The following environmental requirements for photosynthesis are
explained in detail below:
All living organisms require organic matter as food to build up their
structure and to provide chemical energy to fuel their activities. Whilst
photosynthesis is the crucial process, it should be remembered that a
multitude of other processes are occurring all over the plant. Proteins
are being produced, many of which are enzymes necessary to speed up
chemical reactions in the leaf, the stem, the root, and later in the flower and
fruit. The complex carbohydrate, cellulose, is being built up as cell walls
of almost every cell. Nucleo-proteins are being provided in meristematic
areas to enable cell division. These are three examples of many, to show
that growth involves much more than just photosynthesis and respiration.
All the complex organic compounds, based on carbon, must be produced
from the simple raw materials, water and carbon dioxide. Many
organisms are unable to manufacture their own food, and must therefore
feed on already manufactured organic matter such as plants or animals.
Since large animals predate on smaller animals, which themselves feed
on plants, all organisms depend directly or indirectly on photosynthesis
occurring in the plant as the basis of a food web
A summary of the process of photosynthesis is given in Table 8.1 as
a word formula and as a chemical equation. This apparently simple equation represents, in reality, two different stages in the production
of glucose. The first, the ‘light reaction’ occurs during daylight, and
splits water into hydrogen and oxygen. The second, the ‘dark reaction’
occurring at night, takes the hydrogen and joins it to carbon dioxide to
|Table 8.1 Two ways to represent the chemistry involved in photosynthesis
Most plant species follow a ‘C–3’
process of photosynthesis where the
intermediate chemical compound contains three carbon atoms (C – 3)
before producing the six-carbon glucose molecule (C – 6). Many C – 3
plants are not able to increase their rate of photosynthesis under very
high light levels.
In contrast, a ‘C– 4’
process is seen in many tropical families, including
the maize family, where plants which use an intermediate compound
containing four carbon atoms (C4) are able to continue to respond to
very high levels of light, thus increasing their productivity.
A third process, called ‘CAM’
(Crassulacean Acid Metabolism) was first
discovered in the stonecrop family. Here, the intermediate chemical is a
different four-carbon compound, malic acid. This third process has more
recently been found in several other succulent plant families (including
cacti), all of which need to survive conditions of drought. Such plants
need to keep their stomata closed during the heat of the day, but this
prevents the entry of carbon dioxide. During the night, carbon dioxide
is absorbed and stored as malic acid, ready for conversion to glucose
the next day. CAM plants do not normally grow very fast because they
are not able to store large quantities of this malic acid, and thus their
potential for glucose production is limited.
All living organisms, from viruses to whales, have the element carbon at the centre of their chemistry. The study of this element’s
chemical activity is called organic chemistry.
Originally, it was thought that all organic compounds came from living organisms, but
modern chemistry has brought synthetic urea fertilizer and DDT, which are organic.
Unusually in the range of chemical elements, carbon (like silicon) has a combining power, or valency, of four. This means that each carbon atom can react with four other atoms, whether they are atoms of other elements such as
hydrogen, or with more carbon atoms. Since the four chemical bonds from the carbon atom point in diametrically opposite directions, it
can be seen that molecules containing carbon are three dimensional, a feature which is very important in the chemistry of living things.
Carbon normally forms covalent
(non-ionic) bonds in its molecules. Here, the bond shares the electrons, and so there is no chemical
charge associated with the molecule (see also ionic bonds). Two of the simplest molecules to contain carbon are carbon dioxide and
methane (see Figure 8.2).
Carbon dioxide, or CO2
, is a constituent of the air we breathe and the sole source of carbon for almost all plants. In this compound, the
carbon attaches to two oxygen atoms, each with a valency of two.
|Figure 8.2 Carbon dioxide and methane
|Figure 8.3 Sugar molecule
Methane, or CH4
, is often referred to as ‘marsh gas’ and is the major constituent of the
North Sea gas supply. In this compound, the carbon atom is bonded with four hydrogen
atoms, each of which has a valency of one. Most fuels, such as petrol, are chemically
related to methane, but with more carbon atoms in the molecule. This family of chemicals
(containing progressively more carbons in the molecule are methane, butane, propane and
octane) is collectively called the hydrocarbons.
The molecules in petrol mainly contain
eight carbon atoms, hence the term ‘octane’.
