Substance produced by a plant that has a marked effect on its growth, flowering, leaf fall, fruit ripening, or some other process. Examples include auxin, gibberellin, ethene, and cytokinin.
Unlike animal hormones, these substances are not produced by a particular area of the plant body, and they may be less specific in their effects. It has therefore been suggested that they should not be described as hormones at all.
Plants lack a nervous system and so, for the most part, are unable to respond quickly to stimuli in the way animals can. The two important stimuli for plants are light and gravity, and plants respond by growing either towards or away from the stimulus (this is termed tropism). For example, the shoot of a plant grows toward light (phototropism), while root growth is governed by the pull of gravity (geotropism). Tropism plays a very important part in plant development and survival. Shoots need to grow upwards toward the light so that the leaves can photosynthesize, and flowers can be pollinated; whereas roots need to grow downwards into the soil in order to anchor the plant and absorb water and minerals. The response to stimuli is controlled by plant hormones. Plant hormones are organic chemicals, and are usually referred to as ‘growth substances’. These are synthesized by plants, and the five major types are abscisic acid, auxin, cytokinin, ethylene (ethene), and gibberellin. These substances regulate growth and development, and are usually produced in a particular part of the plant, such as the shoot tip, and transported to other parts where they take effect.
The responses of plant roots and shoots to light, gravity, and moisture are the result of unequal distribution of hormones, such as auxin, causing unequal growth rates. For example, plant shoots bend towards the light. This is due to auxin stimulating more growth on the shaded side of the shoot.
The hormones that control the processes of growth and reproduction in plants can be used by humans to produce large numbers of plants quickly by stimulating the growth of roots from cuttings, regulate the ripening of fruits on the plant and during transport to consumers, and kill weeds by disrupting their normal growth patterns.
Tropisms and plant hormones
Making Use of Plant Hormones
Commercial uses of plant hormones