Plant hormones
Hormones are produced by and transported around the entire plant. Put simply, hormones are like signals that can be sent and received throughout the entire plant. So a leaf can transmit a signal to the end of the stem telling it to form flowers for example. The most well-known plant hormones are auxin, gibberellin, cytokinin, ethylene and abscisin. It has also been demonstrated that brassino-steriods, salicylates and jasmonates function in a similar way to hormones. Hormones can also be found bonded to sugars or amino acids. In this form they are inactive and serve as storage. These hormones can be released again and become active under the influence of gravity or light for example.
Auxin
In the 1880s, Charles Darwin and his son Francis conducted experiments that finally confirmed the existence of plant hormones. They experimented with oats and the influence of light on the direction of growth. These experiments demonstrated the role of auxin. Auxin is a hormone that is produced in the plant’s growing tips both above ground and in the roots. It influences water absorption, cell division and cell stretching (it softens cell walls), among other things. Because auxin promotes the formation of roots on stems it is used in a variety of forms in rooting hormones.
Experiments carried out by CANNA have shown that the effect of administering auxin depends very much on the concentration and method of application used for each plant type. With weak concentrations, flower formation is promoted slightly and ripening takes longer. With high concentrations there is an inhibiting effect on growth accompanied by deformities and tumour-like symptoms.
Auxin produced in the growing tips of plants is capable of inhibiting the development of side shoots. This symptom is known as apical dominance. Removing the main growing tips stops this inhibiting effect and allows side shoots to develop, eventually resulting in a broader, bushier plant. If you only plant a few plants per square metre, it’s worthwhile removing the tops since this makes it possible to use the light better. You also need to remove the tips regularly to achieve a good mother plant with a lot of side shoots.
Gibberellin
Gibberellin was first isolated in 1935 in Japan by Yabuta. The gibberellin was acquired from a fungus that had been causing reduced productivity for Japanese rice farmers for centuries. The gibberellin initially caused better growth but later in the season it caused sterile fruits.
Generally speaking, gibberellins work as growth accelerators, causing cell stretching and cell division. They ensure that seeds germinate and that flowers form in plants that need long days. Gibberellin is often used in the cultivation of fruit to help unfertilised pears and apples to mature fully.
Administering gibberellin to short-day plants, or autumn flowerers as they are also known, very quickly produces clear results even at low concentrations. The plants become light green in colour and stems split open because of the fast growth (photo 1). The plant’s speed of growth can reach 10 cm per day!
Administering gibberellin during the vegetative phase will delay and slow down flowering.
For short-day plants, gibberellin has a similar effect to testosterone in humans. It stimulates the formation of typically male organs and taller plants, longer internodes and male flowers in dioecious plants. When pollen from these flowers is used to fertilise female flowers, the seeds created always produce female plants.
Photo 1: Stem that has torn open because of growing too fast after gibberellin was administered.
Certain environmental influences can also cause the production of extra gibberellin. Plants will produce more gibberellin under poorly lit conditions, which causes them to become tall, sparse and lanky.
Cytokinin
The action of cytokinin was first demonstrated in 1913. 30 years later, it was discovered that a natural substance present in coconut milk was capable of helping plant cells to multiply. Cytokinin is the hormone responsible for this.
Cytokinin is known as the hormone responsible for cell division. It stimulates the metabolism and the formation of flowers on side shoots, and as such it is a counterpart to auxin. Cytokinin is most concentrated in the youngest parts of the plant, such as the seeds, fruits, young leaves and root tips. A high concentration of cytokinin in organs or tissues stimulate the transport of sugars to those organs or tissues. Administering cytokinin leads to a greater leaf surface area and faster flower formation. However, the time at which flowering finishes remains comparable to that in untreated plants. Cytokinin can be seen as a counterpart to gibberellin in this respect because it stimulates the formation of female flowers on male plants.
Ethylene
The practical application of ethylene dates back to the time of Ancient Egypt, when figs were scored to make them ripen faster. In 1934, it was discovered that plants produce ethylene themselves, which enables them to regulate fruit ripening.
Ethylene is the least complex plant hormone from the molecular point of view and is produced by all organs. It is a gaseous hormone which is transported via the spaces between plant cells. It is responsible for fruits ripening, inhibiting lengthways growth and causing leaves to be shed.
Ethylene promotes flower formation in certain types of plants such as pineapples, mangoes and lychees. Administering ethylene results in smaller plants and flowering finishes a lot quicker. The flowers ‘ripen’ too quickly and consequently remain small.
Because plants can be very sensitive to ethylene, the concentration is expressed in parts per billion parts of air (ppb). Concentrations of just 10 ppb can cause abnormalities in tomatoes. In situations where ripening flowers come into contact with young plants there is a risk of accelerated ripening in the young plants. The ethylene that is produced can reach the young plants via the air. Ventilating occasionally (once daily) will remove any ethylene that has accumulated. Higher concentrations of ethylene cause leaves to turn yellow immediately.
Ethylene can also accumulate around roots if they are wet for too long. This can lead to leaf chlorosis, stem thickening, leaves bending towards the stem and increased susceptibility to diseases.
In situations of stress, such as disease or damage to the plant, the plant produces more ethylene, which causes it to remain smaller and finish flowering faster. Mechanical stress such as air movement can also cause the plants to produce extra ethylene, which will result in smaller plants with thicker, sturdier stems. If fans are placed too close to plants, they will cause stress and this will adversely affect the yield.
Abscisin
Abscisin was first isolated in 1963 and its name is derived from the Latin word abscissio, meaning ‘breaking off’. This is because people thought that abscisin was responsible for the breaking-off (shedding) of leaves and fruits, however, it was later shown that ethylene in fact plays a much more direct role in this process.
Abscisin is produced in the chloroplasts of older leaves and in fact has both inhibiting (growth) and stimulating (protein storage) characteristics. When there is a large supply of abscisin to the growing points of the stem and roots, cell division stops and the plant enters a rest period.
Abscisin is an important hormone as far as situations of stress are concerned. It is responsible for closing the stomata when there is water stress due to prolonged high temperatures, low atmospheric humidity and an EC in the feeding medium that is too high for example.
Flower formation in short-day plants
Although a lot of research has been done into the transition from growth to flowering in plants, it is still not clear exactly how this mechanism works. In the case of short-day plants, the formation and development of flowers depends on the length of the night in particular. Short-day plants will flower when the night-time period is longer than 12 hours. However, it is important that it is really dark during this period because the plant is only sensitive to the period of darkness and not the period of light. This is measured in the leaves, which then send a signal to the ends of the branches instructing them to form flowers. The hormone that sends this signal is called florigen. So it is theoretically possible, for example, to use material from flowering plants to stimulate other plants to flower under 18 hours of light.
Different hormones play an important role in the phase following the first growth of flower buds. Cytokinin and auxin play an important role in the further formation and growth of the flowers, for example, while abscisin and ethylene are important during ripening.
Using hormone preparations
If you want to experiment with plant hormone preparations, pay close attention to how, when and how much you use. The final effect will depend on factors such as the time of administering (which phase, which time of the day), the route chosen for administering (leaves or roots) and the concentration. For example, administering auxin depends very much on the concentration used: weak concentrations stimulate root growth while strong concentrations cause extra ethylene production, which causes the plant to finish flowering faster.
