Download LEB 05.01 Phototropism and geotropism

LEB
05.01
Phototropism and geotropism
Related topics
Photoperiodism, phototaxis, clinostat, negative and positive phototropic response, geotropism, geotaxis, statoliths
Principle and task
To test the reaction of shoots, leaves and roots of higher
plants to light stimuli (photoperiodism) and to stimuli
caused by the earth’s field of gravity (geotropism).
To measure the alignment of shoot, leaf and root of bean
seedlings which sprout in a vessel standing on the ground
or Iying on its side.
To eliminate the effect of light or gravity by rotation in a clinostat aligned in the vertical or horizontal position.
Equipment
Clinostat with electromotor 230 V
Plastic beaker, 400 ml
Petri dish, d = 100 mm
Support base, variable
Right angle clamp (bosshead)
Support rod, l = 250 mm, stainless
Bean seedlings
Potting mould
Goniometer
65964.93
36014.01
64705.00
02001.00
02043.00
02031.00
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Set-up and procedure
STUDY OF PHOTOTROPIC REACTIONS
— Two to three days before the start of the experiment
about 20 beans are laid out on a moistened sheet of
filter paper in a Petri dish. The cover of the Petri dish
is put on in order to maintain a humid atmosphere and
lifted for a few minutes twice daily for fresh air supply.
— Two beakers are filled with garden mould and 8 bean
seedlings having a root of about 1 cm in length are
planted in each beaker. One beaker is placed on the
horizontally aligned rotating disc of the clinostat (Fig.
1) and the other positioned near the instrument. Both
beakers should stand close to the window and be
watered daily. The clinostat is then switched on.
— When the bean plants have reached a length of 5-8
cm (after 1-2 weeks), the alignment of the shoot, roots
and leaves in the two beakers are measured or estimated.
STUDY OF GEOTROPLC REACTIONS
— The pre-germination of beans and the planting of
beans seedlings is carried out as described above.
Fig. 1. Examination of phototropism
Phywe Series of publication • Laboratory Experiments Biology • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P4050100
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LEB
05.01
Phototropism and geotropism
— One beaker is placed on the vertically positioned
rotating disc of the clinostat and the other beaker is
laid on its side beside the clinostat. Both vessels must
face the window and be kept permanently moistened
(Fig. 2)
— In this experiment, too, the direction taken by the
bean plants is measured after 1-2 weeks, by which
time they have reached a length of 5-8 cm.
Results and evaluation
— The phototropic reaction is defined as the bending of
plants which are not freely mobile towards the direction of incidence of the light. Radiation-induced directional movements of freely mobile organisms, on the
other hand, are designated phototaxis.
— A shoot will incline towards the source of light, i.e.
exhibiting positively phototropic behaviour. The roots
react in negatively phototropic (or indifferent) fashion.
The leaves adopt a position perpendicular to the
direction of incidence of the light (transversely phototropic), thus greatly intensifying photosynthesis
activity.
— Phototropic inclinations are caused by non-uniform
lateral growth of the shoot or roots. The short-wave
fraction of the light elicits a change in the growth hormone content of the light and shadow side of the
plant, thus causing a phototropic bending of the plant.
— The bean plants in the beaker on the clinostat grow
vertically upwards, since they are uniformly exposed
to the light from all sides in succession (1 rotation of
the instrument per hour). The bean plants in the
beaker positioned beside the clinostat exhibit a clearly positive phototropic response (Fig. 1 ).
— Movement reactions of rooted plant organisms to
gravity stimuli are designated geotropism. The reaction of freely mobile organisms, on the other hand,
is designated geotaxis.
The bean plants in the beaker Iying on its side (Fig. 2)
exhibit a negative geotropic behaviour, i.e. they turn
away from the centre of the earth. The reaction of the
roots, on the other hand, is positively geotropic, i.e.
they grow towards the centre of the earth, becoming
concentrated on the beaker wall facing the table.
Plants rotating on the clinostat are exposed to the
gravity field from all sides in succession, so that no
geotropic inclination can occur.
— The effect of gravity can be replaced by centrifugal
forces. This indicates that the underlying basis of
geotropism is a sensitivity to the acceleration of gravity. Starch granules and other cell organelles act as
statoliths in the perception of the force of gravity.
Fig. 2. Examination of geotropism
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Phywe Series of publication • Laboratory Experiments Biology • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen