Cytokinins, ethylene and abscisic acid

Cytokinins act mainly on cell division (cytokinesis). Kinetin, the first found, came from autoclaved herring sperm DNA and does not occur naturally in plants. A natural cytokinin, zeatin, was found in corn kernels and coconut milk. Cytokinins are made where cells divide fast, such as root tips. They help form new leaves, chloroplasts, and side and adventitious shoots; they overcome apical dominance; and they delay leaf ageing by drawing nutrients to the leaf. In short, they keep leaves young.

Ethylene is a gas, made in large amounts by ageing tissues and ripening fruits. Its effects:

• Makes seedlings grow sideways (horizontally), swell, and form a hook at the tip (apical hook).
• Speeds ageing and the falling off (abscission) of leaves and flowers.
• Ripens fruits, with a burst of respiration called the respiratory climacteric.
• Breaks dormancy of seeds and buds.
• Makes the stems of deep water rice grow quickly, keeping the leaves above the water.
• Increases root hairs, and makes pineapple and mango flower.

Ethylene is a gas, so for farm use a liquid, ethephon, is sprayed; it slowly releases ethylene. Ethephon is used to ripen tomatoes and apples, to thin out flowers and young fruits in cotton, cherry and walnut, and to promote female flowers in cucumber (more fruit).

Abscisic acid (ABA) is a general growth inhibitor. It stops seed germination, closes stomata when water is short, and helps plants tolerate stress, so it is called the stress hormone. It keeps seeds dormant, so they can survive drying out until conditions are right. It mostly works against gibberellins.

Growth regulators rarely work alone. Two may work together (synergistic) or against each other (antagonistic). Dormancy, abscission, ageing and apical dominance all involve several PGRs. Outside factors like light and temperature also act on the plant through PGRs.