Q.Give a detailed account of cytokinins, ethylene and abscisic acid as plant growth regulators -- covering, for each, how it was discovered or first characterised, its principal natural source within the plant, and at least three of its major physiological effects -- and explain how ethylene and abscisic acid, in particular, act largely as antagonists to the growth-promoting hormones.

[!TLDR]

Cytokinin (Skoog, kinetin/zeatin): cell division, delays senescence. Ethylene (gaseous): ripening, abscission, dormancy-breaking. ABA (dormin+abscisin): stomatal closure, dormancy -- ethylene and ABA largely oppose auxin/GA/cytokinin's growth-promoting effects. …

Cytokinin was discovered by F. Skoog's tissue-culture research group while searching for a chemical factor capable of reliably inducing cultured plant cells to divide; they found that autoclaved herring sperm DNA promoted this cell division and isolated the active adenine-derivative compound responsible, naming it kinetin, while a genuinely natural cytokinin, zeatin, was later isolated from corn kernels and is also abundant in coconut milk. Cytokinins are synthesised chiefly at root apices, developing shoot buds, and young, developing fruits and seeds. Their central effect, consistent with their discovery, is to promote cell division; beyond this, they actively mobilise nutrients toward the tissue they are present in and, in doing so, delay senescence -- directly demonstrated by the Richmond-Lang effect, in which a cytokinin-treated detached leaf stays green far longer than an untreated one -- and they help overcome apical dominance, promoting the growth of lateral shoots that auxin would otherwise keep suppressed.

Ethylene is set apart from the other plant growth regulators as the only one occurring naturally as a simple gas, and its ripening-related effects were observed (in the form of unusually fast ripening of fruit stored near leaking gas pipes or alongside other ripening fruit) long before the responsible gas itself was correctly identified and characterised. Ethylene is produced particularly abundantly in ripening fruit and senescing tissue. Its principal effects include triggering and accelerating fruit ripening, driving the sharp respiratory climacteric characteristic of fruits like banana and mango; breaking seed and bud dormancy, for instance initiating germination in peanut seeds and sprouting in stored potato tubers; accelerating senescence and promoting the abscission of leaves, flowers and fruits; and promoting predominantly female flower formation in cucumber and related cucurbits, a horticulturally useful effect exploited to increase fruit yield, along with its use in synchronising fruit-set in pineapple plantations. Because pure ethylene gas is impractical to apply directly, growers commonly use ethephon, a liquid that breaks down within plant tissue to release ethylene gas gradually.

Abscisic acid was identified through two initially separate research questions -- one on bud dormancy (yielding a compound named 'dormin') and one on fruit and leaf abscission (yielding a compound named 'abscisin') -- that converged once the two compounds were compared directly and found to be chemically identical, thereafter known as abscisic acid. ABA is synthesised chiefly in mature leaves, stems, root caps and green fruit, and is often called the plant's stress hormone because its level rises sharply under conditions such as water shortage; its principal effects include triggering stomatal closure by acting directly on guard cells, reducing water loss under water stress; inhibiting seed germination; and inducing and maintaining both seed and bud dormancy. …

Cover cytokinin, ethylene and ABA in three clearly separated blocks (discovery, source, at least three effects each), then close with an explicit paragraph reasoning through how ethylene and ABA's effec …

Omitting the closing antagonism discussion the question explicitly asks for, or describing ethylene as purely inhibitory when several of its effects (ripening, deep-water i …