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Friday, February 20, 2009

Aspects of Flowering

Flowering is a fundamental process in plant development and reproduction. It is also important to all life forms as we depend on plants for food as well as aesthetical value. Understanding the process of the initiation of flowering is vital to growers, as plants have great commercial and economic importance, both in agriculture and horticulture. Being able to control the flowering time enables growers to schedule crops, to meet the demands of the market. However, it is a complex physiological process which requires a good deal of knowledge, as different species respond to different stimuli to initiate flowering. As well as having a good knowledge of a plant’s growth regulators, growers need to be able to control light and temperature in order to induce flowering. This assignment examines the factors controlling flowering, and how they can be manipulated to induce the flowering of a Poinsettia for 1st November, to meet the commercial demands of the retail market.

The apical meristem of a plant goes through a series of developmental stages. This is a collection of cells in the actively growing tip of a plant which are able to develop into either leaves or flowers. Many plants will not flower until they reach the adult reproductive stage of maturity, regardless of external stimuli. Until this stage, the apical meristem has no ability to produce reproductive structures. It is a plant’s hormones that dictate the development of the apical meristem. When the apical meristem reaches the reproductive stage, it will respond to external stimuli such as light and temperature to initiate flowering.

An important stimulus when scheduling flowering is the plants exposure ratio of day light hours to darkness. Each leaf contains a photoreceptor chemical called phytochrome, which absorbs light energy. Phytochrome has an absorption spectrum which is a characteristic light wavelength it can absorb.

Experiments done during the 1950’s by Borthwick and Hendrick, lead to the discovery of two interconvertible forms of phytochrome, (Pr and Pfr). They found that these chemicals had a significant effect on a plants physiological development. By exposing plants to red light and far red light, it was discovered that growth and flowering were both promoted or inhibited. Pfr absorbs red light with an optimum absorption wavelength of 660nm. Pfr absorbs light from the far red end of the spectrum at a wavelength of 730nm.
During day light, phytochromes are constantly converting between Pr and Pfr as the light spectrum contains both red light and far red light. In darkness the Pfr continues to convert at a very slow rate to Pr because it is biologically active; Pr however, is inactive and therefore does not convert to Pfr during darkness. This means that come morning, the level of Pr is significantly greater than Pfr. Flowering is triggered when the level of Pfr reaches a particular point, because as the days become longer the amount of Pfr present in the morning increases, thus indicating to the plant what time of year it is.
Only very brief exposure to a particular light is necessary to trigger the conversion; however it is the final light to which the plant is exposed which has the controlling factor. The experiments carried out in the 1950’s showed that whilst exposure to far red light inhibits the germination of certain types of seed such as lettuce, it can both inhibit and stimulate flowering in plants depending on whether they are long day or short day plants. Appendix A gives a summary of effects of red light and far-red light.
Most plants fall in to three categories; Long-Day Plants (LDP), Short-Day Plants (SDP) or day neutral plants. LDP will only flower when the number of day light hours (or red light) exceeds a certain minimum length. SDP will only flower when the day light hours are below a particular maximum length. Therefore far red light or Pfr induces flowering. Day- neutral plants flower regardless of the day length. Although the word ‘day’ is used to describe these categories, it is actually the length of darkness to which the plant is exposed that is crucial when determining its flowering time. This is because of phytochrome conversions. Table A shows that if a short day plant is exposed to red light its flowering is inhibited. This is because red light is the equivalent to day light so; by interrupting darkness with even a brief period of red light the phytochromes will rapidly begin converting Pr into Pfr. When darkness is reinstated the reverse phytochrome conversion (Pfr to Pr) is much slower. As SDP’s require Pr to flower the reduction of this phytochrome by the brief light exposure will inhibit flowering.
Although exposure to red light during darkness will inhibit flowering in SDP, exposing a LDP to a brief period of darkness during day light will have no effect its flowering schedule due to the slow conversion of Pfr to Pr during the period of darkness.
Temperature is another key factor for some flowering plants such as bulbs, Flowering can be stimulated by exposure to low temperatures, this is called vernalisation. The length of vernalisation required can vary from as little as four days to three months. An optimum temperature range of 1°C to 10°C has been found to be the most effective. Temperature is perceived by the apical meristem, therefore only the actively growing tips of the plant require vernalisation to stimulate flowering.
Vernalisation often accompanies photoperidism, since day length can vary in spring and autumn a combination of stimuli guarantees that flowering occurs at the correct time. This is often seen in summer flowering plants which require a cold spell followed by long days to stimulate flowering.
In 2000 Dr Liz Dennis and Dr Jim Peacock of the CSIRO made an important genetic discovery, a gene called the ‘Flowering Switch Gene’. Dr Dennis found that ‘After the cold of winter this gene naturally switches off to ensure that plants will flower in the spring. By varying the activity of this gene in a trial plant they found that it was possible to control its flowering.’ (CSIRO)
Hormones are chemical substances produced in a plant, which can accelerate, inhibit or modify its growth. Plants rely on hormones for movement and internal communication. There are five groups of plant hormones, each controlling different things. Appendix B shows the groups and their controlling factors.
Gibberellin is one of the most important hormonal groups in the process promoting and inhibiting flowering. Giberallins can be a substitute for red light (present in day light) and can therefore trigger flowering in LDP, and similarly inhibit flowering in SDP. Artificial giberellins can be sprayed on to the foliage of plants to trigger flowering in a number of species which would normally require vernalisation or a certain photoperiod
Gibberellins are produced in leaves when a plant is exposed to a stimulus (vernalisation or photoperiodism) the transmission of a stimuli to the apical meristem is still theoretical. It was thought that a hormone called ‘florgen’ transmited the ‘message’ to the apical meristem, however this hormone has never been isolated. Dimech (2003) however, does not mention florigen but says that;
‘Stimuli trigger the production of gibberellins in the leaf, and these are transferred via the phloem to the apical meristem, which then undergoes physiological changes to allow floral initiation.’ He goes on to say that ‘In some cases, the apical meristem will also receive floral initiation signals from the roots. Taiz & Zeiger (1998) report on experiments where root removal has led to floral initiation, suggesting that roots produce a floral inhibitor in some species. Gibberellic acid (and possibly cytokinins ) are produced in the roots.’
Perhaps the floral initiation stimulus in the leaves is communicated to the roots which synthesises more hormones to regulate flowering?’
The flowering process is an on going investigation for scientists and questions such as the communication of stimuli from receptor to apical meristem, still remain unanswered. Manipulating nature can be unsuccessful unless it is handled with great precision.
When using light manipulation, it is vital that the dark period is not interrupted at any time as this will halt the floral induction process, by reversing the phytochrome conversion within the leaves. Correct temperatures must also be maintained, otherwise irregular growth patterns will occur.
Manipulation of plants does come with drawbacks. Vast greenhouses are expensive to heat and require black out blinds to ensure the correct photoperiodism. Large refrigeration units are required for plants or bulbs which require vernalisation. Ensuring that each plant has sufficient space to allow maximum light to the stem, limits the number of plants produced. A problem with flower manipulation is that, unless the plant is subjected to similar circumstances in future, it will revert to its natural flowering time. However, advances in technology and scientific knowledge have meant that plant propagation is now a huge commercial business and allows growers to meet the increasing demands of the market.
By Jane Keightley of giggleberries

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