The progression from a vegetative to a flowering plant involves profound
physical and chemical changes. The stem apex displays a more complex
appearance under the microscope as flower initiation occurs, and is
followed, usually irreversibly, by the development of a flower. The
stimulus for this change may simply be genetically derived, but often
an environmental stimulus is required which links flowering to an
Photoperiodism is a term used to describe the plants various responses to
day length, explained here in terms of flowering; other responses include
bud dormancy and leaf fall.
Many plant species flower at about the same time each year, e.g. in
the UK Magnolia stellata in April, Philadelphus delavayi in June and
chrysanthemum in September. In many cases, flowering is in response
to the changing day length, which is the most consistent changing
environmental factor, in comparison with above-ground temperature
which is more variable.
In a day length sensitive species, the flowering process is ‘switched on’
by a specific period of daylight (or darkness) called its critical period.
In the chrysanthemum the critical period is sixteen hours of daylight (or
eight hours of darkness), which occurs in September in the UK.
If repeated over several weeks, the internal structure of the buds
begins to change from a vegetative meristem to a flowering meristem.
The Phytochrome ‘Switch’
Since 1920 much research has attempted to explain the photoperiodic flowering response, including using artificial lighting and
investigating genetic and biochemical control. Recently, the following stages have been identified, namely switching on at the leaf,
mobilizing leaf genes, moving the message from leaf to bud, and developing a flowering meristem. The first stage represents one of the
best examples of the horticulturist manipulating the biology of the plant and an understanding of the science has enabled the grower
to control the flowering process with the consequent valuable worldwide industry of year-round flower production. Phytochrome
chemical produced by the plant to operate the switching mechanism.
Phytochrome is a large blue-coloured molecule (molecular weight about 125 000). It is made up of two relatively small colour-sensitive
sub-molecules (chromophores) and two very long protein chains. It is thought that the chromophores change their shape in response
to light, and that this vital ‘day-length message’ is passed through the proteins to the next stage in the flowering sequence described.
Investigations of phytochrome suggest that, in addition to its involvement in the flowering stimulus, the chemical is used in as many as
24 other light-induced reactions ranging from opening a seed’s plumule hook as it emerges from the soil to increasing the
respiration rate of cells.
A two-way chemical process is involved, requiring a different light colour for each direction. Phytochrome Pr660 is sensitive to red
light of wavelength 660 nm, found in daylight from dawn to dusk. Pr660 is changed to a less stable form of phytochrome (Pfr730) after
a days ’ exposure. Pfr730 refers to phytochrome as far red light, is found in shaded conditions and is the form which brings about the
Day length sensitive plants respond to changing seasons as either long
or short day plants. In species such as Hosta, sweet pea,
Lobelia and radish, long days are essential for flowering, while the
flowering of carnations and snapdragons, among others, is improved.
In these species, the presence of Pfr730 in a concentration above a
critical limit results in the promotion of flowering, because the
summer nights are not long enough to allow sufficient Pfr730 to revert
back to Pr660. A more accurate term here therefore would be ‘short
- Day neutral species are switched on to flowering by a range of
situations involving plant size and development and temperature, e.g.
Begonia elatior and tomato.
- Short day plants, e.g. chrysanthemum, poinsettia and kalanchoe,
respond differently in that the presence of Pfr730 above a critical
limit inhibits flowering. In chrysanthemum the critical period of dark
is eight hours and this condition over a period of several weeks will
induce flowering. To enable all-year-round production as cut flowers
and pot plants, the day length manipulation is sophisticated. Immature
plants must initially be prevented from flowering, then flower buds
must later be induced, often at a time of year when the natural day
length would not be suitable.
Artificial control of flowering
|Figure 11.8 Daylength control provided by
blackout curtains to control flowering in
A long night may be broken artificially using a technique called nightbreak
Incandescent tungsten bulbs produce a high proportion
of red light and are cheap to run. Hung about 1 m above the crop and
spaced to give about 150 lux for four hours ensures that the Pfr730
critical level is not reached. Cyclic lighting
saves electricity and uses a
series of brief alternating light and dark cycles to replace one continuous
break. High pressure sodium lamps are used where they are installed for
supplementary lighting, this saves expense in providing two
systems. Crops such as chrysanthemums can be induced to flower in the
summer by imposing a long night regime artificially, using opaque black
cloth or plastic curtains to cover the crop (see Figure 11.8). A night of
nine to fi fteen hours causes the Pfr730 level to drop below the critical
limit and the flowering process to be initiated.
Flower initiation can be stimulated largely by photoperiodic or
temperature changes, or a complex interaction between temperature
and day length. Cold temperatures experienced during the winter bring
about flower initiation (i.e. vernalization
) in many biennial species
such as Brassica,
lettuce, red beet, Lunaria
and onion. The period for
the response depends on the exact temperature, as with budbreak and
seed dormancy (see stratification). The optimum temperature for many
of these responses is about 4°C. Hormones are involved in causing the
flower apex to be produced. The balance of auxins, gibberellins and
cytokinins is important, but some species respond to artificial treatment
of one type of chemical; for example, the day length requirement
for chrysanthemum plants can be partly replaced by gibberellic
Extended flower life
The flower opens to expose the organs for sexual reproduction. The
life of the flower is limited to the time needed for pollination and
fertilization, but it is often commercially desirable to extend the life of
a cut flower or flowering pot plant. In cut flowers, water uptake must
be maintained and dissolved nutrients for opening the flower bud are
termed an opening solution.
can be extended by the addition of sterilants and sugar to
the water. A sterilant,
e.g. silver nitrate, in the water can reduce the
risk of blockage of xylem by bacterial or fungal growth. Ethylene
has a considerable effect on flower development, and can bring about
premature death (senescence
) of the flower after it begins to open. Cut
flowers should therefore never be stored near to fruit, e.g. apples or
bananas, which produce ethylene. Some chemicals, such as sodium
thiosulphate, reduce the production of ethylene in carnations and
therefore extend their life.
Removal of dead flowers
The removal of dead flowers, an activity called dead-heading,
effective way to help maintain the appearance of a garden border.
Examples of species needing this procedure are seen in bedding plants
which flower over several months, e.g. African Marigold (Tagetes
); in herbaceous perennials, e.g. Delphinium and Lupin; in
small shrubs, e.g. Penstemon fruticosus;
and in climbers, e.g. sweet
pea and Rosa
‘Pink Perpetue’ . As flowers age, they begin to use up a
considerable amount of the plant’s energy in the production of fruits.
Also, hormones produced by the fruit inhibit flower development.
With species such as those mentioned above, the maturation of fruits
will considerably reduce the plant’s ability to continue producing
flowers. The act of dead-heading, therefore, will greatly improve
subsequent flowering. An added bonus is that plants that have been dead-headed may continue to flower many weeks longer than those
allowed to retain their dead flowers.
Many species such as Wax Begonia (Begonia x semperflorens-cultorum
and Busy Lizzie (Impatiens wallerana
) used as bedding plants have been
specially bred as F1 hybrids where, in this case, flowers do
not produce fruits containing viable seed. In such cases, there is not such
a great need to deadhead, but this activity will help prevent unsightly
rotting brown petals from spoiling the appearance of foliage and newlyproduced