Article
3-3 Hormones
by Erik Biksa
Growers manipulate or take advantage of the enviroment to produce responses
in plants. These include rooting, vegetative growth, branching, flowering,
setting seed or fruit, and dormancy. Hormones in plants influence these
reactions. They can be produced within the plant as a result of environment. In
some instances, they are supplied to the plant by the grower. There is some
debate as to the roles of hormones in plants, and some debate as to whether or
not additional hormones have been documented.
There are five generally accepted types of hormones that influence plant
growth and development. They are: auxin, cytokinin, gibberellins, abscic acid,
and ethylene. It is not one hormone that directly influences by sheer quantity.
The balance and ratios of hormones present is what helps to influence plant
reactions. The hormonal balance possibly regulates enzymatic reactions in the
plant by amplifying them, leading to the results the grower wants to see.
Following is a brief description of the role of hormones in plant responses:
- Auxins:
Auxins are produced in significant quantities in the upper growth regions of
plants, promoting cell elongation. It travels from the shoot tip to the base
when the plant is actively growing. It plays a role in the formation of
plant roots.
IAA is an auxin in it's natural state. Synthetic rooting hormones contain
compounds such as IBA typically ranging in concentrations from 1000-10,000
ppm. When cuttings are taken, and dipped for rooting, here's part of what
happens:
The plant stops growing stem tissue. The cells that have been developing
but have not yet been dedicated to any specific type of growth (i.e. stem
development) are stimulated by the auxin such as IBA to become roots. These
cells are now set to grow roots, and will further multiply and develop to
produce a healthy root system, which will develop hormones that influence
the upper development of the plant. Synthetic auxins sprayed on tomato vines
will allow fruit to develop without pollination. Auxin that is usually
produced by the seeds has been replaced, so no seeds will develop.
An overdose of auxin will actually inhibit cell elongation, because the
plant will produce another hormone to try and balance the act. When applied
to seeds, auxins also help to promote femaleness in dioecious plants (plants
having females and males). The concentration of auxin is usually highest at
the main growing point of the plant, surpressing lateral/side shoot growth.
Growers have often pinched the tips of the plants in order to promote
extensive branching and to keep plants short and sturdy. Bending and tying
the growing point downwards will also have a similar effect without damaging
the concentration of auxins within the plant.
- Cytokinins:
Cytokinins are produced in the plant's roots and move upwards through the
plant to the growing tips. As the roots system grows larger, it produces
more cytokinins, which in turn, signals the plant to grow and branch more.
As the plant continues to grow and branch, it produces more auxin in the
growing points. Remember auxin influences root development, so the plant
grows more roots, producing more cytokinin, etc. Less cytokinin with more
auxin signals root growth. More cytokinin, less auxin triggers more shoot
growth.
Kelp meal is a natural source of cytokinins. Kelp based products contain
kelp extracts in easy to apply liquids. Additions of these products at the
roots during the vegetative stage will result in rapid growth and branching.
It is best applied to the root zone, but a quick foiliar spray can be used
to perk up growth. A fine spray in early flowering will help to build the
framework for a bountiful harvest, as it plays a role in longevity of
plants. Flowering might be slightly extended, adding to the quality and
results. During the vegetative stage an abundance of cytokinins will help to
remove dominance (due to higher concentrations of auxin) from the main
growing tip, leading to bushier plants.
Applications of cytokinins can reduce aging in plants and can improve the
shelf life of vegetables and cut flowers.
- Giberellins:
Giberellins are produced by the roots and growing leaves. It promotes cell
elongation and cell division. In seeds, it is activated by water and helps
to break seed dormancy leading to germination. Some plants such as lettuce,
in high temperature will "bolt" growing an upright seed stock,
this can be attributed to an abundance of gibberellins.
In dioecious plant species (those types of plants having females and
males) the application of giberellic acid ( typically foiliar sprays) can
cause female plants to develop male flowers, thus, pollinating themselves.
These plants generally turn out to be all female. So if a grower has two
different or same types of a favorite specimen, they can apply giberellic
acid as a spray once or twice during flowering. The plants should only be
harvested for their pollen for transfer to an unsprayed and desirable
female. The offspring might be more susceptible to hermaphordism (both male
and female flowers on the same plant) than their parent under environmental
stress such as photoperiod interruptions or being root bound.
Giberellins also act against abscic acid in breaking the dormancy in the
growing points of plants.
- Abscic Acid:
Abscic acid is produced in the buds/shoot tips of the plants. It plays a key
role in slowing plant growth and promotes dormancy (overwintering) in
plants. This function also helps to slow cell elongation, possibly promoting
tighter internodal spacing (distance between growing points) for tight
flower spikes. ABA (abscic acid) plays an important role in plant survival.
It can prevent plant wilt, as it triggers stomata (tiny holes in leaves
plants used to take in CO2 and release O2 for photosynthesis) to close
during drought conditions, so the plant requires less water because it has
shut down its growth process.
This creates the assumption that abscic acid is a "stress"
hormone. Furthermore, abscic acid plays a role in leaf abcision, the process
where the lower leaves of the plant begin to drop near maturity or from
severe stress.
- Ethylene:
Ethylene is present as a gas produced by the plant. As levels of ethylene
increase with plant development while other hormones are balancing in
accordance, the effects of ethylene become more pronounced. Ethylene
promotes fruit ripening, or "maturing". This signals the plant
that it's life cycle is changing or ending. Tomatoes turn red and small
flowers begin to wither. It will signal the plant to transfer it's nutrients
back into the stem tissue from the leaves and other storage tissue.
Abcission (the process where leaves drop) is attributed to decreasing
levels of auxin produced by the leaves, allowing further expression of
abscic acid and ethylene. This response is brought on by shortening days and
cooler nights.
Tomatoes can be grown with an ethylene inhibitor, shipped green for shelf
life and resistance to handling damage, and then exposed to a sealed
environment with controlled levels of ethylene to ripen the fruit at their
point of destination. Burning fossil fuels, such as CO2 generators produce
small amounts of ethylene. Inefficient and un-inspected burners may produce
ethylene levels high enough to seriously harm plants. Ethylene is found in
abundance in the skin of ripening fruit such as apples or green bananas. By
placing unripened plant material in a paper bag with a green banana you can
hasten ripening, bringing out flavour and aroma. If done at warmer
temperatures fresher material will convert stored starches to sugars.
Contrary to CO2 burners, bottled CO2 will help to decrease ethylene
levels with good air circulation. By adding CO2 to sealed containers for
storage, you can help to increase the shelf life and qualities you desire,
as it will counteract the ethylene driving the aging process after harvest.
The above helps to shed some light on the role of hormones in plants. Growers
should be cautious when applying hormones to plants. Excessive amounts could be
detrimental to the plants development, causing serious set backs in plant
functions. Synthetic hormones should also be handled with caution. Some are
known to me mutanagenic and carcinogenic in mammals. However, used safely, at
the right time, and in the right amount, hormones provide the grower with an
additional means of plant manipulation. Sterilized coconut milk is used in
tissue culture for stimulating shoot development as it contains cytokinins. Most
agar base recipes call for 100ml per liter of medium. Malt can be used to
provide auxin for root growth. A great deal of information regarding hormones
can be found in publications dealing with tissue culture. The key to remember is
that no one hormone controls plant processes, it is the balance of all hormones
influenced by environmental factors which the grower must control in order to
achieve the results they desire.
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