Plant Growth Regulators

Ethylene is a simple gaseous plant growth regulator with a wide range of effects on plant physiology. Physiological Effects:

Fruit Ripening: It is the most widely known effect. Ethylene initiates and promotes the ripening process in climacteric fruits (e.g., bananas, apples, tomatoes), leading to changes in color, texture, and aroma.
Senescence and Abscission: It accelerates the aging (senescence) of leaves and flowers and promotes the formation of an abscission layer, leading to the shedding of leaves, flowers, and fruits.
Breaking Dormancy: It can break seed and bud dormancy in certain plants.
Flowering: While it generally inhibits flowering, it induces it in some plants like pineapples and mangoes.
Triple Response: In seedlings growing in the dark, ethylene causes a 'triple response' to mechanical stress: inhibition of stem elongation, promotion of radial swelling of the stem, and horizontal growth (diageotropism).
Applications:

Artificial Ripening: Ethylene is used commercially to ripen fruits that are harvested when green. A compound called Ethephon is often used, which releases ethylene when sprayed on plants. This allows for controlled ripening of fruits like tomatoes and bananas.
Inducing Flowering: It is used to synchronize flowering and fruit set in pineapple plantations.
Promoting Sprouting: Used to promote the sprouting of potato tubers.
Thinning Agent: It can be used as a thinning agent to reduce the number of fruits in crops like cotton, cherry, and walnut, leading to larger remaining fruits.

Abscisic acid (ABA) is a plant hormone that functions in many plant developmental processes, including bud dormancy. It is also known as the "stress hormone" because it is involved in the response to environmental stresses like drought. A primary function of ABA is to inhibit growth, acting as an antagonist to growth-promoting hormones like auxins and gibberellins. It promotes stomatal closure, inhibits shoot growth, and induces seed dormancy.