
more complex organic compounds. All
these compounds are transported to the
growing parts, to become part of the plant's
structures, contributing to the generation of
biomass.
The majority of plants cultivated in
greenhouses are of the C3 type (metabolism
in C3), so called because of the type of
chemical reactions in their photosynthetic
process. Other plants, called C4, are less
responsive to atmospheric CO
2
content;
they are usually plants from tropical areas.
From the cultivated species, C4 plants are
species such as sugarcane, maize, millet
and sorghum. A third type of photosynthe-
sis is CAM metabolism, undertaken by suc-
culent plants, which are characterized by
their ability to pre-fix CO
2
in the dark, dur-
ing the night. Therefore, the stomata do not
have to open during the day, avoiding the
loss of precious water supplies.
6.3
Photosynthesis
6.3.1
Introduction
Photosynthesis is the process by which plants,
using the Sun's energy, synthesize organic
compounds from inorganic substances.
Every existing living being needs energy
for its growth and conservation. In the vege-
table kingdom the energy used comes from
the Sun, whereas animals, being unable to
directly use the Sun's energy, use energy
stored in plants or in other animals on