Slightly more complicated is the glucose molecule (C6
). Here there is a circular
molecule and Figure 8.3 indicates its three-dimensional structure. Glucose is the molecule
produced by photosynthesis. It is the starting point for the synthesis of all the many
molecules used by the plant, i.e. starch, cellulose (see Figure 8.4), proteins, pigments,
auxins, DNA, etc. These molecules may contain chains with hundreds of carbon atoms
joined in slightly different ways, but the basic chemistry is the same. All the valency requirements of carbon, oxygen and hydrogen are still met.
|Figure 8.4 Starch and cellulose molecules
Requirements for photosynthesis
In order that a plant may build up organic compounds such as sugars,
it must have a supply of carbon which is readily available. Carbon
is present in the air in concentrations of 330 ppm (parts per
million) or 0.03 per cent, and can diffuse into the leaf through the
stomata. Carbon dioxide gas moves ten thousand times faster in air than
it would in solution through the roots. The amount of carbon dioxide in
the air immediately surrounding the plant can fall when planting is very
dense, or when plants have been photosynthesizing rapidly, especially in
an unventilated greenhouse.
This reduction will slow down the rate of photosynthesis, but a grower
may supply additional carbon dioxide inside a greenhouse or polythene
tunnel to enrich
the atmosphere up to about three times the normal
concentration, or an optimum of 1000 ppm (0.1 per cent) in lettuce. Such
practices will produce a corresponding increase in growth, provided
other factors are available to the plant. If any one of these is in short
supply, then the process will be slowed down. This principle, called the
law of limiting factors, states that the factor in least supply will limit the
rate of the process, and applies to other non- photosynthetic processes in
the plant. It would be wasteful, therefore, to increase the carbon dioxide
concentration artificially, e.g. by burning propane gas, or releasing pure
carbon dioxide gas, if other factors were not proportionally increased.
|Figure 8.5 CO2 burner enriches the
glasshouse environment, so helping
Light is a factor required for photosynthesis to occur. In any
series of chemical reactions where one substance combines with
another to form a larger compound, energy is needed to fuel the
reactions. Energy for photosynthesis is provided by light from the
sun or from artificial lamps. As with carbon dioxide, the amount
of light energy present is important in determining the rate of
photosynthesis – simply, the more light or greater illuminance
(intensity) absorbed by the plant, the more photosynthesis can take
place. Light energy is measured in joules/square metre, but for
practical purposes the light for plant growth is measured according
to the light falling on a given area, that is lumens per square metre
(lux). More recently, the unit ‘ microwatts per sq. metre’ has been
introduced. One lux, in natural sunlight, is equal to microwatts
per sq. metre. Whilst the measurement of illuminance is a very
useful tool for the grower, it is difficult to state the plant’s precise
requirements, as variation occurs with species, age, temperature,
carbon dioxide levels, nutrient supply and health of the plant.
However, it is possible to suggest approximate limits within which
photosynthesis will take place; a minimum intensity of about 500–1000
lux enables the plant’s photosynthesis rate to keep pace with respiration,
and thus maintain itself. The maximum amount of light many plants
can usefully absorb is approximately 30 000 lux, while good growth in many plants will occur at 10 000–15 000 lux. Plant species adapted to
shade conditions, however, e.g. Ficus benjamina , require only 1000 lux.
Other shade-tolerant plants include Taxus spp., Mahonia and Hedera.
In summer, light intensity can reach 50 000–90 000 lux and is therefore
not limiting, but in winter months, between November and February, the
low natural light intensity of about 3000–8000 lux is the limiting factor
for plants actively growing in a heated greenhouse or polythene tunnel.
Care must be taken to maintain clean glass or polythene, and to avoid
condensation that restricts light transmission. Intensity can be increased
by using artificial lighting, which can also extend the length of day, which
is short during the winter, by supplementary lighting
. This method is
used for plants such as lettuce, bedding plants and brassica seedlings.
Total replacement lighting
Growing rooms which receive no natural sunlight at all use controlled
temperatures, humidities, and carbon dioxide levels, as well as light.
Young plants which can be grown in a relatively small area, and which
are capable of responding well to good growing conditions in terms of
growth rate, are often raised in a growing room.
The type of lamp
Lamps are chosen for increasing intensity, and therefore more
photosynthesis. All such lamps must have a relatively high efficiency
of conversion of electricity to light, and only gas discharge
able to do this. Light is produced when an electric arc is formed across
the gas filament enclosed under pressure inside an inner tube. Light,
like other forms of energy, e.g. 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)
to 350 nm (blue light – in the short wavelength area), and a combination
of different wavelengths (colours) appears as white light. Each type of
lamp produces a characteristic wavelength range and, just as different
coloured substances absorb and reflect varying colours of light, so a
plant absorbs and reflects specific wavelengths of light.
|Figure 8.6 High-pressure sodium lamp is
used for supplementary lighting
Since the photosynthetic green pigment chlorophyll absorbs mainly
red and blue light and reflects more of the yellow and green part of
the spectrum, it is important that the lamps used produce a balanced
wavelength spectrum to include as high a proportion of those colours
as possible, in order that the plant makes most efficient use of the light
provided. The gas included in a lamp determines its light characteristics.
