Plant life cycles - Principles of Horticulture

Agriculture Reference

In-Depth Information

(a)

(b)

(c)

Figure 5.11 Autumn colour in (a) Vaccinium

corymbosum (blueberry); (b) Viburnum ; and

(c) Photinia, showing loss of chlorophyll and

emergence of xanthophylls

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 - for example, the carotenoids (yellow)

and xanthophylls (red) - but usually the quantities are

so small as to be masked by the chlorophyll. In some

species, such as Fagus sylvatica Purpurea Group

(copper beech), other pigments predominate, masking

chlorophyll. Many colours are displayed in the leaves

at this time in such species as Acer platanoides ,

turning gold and red, Prunus cerasifera 'Pissardii' with

light purple leaves, Larix decidua (European larch) with

yellow leaves, Parthenocissus quinquefolia (Virginia

creeper) and Vitis spp. with red leaves, beech with

brown leaves. Coloured autumn fruits are also prized

in the garden such as the berries in Cotoneaster and

Pyracantha and the colouring of stems can become

more intense, such as in Cornus (dogwoods). These

are used in autumn colour displays at a time

when fewer flowering plants are seen outdoors

(Figure 5.12).

In deciduous woody species, the leaves drop in the

process of abscission , which can be triggered by

shortening of day length. To reduce risk of water loss

from the remaining leaf scar, a corky layer is formed

before the leaf falls.

In many fruits , ripening is associated with an increase

in respiration and changes in colour, sweetness,

Figure 5.12 Autumn interest in the garden: acers with

bright leaves, birch with silver bark and yellow leaves,

fl owering grasses, leaves and bark of Eucalyptus

flavour, texture and scent. This is soon followed by

natural progression to a senescent stage in which the

fruit deteriorates and the seed ripens before it is shed,

although growers aim to harvest fruit before this stage

is reached and arrest further development through

a range of post-harvest techniques such as the use

of reduced temperatures. Eventually, senescence is

followed by death at the end of the season in annuals

or at the end of the plant's life in perennials (or death

of the plant organ such as a leaf where this undergoes

senescence). All metabolic processes cease and the

plant matter is returned to the soil to be taken up by

and to sustain future plants.