Agriculture Reference
In-Depth Information
carbon dioxide at levels above that in the atmosphere.
In fact the atmosphere within a glasshouse or
polytunnel can be increased to levels well above
ambient concentrations, typically three times greater,
(e.g. up to 1,000 ppm (0.1 per cent) in lettuce), with
a resulting increase in the rate of photosynthesis
leading to improvements in yield and quality of many
glasshouse crops.
Light wavelengths
Light, like other forms of energy, such as heat,
X-rays and radio waves, travels in the form of
waves, and the distance between one wave
peak and the next is termed the wavelength.
Light wavelengths are measured in nanometres
(nm): 1 nm = one-thousandth of a micrometre.
Visible light wavelengths vary from 800 nm
(red light, in the long wavelength area) through
the spectrum to 350 nm (blue light, in the short
wavelength area). A combination of different
wavelengths (colours) appears as white light.
Photosynthetically Active Radiation or PAR
contains wavelengths useful for photosynthesis,
between 400 nm and 700 nm.
Other light-absorbing systems in the plant
are responsible for developmental changes
through the plant's life cycle. Blue light, with
wavelengths around 400 nm, is important for
vegetative growth, stimulating leafy growth and
sturdy plants, and is involved in the directional
growth responses to light (see phototropism
p. 70). Red light, with wavelengths around 580
nm to 700 nm controls flowering. For successful
plant growth, therefore, artificial lighting
contains a mix of red and blue wavelengths
which aim to mimic sunlight, optimizing
overall increase in plant material (through
photosynthesis) and correct development from
early growth through to flowering and fruiting
(through other light-absorbing systems).
Light
In any series of chemical reactions where one
substance combines with another to form a larger
compound, energy is needed to fuel the reactions.
In plants this energy is provided by light from the
sun. As with carbon dioxide, the amount of light
energy present is important in determining the rate
of photosynthesis; simply, the more light, or greater
the
light intensity
supplied to the plant, the more
photosynthesis can take place. Beyond a certain light
intensity, however, the rate of photosynthesis levels
off as the chloroplasts are fully engaged. This is called
the
saturation point
and will vary from plant to plant,
shade lovers such as
Ficus benjamina
having lower
saturation points than those adapted to high light
conditions. Light levels also affect stomatal opening:
stomata close as light levels reduce, which restricts
carbon dioxide uptake. Care must be taken to maintain
clean glass or polythene and to avoid condensation
that restricts light transmission. Light intensity can be
increased by using artificial lighting (
supplementary
lighting
) to boost light levels, particularly in the winter
when light is the rate-limiting factor. The
duration
of lighting will naturally influence the length of time
that photosynthesis can continue, longer in the
spring and summer than during the winter months.
Supplementary lighting can also be used to
extend the duration of the daylight hours in
winter.
As well as light intensity and duration,
light quality
is important in optimizing
photosynthesis. Photosynthesis only utilizes
certain wavelengths of light, those in the red and
blue parts of the visible spectrum. Pigments such
as chlorophyll absorb light of certain wavelengths,
in this case the ones useful to photosynthesis,
and reflect the rest such as yellow and green
wavelengths, which is why chlorophyll appears
green. If supplementary lighting is given in a
greenhouse, the lamp chosen, as well as giving
good light intensity, must produce the right
wavelengths of light for photosynthesis (known
as Photosynthetically Active Radiation or PAR).
Temperature
The complex chemical reactions that occur during
the formation of carbohydrates such as glucose from
water and carbon dioxide require the presence of
chemicals called
enzymes
to accelerate the rate of
reactions. Without these enzymes, little chemical
activity would occur. Enzyme activity in living things
increases with temperature from 0°C to 36°C, and
ceases at around 40°C when the enzymes break
down irreversibly. This pattern is mirrored by the
effect of air temperature on the rate of photosynthesis
which increases with increasing temperature up
to an optimum (this varies with plant species from
25°C to 36°C) above which it slows again. At high
temperatures, stomata may close to reduce water
loss (see p. 123) thus preventing carbon dioxide
uptake, while at very high temperatures leaves may
be damaged and photosynthesis ceases altogether.
As with light, in temperate countries, low winter
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