8/13/2019 Plants That Control or Regulate Germination
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Any of various hormones produced by plants that control or
regulate germination, growth,
metabolism, or other physiological activities. Also
calledphytohormone.
Plant hormones(also known as phytohormones) arechemicalsthat
regulateplantgrowth, which, inthe UK, are termed 'plant growth
substances'
.
Planthormonesare signalmoleculesproduced within theplant,and
occur in extremely
lowconcentrations.Hormones regulate cellular processes in
targetedcellslocally and, when moved to
other locations, in other locations of the plant.Hormonesalso
determine the formation
offlowers,stems,leaves,thesheddingofleaves,and the development
and ripening offruit.Plants,
unlikeanimals,lackglandsthat produce andsecretehormones.
Instead, each cell is capable of
producing hormones
. Plant hormones shape theplant, affecting seed growth, timeof
flowering, the sex offlowers, senescenceof leaves, andfruits.They
affect which tissues grow upward and which grow downward, leaf
formation andstem growth, fruit development and ripening,
plantlongevity,and even plantdeath.Hormones
arevitalto plant growth, and, lacking them, plants would be
mostly a mass of undifferentiated cells.
So they are also called as growth factors or growth hormones
Organic compounds other thannutrientsthat regulate plant
development and growth.Plant hormones, which are active in very low
concentrations, are produced in certainparts of the plants and are
usually transported to other parts where
theyelicitspecificbiochemical, physiological,
ormorphologicalresponses. They are also active in tissues
where they are produced. Each plant hormone evokes many
different responses. Also,the effects of differenthormonesoverlap
and may be stimulatory orinhibitory.Thecommonly recognized classes
of plant hormones are the auxins, gibberellins,cytokinins,abscisic
acid, andethylene.Circumstantial evidence suggests that flower
initiation is
controlled by hypothetical hormones called florigens, but these
substances remain to be
identified. A number of natural or synthetic substances such
asbrassin,morphactin, andother growth regulators not considered to
be hormones nevertheless influence plantgrowth and development.
Each hormone performs its specific functions; however, nearlyall of
the measurable responses of plants toheredityor environment are
controlled byinteraction between two or more hormones. Such
interactions may occur at variouslevels, including the synthesis of
hormones, hormone receptors, and second messengers,
as well as at the level of ultimate hormone action. Furthermore,
hormonal interactionsmay be cooperative, antagonistic, or in
balance.The term plant growth regulator is usually used to denote a
synthetic plant hormone, but
most of the synthetic compounds with structures similar to those
of the naturalhormones have also been called hormones. For
instance, the syntheticcytokininkinetinis considered a hormone. See
alsoAbscisic
acid;Abscission;Auxin;Cytokinins;Ethylene;Gibberellin.There are
a number of applications of plant hormones in
agriculture,horticulture,andbiotechnology.Synthetic auxins are used
asweedkillers. Auxins are also usedtocounteractthe effects of
hormones that promote the dropping of fruit from trees.Gibberellins
are used extensively to increase the size of seedless grapes: when
applied
at the appropriate time and with the proper concentration,
gibberellins cause fruits
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