Plant Growth and Development: Hormones and Flowering

The three phases of plant growth

Plant growth proceeds through three sequential phases, each dominated by a different cellular process. The meristematic phase, at root and shoot tips, is dominated by active cell division. The elongation phase, just behind the meristematic zone, is where newly divided cells increase dramatically in size, chiefly by taking up water into a large central vacuole, driving the visible lengthening of the root or shoot. The maturation phase, furthest from the tip, is where cells stop elongating and instead differentiate into their final, specialised structure and function.

The five classical plant hormones

Five hormones, each discovered independently and each named for a distinct function, together regulate nearly every aspect of a plant's growth and development.

Auxin was the first to be identified, its existence traced to observations of phototropism (a plant's tendency to bend toward light) going back to Charles Darwin's own experiments on grass coleoptiles: covering or removing a coleoptile's tip prevented the characteristic bending altogether, showing that some influence generated at the tip was responsible for the bending lower down. The Dutch botanist Frits Went, in the 1920s, isolated the actual chemical signal responsible, demonstrating that this substance (auxin) moves through plant tissue in one fixed direction, away from the tip. Auxin's core function is promoting cell elongation in the shoot, and it also maintains apical dominance, the suppression of lateral (side) bud growth by the actively growing main shoot tip, which is exactly why pinching off a plant's main growing tip causes its side branches to grow out instead.

Gibberellin was discovered through a genuinely unusual route: Japanese rice farmers, observing paddy seedlings that grew abnormally pale, unusually tall, spindly and sterile, named the condition bakanae ("foolish seedling") disease. In 1926, the scientist Kurosawa traced the cause to a chemical secreted by the fungal pathogen Gibberella fujikuroi infecting the rice plants, a substance later identified and named gibberellin after the fungus itself. Gibberellins promote stem elongation (bolting), stimulate seed germination, and can induce flowering in certain plants.

Cytokinin, whose first naturally occurring form (zeatin) was isolated from corn in 1964, is also found richly in coconut milk, still the standard natural source used in plant tissue culture media today. Cytokinin's core function is promoting cell division, and it also delays leaf senescence (ageing and yellowing), the opposite effect to abscisic acid below.

Ethylene, the only plant hormone that is a simple gas at normal temperature, is best known for controlling fruit ripening, and it is also involved in triggering senescence and the abscission (dropping) of leaves and fruit.

Abscisic acid (ABA), often called the plant's own "stress hormone," induces seed and bud dormancy and triggers stomatal closure under water stress, generally acting to slow growth and conserve resources under unfavourable conditions, the functional opposite of the other four, growth-promoting hormones.

Photoperiodism and vernalisation: two different environmental triggers for flowering

Photoperiodism is a plant's flowering response to the relative length of light and dark periods in a 24-hour cycle, and plants are accordingly classified as short-day plants (flowering when the night period exceeds some critical length), long-day plants (flowering when the night period is shorter than some critical length), or day-neutral plants (flowering independent of day length altogether). Vernalisation is a genuinely different phenomenon, a flowering response to temperature rather than light: certain plants require a prolonged period of cold exposure (mimicking passage through winter) before they become capable of flowering the following spring, which is exactly why some crops must be deliberately cold-treated if they are to be grown and made to flower outside their normal seasonal cycle.

Quick revision points

• Three growth phases: meristematic (cell division, at tips), elongation (cell enlargement via water uptake), maturation (cell differentiation into final specialised form).
• Auxin: discovered via coleoptile phototropism experiments (Darwin's observations, Went's isolation of the chemical signal); promotes cell elongation and maintains apical dominance (suppresses lateral bud growth).
• Gibberellin: discovered via rice "bakanae" (foolish seedling) disease, traced to the fungus Gibberella fujikuroi (Kurosawa, 1926); promotes stem elongation/bolting, seed germination, and flowering in some plants.
• Cytokinin: first isolated as zeatin from corn (1964); rich in coconut milk (still used in tissue culture media); promotes cell division, delays leaf senescence.
• Ethylene: the only gaseous plant hormone; controls fruit ripening, triggers senescence and abscission.
• Abscisic acid (ABA): the "stress hormone"; induces dormancy and stomatal closure under water stress, opposite in effect to the growth-promoting hormones.
• Photoperiodism (response to day/night length): short-day, long-day, day-neutral plants. Vernalisation (response to cold temperature exposure): required by certain plants before they can flower, a genuinely different trigger from photoperiodism.