identical.
The word “cultivar” should not be confused with the word cultigen, the name given
to a plant species which is found only in cultivation and which did not originate from a
wild type.
Photosynthesis is the process by which green plants synthesise carbohydrates from water
and carbon dioxide. The energy from sunlight is utilised to achieve this, and it takes
place in the chlorophyll (green pigment) of the plant. The process is sometimes known
as carbon assimilation (ie a gas, carbon dioxide, is converted to a solid, carbon).
Respiration is the process of the oxidation of carbon and hydrogen and the release of
energy, carbon dioxide and water—ie the reverse process to photosynthesis, and is
continuous in living plant cells. Respiration provides the energy for plant growth during
the hours of darkness when photosynthesis virtually comes to a halt. As a result, the dry
weight of a growing plant decreases slightly each night, but this loss is more than
replaced by photosynthesis the next day.
The heavily shaded lower leaves often consume more carbohydrates by respiration
than they can manufacture by photosynthesis. In these cases removing the lower leaves
(of tomatoes, for example) may help the plant, as well as facilitating weeding, irrigation
and harvest.
Transpiration is the process by which water is extracted from soil by root hairs, moves
upwards through the plant and is given off through the leaves and stems as water vapour
which enters the atmosphere.
Transpiration may occur either through the plant’s cuticle, the moist membrane on
the outer layer of the plant, or through the stomata, very small holes, found most
commonly on the underside of leaves.
New evidence suggests that transpiration may be a leaf cooling mechanism to
prevent the denaturation of leaf protein.
A plant appears to wilt when its cells lose turgor (rigidity) as a result of water loss; its
leaves and young stems become weak and drooping. Wilting normally occurs when
water loss by transpiration exceeds water intake through the roots.
Young plants are more susceptible to wilting than older plants because their root
systems are less well developed, especially soon after they have been transplanted.
- Cultivar (cv.)—for botanical distinctions between closely related species,
- Variety (var.)—a group of agricultural or horticultural plants which are essentially
GROWING FOOD – THE FOOD PRODUCTION HANDBOOK 39
1Ed. Photosynthesis / Respiration