131458
Beschreibung
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Plant growth factors
- Are chemicals that occur naturally at extremely low concentrations. They regulate various
aspects of plant growth and development from seed formation and germination through the
aging and death of a plant. They also coordinate many plant responses to enviromental stimuli
- Often, plant growth factors, like animal hormones, carry information from on
epart of the organism to another. Plant growth factor are transported in the
transpiration stream of the Xylem and by mass flow in the phloem. From the
xylem and phloem, they usually reach their target cels by diffusion.
- Unlike hormones, plant growth factors are not synthesized
in special organs, and they sometimes act in the immediate
vinictiy of their production. Ethene for example, usually acts
on the tissues from which it is released.
- Ethene is a gas released from ripening fruits, nodes of stems, aging leaves,
and flowers. It is involved in seed dormancy, fruit ripening and leaf
abscisson. Other plant growth substances include gibberellins and cytokinins
which are growth promoters while abscisic acid (ABA) is a powerful growth
inhibitor that often acts antagoniscally to growth promoters. ABA appears to
promote dormancy in some seeds, and it stimulates the closing of stomata.
- The main plant growth factor involved in phototropism is
indoleacetic acid (IAA) which is synthesized mainly in the
shoot tips. Its major effect is to promote growth by
increasing the rate of cell elongation.
- Phtotropisms. Most of our knowledge about
phototropism has been gained from experiments
on coleoptiles, the specialised, protective sheaths
around the germinating shoots of grasses.
Coleoptiles have been used because their
response to light is easy to observe, they are
small, and they are easy to grow in large numbers
- If a grass seeding is exposed to light from one side, it grows towards that light. microscopic examination
reveals that the cells on the shaded side of the coleoptiles are significantly longer than those on the lit
side. It seems that coloeptiles bend towards the light by differential growth; Cells on the side (shaded)
elongate faster and become longer that those on the lit side. Early exeperiments on phototropism
involved coleoptiles, decapitated coleoptiles, and coleoptiles shaded in various ways. Later experiments
used agar blocks to collect a chemical diffusing downwards from shoot tips.
- In the 1930's, the chemical messenger responsible
for phototropic repsonses was extracted and given
the name auxin. Auxin was later found to consist
mainly of indoleactic acid (IAA)
- Coleoptile tips appear to produce equal amounts
of IAA in the dark or the light, but its distribution
can vary. It seems that when light strikes one side
of the coleoptile, a receptor triggers the
redisdribution if IAA so that more travels down the
shaded side of the coleoptile. The increased IAA
concentration causes cells on the shaded side to
grow longer than those of the lit side
- IAA is thought to stimulate cell elongation by causing target cells to secrete proteins into
their cell walls. The resulting increase in acidity is thought to weaken the bond between
cellulose microfibrils, allowing the cell wall to expand when the cell takes in water
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