Plant growth - Principles of Horticulture - page 112

What is meant by 'growth'? Growth is a difficult
term to define because it really encompasses the
totality of all the processes that take place during
the life of an organism. However, it is useful to
distinguish between the processes that result
in an increase in size and weight, which we can
call 'growth', and those processes that cause the
changes in the plant during its life cycle, which
can usefully be called 'development', as described
in Chapter 5. All living organisms need food and
energy to grow and plants obtain these through
photosynthesis , water and mineral uptake .
Respiration is the process by which this food and
energy is converted into a form which can be used
by the plant. Because one process makes food and
the other breaks it down, for plants to grow, the right
balance between photosynthesis and respiration is
essential.

which is exported from the leaves to other plant
organs and may also be converted to starch (a large
carbohydrate) for storage until it is required. Oxygen
is produced as a waste product and this is released to
the air. All the oxygen that we breathe on earth has
been produced by plants through photosynthesis over
millions of years.

light

carbon dioxide + water glucose + oxygen

chlorophyll

Figure 9.2 Equation for photosynthesis

Photosynthesis is the process in the chloroplasts
by which green plants trap light energy from
the sun, convert it into chemical energy and use
it to produce food in the form of carbohydrates
such as sugars and starch. The raw materials are
carbon dioxide and water. Oxygen is released as
a waste product.

Photosynthesis
Photosynthesis is the process by which green
plants manufacture 'food' in the form of high-energy
carbohydrates such as sugars and starch, using light
as an energy source.
'Food' is needed to build the plant's structure and
to provide energy to fuel its activities such as the
manufacture of: proteins , including enzymes which
speed up chemical reactions in cells; cellulose , used
to build cell walls at meristems; oils and starches ,
laid down in seeds to nourish the embryo when it
germinates.
All the complex organic compounds, based on carbon,
must be produced from the simple raw materials
water and carbon dioxide. Green plants are able to
do this through the process of photosynthesis (see
producers p. 38). Many other organisms are unable to
manufacture their own food, and must therefore feed
on already manufactured organic matter such as plants
or animals (see consumers p. 38). Since large animals
predate smaller animals, which themselves feed on
plants, all organisms depend directly or indirectly on
photosynthesis as the basis of a food web or chain
(see p. 39).
Photosynthesis (Figure 9.2) involves the conversion
of water and carbon dioxide into glucose (a simple
carbohydrate or sugar) and oxygen. Light energy from
the sun is the fuel which drives the process and is
captured by the green pigment chlorophyll in the
chloroplasts , mainly in the leaf. Water is supplied by
the roots and transported to the leaves while carbon
dioxide is taken in by leaves from the air. The glucose
produced is converted to another sugar, sucrose,

Factors affecting photosynthesis
The following environmental requirements for
photosynthesis are explained in detail below:
X carbon dioxide
X light
X adequate temperature
X water (see also Chapter 10)
X mineral nutrients (see also Chapter 10).

Carbon dioxide

In order that a plant may build up organic compounds
such as sugars, it must have a supply of readily
available carbon. Carbon dioxide is present in the
air in concentrations of around 400 ppm (parts per
million) or 0.04%, and can diffuse into the leaf through
the stomata (see p. 123). If no other factors are
limiting, the rate of photosynthesis increases as levels
of carbon dioxide in the surrounding air increase.
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.
Ventilation can rectify this, replacing the carbon
dioxide used up. Alternatively the atmosphere can
be enriched in commercial glasshouses by supplying