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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 1 2 1 1 1 1 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 45 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 46 P4050100 Phywe Series of publication • Laboratory Experiments Biology • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen