*2.4.2 Functions of ethylene*

In plants, ethylene is used for a variety of purposes. Seed germination, shoot and root growth, root development, abscission of leaves and fruits, the creation of adventitious roots, senescence of leaves and flowers, and sex determination are a few of the crucial tasks that ethylene performs. For instance, in plant tissue, ethylene stimulates the development of air-filled cavities known as aerenchym tissues during floods, which aids in the oxygenation of plants. However, the ripening of climacteric fruits, such as peaches, bananas, apples, and tomatoes, is ethylene's most significant role. For instance, putting a ripe banana in a bag of immature avocados would speed up the avocados' ripening process. The build-up of ethylene in the bag is to blame for this.

In summary Ethylene's important uses are the following:


grows more than the bottom ones. This induces the leaves to drop and rather than being horizontal the leaves become more vertical. This is specially induced by ethylene when it is converted to ACC and transported from the xylem to the tissues of leaves on the upper part.


#### **2.5 Abscisic acid**

#### *2.5.1 Introduction*

Abscisic acid is the plant stress hormone (ABA). It inhibits plant development and regulates abscission and dormancy. The naturally occurring Abscisic acid is dextrorotatory (+), but commercially available synthesised ABA is a racemic combination. ABA is transported by the xylem, phloem, and parenchyma cells [24].

Discovery of ABA took place between 1950 and 1960, scientists had a hunch that when a growth stimulating endogenous hormones are present in the plant cell, growth inhibiting hormones which causes the senescence or abscission of fruits must be governed by other hormones namely abscisic acid(ABA). ABA does not cause abscission, they just inhibit growth [25]. Violaxanthin and neoxanthin are Xanthophylls that are used to synthesise ABA. Epoxidation, or the presence of epoxycarotenoids, is required for ABA production. The synthesis however initiates from IPP forming GGPP further leads to the formation of Zeaxanthin produces violaxanthin. Violaxanthin forms cis—neoxanthin followed by cis—xanthin produces ABA Aldehyde leads to ABA [26]. Synthesis occurs in mature leaves and stems, as well as developing fruits, seeds.

Abscisic acid is referred to as a stress hormone since its production is promoted by environmental challenges such as drought and water logging. It is crucial in the tolerance of abiotic stress. ABA is important in a variety of developmental and physiological processes, including:

• ABA causes stomata to close when excessive salinity, water stress, and lowers water loss through transpiration. To stimulate stomatal closure, ABA interacts with other phytohormones such as jasmonates, nitric oxide, and signalling molecules.

