Hormones (often called phytohormones) are the means by which plant cells communicate. Hormones are described as "any substance that has a marked and specific effect on plant growth and that produces this effect when present in very small concentrations" (Bailey 1999).
Hormones are naturally-produced in plants and have a number of roles. In summary (and as a generalisation), a list of hormone types and their physiological roles is shown below:
|Gibberellins||Cell elongation, floral initiation|
|Auxins||Cell elongation, lateral root development.|
|Cytokinins||Cellular division and bud and root initiation (in the presence of auxin).|
|Brassinosteroids||Inhibition of root growth|
|Jasmonates||Inhibition of root growth, promote root initiation|
|Polyamines||Root emergence in some species. Possible floral initiation in some species.|
|Salicyclic acid||Pathogen resistance (in some species)|
One of the most important groups of hormones for floral initiation are the gibberellins.
There are more than 100 different gibberellins, however only a few are biologically active (meaning that they have an effect on the plant). The rest are intermediates between active forms of gibberellic acid.
The application of exogenous gibberellins (synthetic gibberellins sprayed on the foliage of plants) can trigger flowering in a number of species which would otherwise require some other stimulus such as vernalisation or a certain photoperiod. Exogenous gibberellins can also substitute for age in autonomous flowering species.
The chemical structure of Giberellic Acid 3 (GA3), a phytohormone
An example of such commercial use is for producing chrysanthemums on Mothers' Day. Both chilling (vernalisation) and an application of giberellic acid are required to promote flowering in this short-day (SD) plant.
Leaves are the receptors of many of the floral stimuli which influence floral initiation (such as photoperiod). These stimuli trigger the production of gibberellins in the leaf which are transferred via the phloem (vascular tissue) to the apical meristem (shoot tip). When the hormones reach the apical meristem, they trigger physiological changes that allow floral initiation. Sometimes other hormones such as cytokinins and polyamines are also required for floral initiation, but these are ineffective if used endogenously in the absence of gibberellins.
In some cases, the apical meristem will also receive floral initiation signals from the roots, which is where giberellins are synthesised.
Giberellins aren't the only hormones which have a role in floral initiation. Another key group of hormones are the auxins.
Auxin is an important regulator of many aspects of plant growth and development. During reproductive development, auxins specify the site of flower initiation and subsequently regulate organ growth and development.
Plants naturally synthesise the following four auxins:
- indole-3-acetic acid (IAA)
- 4-chloroindole-3-acetic acid (4-Cl-IAA)
- indole-3-butyric acid (IBA)
- phenylacetic acid (PAA)
The chemical structure of indole-3-acetic acid, an auxin.
In summary: Hormones are the method by which plants communicate internally. The floral stimulus is received in the leaves and this leads to the biosynthesis of hormones (usually gibberellins) in various parts of the plant. These travel through the phloem to the apical meristem and cause the cells to initiate floral production by altering patterns of gene expression and cell morphology.