“The role of ethylene and abscisic acid is both positive and negative.”
Justify the statement.

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Hint: Ethylene that is used as a plant hormone is referred to as gas. The chemical formula of the compound is $C_2H_4$. It is a flammable gas that is colourless at room temperature. Abscisic acid is a plant hormone having the chemical formula of $C_{15}H_{20}O_{4}$. It is also referred to as the stress hormone for the plants.

Complete answer: Ethylene:
Positive- Ethylene is associated with ripening fruit, wilting flowers, and dropping leaves. Ethylene is rare because the product ($C_2H_4$) is volatile. When gas street lamps were built in city streets hundreds of years ago, trees that grew near to the lamp posts formed bent, thickened trunks that shed their leaves earlier than expected. Such symptoms were caused by the volatilization of ethylene from the lamps.
Negative-This stimulates maturation, allows leaves to slip (epinasty) and decrease (abscission), and fosters senescence. Plants also increase the production of ethylene in response to stress, and at the end of a plant's life ethylene is often present in high concentrations within cells.

Abscisic Acid:
Positive- The plant hormone abscisic acid (ABA) was first discovered as the agent which causes the cotton bolls to be abscissed or lowered. More modern research, however, suggests that ABA plays only a minor role in the cycle of abscission. ABA accumulates as a reaction to adverse weather factors, such as fatigue, freezing temperatures, or reduced daytime periods.
Negative- Abscisic acid ( ABA) is a general inhibitor of plant growth. It stimulates dormancy and inhibits germination of seeds; causes leaves, fruits, and flowers to be abscessed; and causes the stomata to close. Large doses of ABA in guard cells during times of drought stress undoubtedly play a role in the closing of the stomata.

Note: Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with the application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.