The flowering plant

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 appropriate season.

Photoperiodism
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 is the 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 plant response.

Day length sensitive plants respond to changing seasons as either long day plants 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 night plant’.

• 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
A long night may be broken artificially using a technique called nightbreak lighting. 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
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 acid sprays.

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.

Vase life 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, is an 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 erecta); 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 flowers.