370 Produce Degradation: Reaction Pathways and their Prevention
agents include the mentioned synthetic auxins NAA and NAD, which induce for-
mation of an abscission zone in peduncle [5], and carbamate insecticide carbaryl,
which also possesses growth regulation properties [82], but its use may create
potential mite problems by killing natural mite predators.
Gibberellins stimulate cell division and elongation. Gibberellic acid (GA) was
first isolated in 1926 from the fungus Gibberella fujikuroi, and since then more than
70 gibberellins have been found in a wide variety of plant species [5,80]. GA is
produced in the seed embryo, where it initiates growth, possibly by stimulating RNA
and protein synthesis and increasing the transport of carbohydrates from the
endosperm to the embryo [1]. The most significant effect of gibberellin treatment
consists of stem growth stimulation. GA has a wide variety of applications, such as
to improve fruit setting in pears, blueberries, and some other fruits; to control fruit
maturity by delaying the yellow color in lemons; to loosen and elongate clusters
and increase berry size in grapes; to break dormancy and stimulate sprouting in seed
potatoes; to produce brighter-colored, firmer, and larger cherries; and to advance
flowering and increase the yield of strawberries, among other purposes [1,4,5].
Cytokinins are derivatives of the purine adenine, and they are essential for the
cell division process [80,83]. The first identified cytokinin was zeatin, isolated from
maize grain in 1964. Examples of synthetic cytokinins include kinetin and 6-ben-
zylaminopurine (6-benzaldenine). Their application range is also relatively wide.
For instance, cytokinins delay senescence and thus can be used to prolong shelf life
of fresh vegetables; they also stimulate fruit set, flower bud formation, and regular
bearing in fruit trees or promote seed germination [1,5].
Ethylene is the simplest plant hormone and affects various plant growth pro-
cesses [84]. It has been demonstrated that IAA promotes ethylene production in
plants, which may account for some of the effects caused by auxins [1]. Ethylene
releasers, such as ethephon, can be sprayed onto plants; they penetrate into the plant
tissues and decompose to ethylene. Depending on the time of application, ethylene
(from ethephon) may control different phases of plant development. Ethylene is used
to promote preharvest ripening in apples, tomatoes, citrus fruits, etc.; to accelerate
postharvest ripening in bananas, mango, and citrus fruits; to induce flowering and
regulate ripening in pineapples; to increase fruit setting and yield in cucumbers; or
to act as a fruit thinning agent [1,5].
Abscisic acid (ABA) is the best-known natural plant growth inhibitor that
induces bud dormancy and probably also leaf fall in deciduous trees [1,85]. Examples
of important synthetic growth inhibitors or retardants include compounds that act
as sprouting inhibitors, such as maleic hydrazide, defoliants (dimethipin, endothal,
tribufos, or thidiazuron), or agents that (among other functions) prevent premature
fruit drop, such as chlormequat or trinexapac-ethyl. Chlormequat and trinexapac-
ethyl inhibit gibberellin biosynthesis, which results in inhibition of cell elongation,
mainly shortens and strengthens the stem and produces a sturdier plant [86]. Thus, these
PGRs are also used to prevent lodging in cereals (wheat, rye, oats, and triticale) [5].
Sprouting inhibitors control postharvest sprouting in root (carrots, beets), tuber
(potatoes), and bulb (onions) crops [68]. The most important sprout suppressants
include preharvest-applied maleic hydrazide or postharvest-applied chlorpropham,
propham, or tecnazene. Maleic hydrazide inhibits cell division in meristematic