With all of the floral induction mechanisms examined so far (eg: phase changes, circadian rhythms, photoperiodism and vernalisation), the influencing mechanism has been discussed. But for it to work, there must be communication between the site of the stimulus (eg leaves) and the site of activity (the apical meristem).

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:

Hormone Physiological Role
Gibberellins Cell elongation, floral initiation
Auxins Cell elongation, lateral root development.
Cytokinins Cellular division and bud and root initiation (in the presence of auxin).
Ethylene Fruit ripening
Abscisins Anti-transpirant
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.
Giberellic Acid molecular structure
The chemical structure of Giberellic Acid 3 (GA3), a phytohormone
Commercial growers often use gibberellins to force plants to flower for a particular season, by spraying the foliage. Different gibberellins can have distinctly different effects, and no two plant species will react in the same way to any gibberellin treatment. The concentration of gibberellin is very important when it is applied exogenously because a solution which is too  concentrated may lead to other undesirable outcomes.

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)
By far, IAA has the biggest physiological impact on plant growth and development. Plant scientists have known about the influence of auxins on flowering since the 1930's (Leopold & Thimanm 1949).
Chemical structure of indole-3-acetic acid
The chemical structure of indole-3-acetic acid, an auxin.
Water stress (aridity) can sometimes limit flowering, depending on the species. When a plant is water-stressed, the roots synthesise a lot of abscisins which are transferred to the apical meristem. Abscisins can inhibit floral initiation. However this system does not exist in all species. Mangoes and Geophylla remnaris are examples of species that will flower if exposed to short periods of water-stress.

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.