The two most commonly used gases for horticultural lighting are
mercury vapour, producing a green blue light with no red, and sodium,
producing yellow light. This limited spectrum may be modified by the
inclusion of fluorescent materials in the inner tube, which allow the tube
to re-emit wavelengths more useful to the plant emitted by the gas and
re-emit the energy as a shorter wavelength. Thus, modified mercury
lamps produce the desirable red light missing from the basic emission. Low-pressure
mercury-filled tubes produce diffuse light and, when
suitably grouped in banks, provide uniform light close to plants. These
are especially useful in a growing room, provided that they produce
a broad spectrum of light as is seen in the ‘ full spectrum fluorescent
tubes ’ . Gas enclosed at high pressure
in a second inner tube produces
a small, high intensity source of light. These small lamps do not greatly
obstruct natural light entering a greenhouse and, while producing
valuable uniform supplementary illumination at a distance, cause no
leaf scorch. Probably the most useful lamp for supplementary lighting
in a greenhouse is a high-pressure sodium lamp, which produces a high
intensity of light, and is relatively efficient (27 per cent).
Carbon dioxide enrichment
should be matched to artificial lighting in
order to produce the greatest growth rate and most efficient use of both
The complex chemical reactions which occur during the formation of
carbohydrates 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 40°C. This pattern is mirrored by the effect of air temperature on the
rate of photosynthesis. But here, the optimum temperature varies with
plant species from 25°C to 36°C as optimum. It should be borne in
mind that at very low light levels, the increase in photo-synthetic rate
with increased temperature is only limited. This means that any input of
heating into the growing situation during cold weather will be largely
wasted if the light levels are low.
Integrated environmental control
in a greenhouse is a form of
computerized system developed to maintain near-optimum levels of
the main environmental factors (light, temperature and carbon dioxide)
necessary for plant growth. It achieves this by frequent monitoring of
the greenhouse using carefully positioned sensors. Such a system is
able to avoid the low temperature/light interaction described above. The
beneficial effects to plant growth of lower night temperatures compared
with day are well known in many species, e.g. tomato. The explanation
is inconclusive, but the accumulation of sugars during the night appears
to be greater, suggesting a relationship between photosynthesis and
respiration rates. Such responses are shown to be related to temperature
regimes experienced in the areas of origin of the species.
|Figure 8.7 The glasshouse environment may be controlled by means of
(a) a computer
situated in the glasshouse, while
(b) conditions are monitored throughout the glasshouse
Adaptation to extremes in temperature can be found in a number of
species; for example resistance to high temperatures above 40°C
; resistance to chilling injury
is brought about by
lowering the freezing point of cell constituents. Both depend on the
stage of development of the plant, e.g. a seed is relatively resistant, but the hypocotyl of a young seedling is particularly vulnerable.
Resistance to chilling injury is imparted by the cell membrane, which
can also allow the accumulation of substances to prevent freezing of
the cell contents. Hardening off
of plants by gradual exposure to cold
temperatures can develop a change in the cell membrane, as in bedding
plants and peas. Examples of plant hardiness are found in Table 4.3.
Water is required in the photosynthesis reaction but this represents
only a very small proportion of the total water taken up by the plant
(see transpiration). Water supply through the xylem is essential to
maintain leaf turgidity and retain fully open stomata for carbon dioxide
movement into the leaf. In a situation where a leaf contains only 90
per cent of its optimum water content, stomatal closure will prevent
carbon dioxide entry to such an extent that there may be as much as 50
per cent reduction in photosynthesis. A visibly wilting plant will not be
photosynthesizing at all.
Minerals are required by the leaf to produce the chlorophyll
that absorbs most of the light energy for photosynthesis. Production of
chlorophyll must be continuous, since it loses its efficiency quickly.
A plant deficient in iron, or magnesium especially, turns yellow (chlorotic)
and loses much of its photosynthetic ability. Variegation
similarly results in a slower growth rate.
|Figure 8.8 Cross-section of Ligustrum leaf showing its structure as an efficient
The leaf is the main organ for photosynthesis in the plant, and its cells
are organized in a way that provides maximum efficiency. The upper
epidermis is transparent enough to allow the transmission of light into
the lower leaf tissues. The sausage-shaped palisade mesophyll cells
are packed together, pointing downwards, under the upper epidermis.
The sub-cellular chloroplasts
within them carry out the photosynthesis
process. The absorption of light by chlorophyll occurs at one site and the
energy is transferred to a second site within the chloroplast where it is
used to build up carbohydrates, usually in the form of insoluble starch.
