Ethylene and abscisic acid

Ethylene is a simple gaseous PGR, made in large amounts by tissues undergoing senescence and by ripening fruits.

On seedlings it causes horizontal growth, a swollen axis and, in dicots, the apical hook. It speeds senescence and abscission, especially of leaves and flowers.

Ethylene is highly effective in fruit ripening and raises the rate of respiration during ripening; this rise is called the respiratory climacteric.

It breaks seed and bud dormancy, starts germination in peanut seeds and sprouting of potato tubers. It promotes rapid internode and petiole elongation in deep-water rice, keeping the upper shoot above water. It promotes root growth and root hair formation, increasing the absorbing surface.

Ethylene is used to start flowering and synchronise fruit-set in pineapple, and induces flowering in mango. Its most widely used source is ethephon, which is absorbed from aqueous solution, moves through the plant and releases ethylene slowly.

Ethephon hastens ripening in tomato and apple, speeds abscission of flowers and fruits (thinning of cotton, cherry, walnut) and promotes female flowers in cucumber, raising the yield.

Abscisic acid (ABA) was found for its role in abscission and dormancy. It acts as a general inhibitor of plant growth and metabolism and inhibits seed germination.

ABA stimulates stomatal closure and raises tolerance to many kinds of stress, so it is called the stress hormone. It is important in seed development, maturation and dormancy; by inducing dormancy it helps seeds withstand desiccation. In most situations ABA is an antagonist of GAs.

PGR roles can be complementary or antagonistic, individual or synergistic. Events such as seed and bud dormancy, abscission, senescence and apical dominance involve more than one PGR.

PGRs are just one layer of internal control. Genes and outside factors act too, and many outside factors, such as light and temperature, work by changing PGR levels.