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PLANT HORMONES

It is said that plant growth and development are regulated by hormones.

What is the difference between growth and development?

Growth is an increase in size of basic parts of plants (due to production of extra cells), whereas development is the production of new structures in the progress towards maturity - flowers, fruits etc.

In what ways are plant hormones similar to animal hormones?
Both : are "chemicals"
produced in one region
move - by diffusion not not blood flow!
to another ("target") region
have major physiological effects.

Which parts of a plant are involved in "growing"?

meristems - cells dividing at tips of roots and shoots.

Regulation of Plant Growth

In that green plants make growth in response to stimuli from their surroundings, they show sensitivity, i.e. one of the seven characteristic processes of living organisms.
However, green plants respond to fewer stimuli than animals, and they make a slower and more restricted response.
Name 3 stimuli to which green plants respond.
   > light       
   > gravity      
   > water/touch (peasticks etc)
    NOT CO2

If a plant responds by growing in a particular direction, it is called a > tropism.

Positive tropisms are towards the stimulus, negative tropisms are away from the stimulus.

A plant's response to light is called > phototropism.

Why do green plants need light?
   > as energy for photosynthesis
Specifically, which parts of the green plant need light?
   > leaves
If the shoot - the tip of a green plant's stem - is growing toward light, what happens to these structures?
   > leaves spread out sideways
What will be the advantage to the plant?
   > maximise exposure gaining more light for photosynthesis

In what way do the following parts of plants respond to gravity, and what is the advantage (adaptive significance) to the plant?
   roots   > grow downwards    > into ground, towards water?

   shoots   > grow upwards   > towards light

The way that plants respond to the stimuli of light and gravity have been explained in terms of a chemical called auxin. Auxin functions like a plant hormone, but it may also be known as a plant growth substance.

Auxin is said to have different effects on stems and roots.

In shoots, what does auxin do?
   > causes it to bend up

In shoots, where is auxin made?
   > at the tip

Regulation of development



Stimulation of roots in cuttings


When cuttings are taken, the shoot which is selected is removed from the plant and disconnected from a supply of water and minerals from the roots.
It is therefore important to encourage the development of roots, to aid the establishment of the "cutting" as an independent plant.

"Hormone rooting powders" contain the substance indoleacetic acid IAA which has been identified as the same chemical as "auxin" which causes curvature in roots and shoots. Usually the newly exposed surface of the cutting is dipped into a powder containing the hormone, mixed with inert powder, and then it is placed into a pot containing moist compost or soil, which is gently firmed around the cutting and placed in a cool moist environment.
There are a variety of other chemically similar compounds which may be used for this purpose.


Hormone weedkillers


Although there are a wide variety of chemicals which can be toxic to plants or which damage their exposed parts, such as leaves and shoots, these are of limited use due to problems of toxicity to humans and other animals, and danger due to their oxidising nature. These substances may not affect parts of plants which are underground, and there is a likelihood that new growth will sprout again from these parts.
There are also many situations in which more specificity is required: killing some plants but not others, and leaving soil fit for subsequent growth of plants which are required.

Hormone weedkillers have a variety of advantages over more traditional substances:
- They should be more toxic to plants than animals.
- They should be carried to underground plants and kill then too
- Some weedkillers have a greater effect on "broad-leaved weeds" (mainly dicots) than on grasses and also cereal crops (monocots). For this reason they are called selective weedkillers. These may be used to control weeds amongst the grass on lawns, or to reduce competition from weeds in cereal crops. Examples include compounds with complicated chemical names, popularly abbreviated to 2.4-D and 2,4,5-T. Other compounds such as Dalapon are more effective against monocots than dicots, so they could be used in the opposite situation. In fact these weedkilllers cause sensitive plants to grow too fast, damaging their cells so that their conducting tissue does not develop properly, or they use up their food reserves and die.

What is the definition of a weed?
A plant growing in the wrong place - i.e. not planted by a gardener or grower. In other words, a plant which is able to compete and grow well in the prevailing conditions - probably in its own natural habitat!

Why do the directions for "Lawn feed and Weed" say "Use between April and September. Apply in fair weather conditions when the soil is moist"?
In order to be effective, weather conditions must favour fast plant growth. Otherwise there will be much less chance of obtaining the weedkilling effect, not just a slow effect.

Economic and Ecological consequences of using selective weedkillers

It must be admitted that the widespread use of selective weedkillers in European and Western agriculture has improved the efficiency of crop production (productivity of arable farms), in response to policies promoted by government ministries. This has been quite successful. resulting in the excessive production of cereal and other crops - so called "grain mountains".

However this apparent economic improvement has been at the expense of biodiversity in the countryside, as many "weed" species are food sources and act as shelter for many species of wildlife. Many plants which used to flower alongside crops and weeds, adding to the visual impact of the countryside, have been adversely affected by herbicides sprayed onto fields. Similarly, the survival of animal species, such as birds and butterflies, which add immeasurably to the public enjoyment of the wider environment, are consequently threatened more by the lack of food and protection than by the killing power of the chemicals involved.
Recently, there have been a series of developments aimed at reversing this trend, including "set-aside" where farmers are paid compensation in order to leave sections of their farms fallow in order to revert to "natural" conditions which should support normal fauna and flora. Regulations about maintenance of hedges and borders around fields have also been relaxed in order to increase the areas which can act as habitats for British wildlife.

The term pesticide is used to describe a chemical which kills pests. What term describes the sub-category of this which is used to kill weeds?
herbicide

What is meant by the terms flora and fauna?
flora covers all the plant life, and fauna means the animal life, in an area

Development of fruits


The ripening of fruits is known to be affected by a number of plant hormones.

Commercially, growers sometimes use chemical sprays to prevent fruit from falling from trees. The substances used are auxins such as IAA or substances such as 2,4-D which is also used as a herbicide. Depending on weather conditions, this normally happens with a proportion of the fruit crop, leading to uncertainty as to the likely productivity of the crop. The consistency of cropping and ease of picking can thus be increased.

Normally, a flower which is pollinated develops into a fruit, containing seeds derived from the fertilised egg cell. However, in some circumstances, fruit develops without pollination taking place. In seedless varieties of fruits this parthenocarpic fruit development occurs when other parts of the fruit produce auxins which would otherwise come from the seeds.

Other chemical substances acting as growth regulators can be used to induce flowering and alter the number and shape of fruit forming.

The gas ethene (ethylene) has also been shown to have a hormone-like activity on fruit and is used to control the ripening of stored fruit such as bananas and apples which are best picked and shipped before maturity.



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