The spongy mesophyll, below the palisade mesophyll, has a loose
structure with many air spaces. These spaces allow for the two-way
diffusion of gases. The carbon dioxide from the air is able to reach the
palisade mesophyll; and oxygen, the waste product from photosynthesis,
leaves the leaf. The numerous stomata on the lower leaf surface are
the openings to the outside by which this gas movement occurs. The
numerous small vascular bundles (veins) within the leaf structure
contain the xylem vessels that provide the water for the photosynthesis
reaction. The phloem cells are similarly present in the vascular bundles,
for the removal of sugar to other plant parts. Figure 8.8 shows the structure of the leaf and its relevance to the process of photosynthesis.
A newly expanded leaf is most efficient in the absorption of light,
and this ability reduces with age. The movement of the products of
photosynthesis is described in Transport
in the plant
Gases in the air, which are usually products of industrial processes or
burning fuels, can cause damage to plants, often resulting in scorching
symptoms of the leaves. Fluoride can accumulate in composts and be
present in tap water, so causing marginal and tip scorch in leaves of
susceptible species such as Dracaena and Gladiolus.
and carbon dioxide may be produced by faulty heat exchangers in
glasshouse burners, especially those using paraffin. Scorch damage over
the whole leaf is preceded by a reddish discolouration.
In order that growth can occur, the food must be broken down in
a controlled manner to release energy for the production of useful
structural substances such as cellulose, the main constituent of plant cell
walls, and proteins for enzymes. This energy is used also to fuel cell
division and the many chemical reactions that occur in the cell.
The energy requirement within the plant varies, and reproductive organs
can respire at twice the rate of the leaves. Also, in apical meristems, the
processes of cell division and cell differentiation require high inputs of
energy. In order that the breakdown is complete, oxygen is required in
the process of aerobic respiration. A summary of the process is given in
|Table 8.2 A summary of the process of aerobic respiration
It would appear at first sight that respiration is the reverse of
photosynthesis. This supposition is correct in the sense
that photosynthesis creates glucose as an energy-saving strategy, and
respiration breaks down glucose as an energy releasing mechanism. It is
also correct in the sense that the simple equations representing the two
processes are mirror images of each other.
It should, however, be emphasized that the two processes have two
notable differences. The first is that respiration in plants (as in animals)
occurs in all living cells of all tissues at all times in leaves, stems,
flowers, roots and fruits. Photosynthesis occurs predominantly in the palisade mesophyll tissue of leaves. Secondly, respiration takes place
in the torpedo-shaped organelles of the cell called mitochondria.
Photosynthesis occurs in the oval-shaped chloroplasts. Details of
biochemistry, beyond the scope of this book, would reveal how
different these processes are, in spite of their superficial similarities.
In the absence of oxygen, inefficient anaerobic respiration takes place
and incomplete breakdown of the carbohydrates produces alcohol
as a waste product, with energy still trapped in the molecule. If a
plant or plant organ such as a root is supplied with low oxygen
concentrations in a waterlogged or compacted soil, the consequent
alcohol production may prove toxic enough to cause root death.
Over-watering, especially of pot plants, leads to this damage and
encourages damping-off fungi.
Storage of plants
The actively growing plant is supplied with the necessary factors
for photosynthesis and respiration to take place. Roots, leaves or
flower stems removed from the plant for sale or planting will cease
to photosynthesize, though respiration continues. Carbohydrates and
other storage products, such as proteins and fats, continue to be broken
down to release energy, but the plant reserves are depleted and dry
weight reduced. A reduction in the respiration rate should therefore be
considered for stored plant material, whether the period of storage is a
few days, e.g. tomatoes and cut flowers, or several months, e.g. apples.
Attention to the following factors may achieve this aim:
- Temperature: The enzymes involved in respiration become
progressively less active with a reduction in temperatures from 36°C
(optimum) to 0°C. Therefore, a cold store employing temperatures
between 0°C and 10°C is commonly used for the storage of materials
such as cut flowers, e.g. roses; fruit, e.g. apples; vegetables, e.g.
onions; and cuttings, e.g. chrysanthemums, which root more readily
later. Long-term storage of seeds in gene banks (see Pollination and
liquid nitrogen at 20°C.
- Oxygen and carbon dioxide: Respiration requires oxygen in
sufficient concentration; if oxygen concentration is reduced, the
rate of respiration will decrease. Conversely, carbon dioxide is a
product of the process and as with many processes, a build-up of a
product will cause the rate of the process to decrease. A controlled
environment store for long-term storage, e.g. of top fruit, is
maintained at 0° C–5°C according to cultivar, and is fed with inert
nitrogen gas to exclude oxygen. Carbon dioxide is increased by up to
10 per cent for some apple cultivars.
- Water loss: Loss of water may quickly desiccate and kill stored
material, such as cuttings. Seeds also must not be allowed to lose
so much water that they become non-viable, but too humid an
environment may encourage premature germination with equal loss of