Understanding plant hormones

Steve and Val explain the role of plant hormones in growth and dormancy

- Steve, Val

YOU might not associate hormones with plants - with people, yes, but not plants. They have them, though, and these hormones govern many of the processes that keep plants growing, or keep them dormant.

Plant hormones are involved in cell production and repair, causing callus formation over wounds and root initiation where it is needed. They also promote and balance growth between roots and shoots, which is why root pruning a plant will cause a slowdown in top growth until the balance is restored.

Chemical messengers

There is a group of five main phytohormones comprising gibberellin, ethylene, cytokinins, auxin and abscisic acid. Auxins are a family of plant hormones found mostly in the tips of the growing stems and roots (the 'apical meristems') and they control growth by boosting cell division and causing existing cells to lengthen, making roots and shoots longer. They are also involved with growth towards a light source (phototropism), downward root growth (geotropism), flower formation, fruit set/growth and apical dominance (when the top bud on a stem suppresses the buds below it).

Gibberellins improve fruit set and size, help break seed dormancy and can speed germination. Growers can use them as a natural way to trick plants into growing fruit without seeds (false fruit) that are much bigger than average.

Cytokinins are found in both plants and animals, unlike other plant hormones. They stimulate cell division and can be used to delay ageing.

Ethylene (ethene) is unique in that it is only found in gaseous form. It induces ripening, causes leaves to droop (epinasty) and drop (abscission).