At the end of an annual plant’s life, or the growing season of perennial
plants, a number of changes take place. The changes in colour associated
with autumn are due to pigments that develop in the leaves and stems
and are revealed as the chlorophyll (green) is broken down and absorbed
by the plant.
are substances that are capable of absorbing light; they also
reflect certain wavelengths of light which determine the colour of the
pigment. In the actively growing plant, chlorophyll, which reflects
mainly green light, is produced in considerable amounts, and therefore
the plant, especially the leaves, appears predominantly green. Other
pigments are present; e.g. the carotenoids (yellow) and xanthophylls
(red), but usually the quantities are so small as to be masked by the
chlorophyll. In some species, e.g. copper beech (Fagus sylvat-ica
pigments predominate, masking chlorophyll. These pigments also occur
in many species of deciduous plants at the end of the growing season,
when chlorophyll synthesis ceases prior to the
|Figure 11.9 Autumn colour in (a) Blueberry,
(b) V iburnum and (c) P hotinia , showing loss of
chlorophyll and emergence of xanthophylls.
abscission of the leaves.
Many colours are displayed in the leaves at this time in such species as Acer platanoides,
turning gold and red, Prunus cerasifera
with light purple leaves, European larch with yellow leaves, Virginia
creeper (Parthenocissus and Vitus spp.
) with red leaves, beech with
brown leaves, Cotoneaster
with coloured berries, and
Cornus species, which have coloured stems. These are used in autumn colour
displays at a time when fewer flowering plants are seen outdoors
(see Figure 11.9).
In deciduous woody species the leaves drop in the process of abscission,
which may be triggered by shortening of the day length. In order to
reduce risk of water loss from the remaining leaf scar, a corky layer is
formed before the leaf falls. Auxin production in the leaf is reduced,
this stimulates the formation of the abscission layer, and abscisic acid is
involved in the process. Auxin sprays can be used to achieve a premature
leaf fall in nursery stock plants thus enabling the early lifting of bareroot
plants. Ethylene inhibits the action of auxin, and can therefore
also cause premature leaf fall, for example, in Hydrangea
prior to cold
treatment for flower initiation.