Plant growth substances
Plant growth regulators include:
- Auxins - stimulate cell elongation in zone of elongation
- Cytokinins - promote cell division
- Gibberellins - stimulate elongation of internodal regions
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- Auxins are produced in the shoot tip and diffuse down to the zone of elongation.
- They stimulate the pumping of hydrogen ions into the cellulose cell wall
- The acidification of the cell wall activates agents which loosen the linkages between cellulose microfibrils - making the wall more flexible
- The cells absorb water by osmosis and the flexible cell walls allow the cells to expand as the extra water exerts increased hydrostatic pressure against them
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Auxin and phototropism
- Auxins are involved in producing the positive phototropic response in plant shoots.
- As the auxins diffuse down from the tip, unidirectional light causes auxin to move towards the shaded side promoting greater cell elongation on the shaded side and a resultant curvature of the shoot towards the light.
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- Plants have phytochromes that detect the photoperiod.
- They are interchangeable: P660 and P730.
- P660 (Pr) maximally absorbs red light, while P730 (Pfr) maximallly absorbs far-red light.
- During darkness, there is a slow conversion of P730 to P660 - P660 accumulates
- During daylight, P730 accumulates
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The control of flowering
- P730 is a physiologically active form, and can either stimulate or inhibit flowering
- LONG DAY PLANTS - require a critically short period of darkness, so that high levels of P730 accumulate and stimulate flowering
- SHORT DAY PLANTS - require a critically long period of continuous (uninterrupted) darkness, so that the inhibitory effect of P730 is removed
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Comparison of coordination in plants and animals
In both plants and animals:
- Chemicals are used to communicate between receptors and effectors
- Receptors receive the stimulus (e.g. phytochromes detects photoperiod in plants, while osmoreceptors detect changes in water potential and retina in eye detects light in animals).
- Effectors bring about the response (e.g. plants may bud into a flower, and cells elongate in response to auxin, while in animals muscle contracts)
- However, plants lack a nervous system, which animals require for rapid communication in the body and is important in coordinating movement.
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