Plant Hormones: Types, Functions, and Effects on Growth

What are Plant Hormones?

Plants require light, carbon dioxide, water, and minerals, including nitrogen, for growth. Utilizing energy from the sun, they produce much of their own substance, transforming simple materials into complex organic compounds. Hormones play a crucial role in regulating plant growth and development by stimulating or inhibiting specific processes. Among the most important internal factors are plant hormones. By definition, hormones are substances produced in one tissue and transported to another, where they exert highly specific effects.

Types of Plant Hormones

Among the many plant hormones identified, the best known are auxins, cytokinins, gibberellins, abscisic acid, and ethylene.

Auxins

Auxins are a group of plant hormones that regulate plant growth, primarily by causing cell elongation. They are synthesized in the meristematic regions at the tips of stems and move from there to other parts of the plant, mainly towards the base, creating a concentration gradient. This movement occurs through the parenchyma surrounding the vascular bundles.

Effect of Auxins

Auxins stimulate plant growth by promoting cell elongation. Consequently, they are produced in greater quantities in areas of active plant growth, such as stem tips, buds, and cambium.

Ethylene

Ethylene is a natural substance associated with fruit ripening. While it was initially believed that heat caused fruit maturation, ethylene gas was later identified as the primary factor. Ethylene influences fruit ripening, leaf fall, and flower wilting. Additionally, it inhibits root and stem growth and delays bud development.

Gibberellins

Like auxins, gibberellins are involved in plant growth, stimulating both cell division and cell elongation. They promote stem elongation by increasing the length of internodes without affecting the number of nodes. Gibberellins also induce flowering in some plants and influence the formation of enzymes that hydrolyze starch during seed germination.

Cytokinins

Cytokinins significantly accelerate the growth of plant embryos and increase cell division in test tubes. Due to this latter effect, this group of growth regulators is called cytokinins, referencing cytokinesis, the term for cell division. The first discovered cytokinin, kinetin, was isolated from DNA preparations, yeast, and animal tissue.

Florigen

In many plant species, flowering can be induced by exposing the plant's leaves to a suitable period of light, a process known as photoinduction. This exposure appears to transmit a signal from the leaf to the bud, causing the apical meristem to form floral buds instead of leaf primordia. This signal is called florigen, although its existence has not yet been definitively proven.

Abscisic Acid

Abscisic acid is a hormone first isolated from dormant ash and potato buds. When seed dormancy is broken, the concentration of abscisic acid decreases.