When plants are placed horizontally they exhibit growth response to the stimulus of one-sided gravity. It is called geotropism or gravitropism. The tip of the stem grows away from the pull of the gravity (negative geotropism) and the root tips grow towards it (positive geotropism). The plant responses to unilateral gravity are growth movements because the resulting curvatures occurs in the region of elongation of stem and root. If plants are held horizontally on a slowly revolving klinostat, they experience the stimulus of gravity equally on all sides and the tips of roots and stem do not exhibit growth curvatures.
Hypothesis Explaining Mechanism of Geotropism
Most study of geotropism is devoted to roots. There is ample evidence that site of gravity perception is the root cap. When the root tips were removed from roots of pea, lentils and broad beans, they did not respond to the gravity until a new root cap produce an inhibitor on the lower side of the roots, stopping or slowing the growth so that the root bends down. From the work on coleoptile tips there is evidence that geotropism, like phototropism may be due to a redistribution of auxin (IAA). Cholodny-Went and Haberlandt and Nemec proposed theories to explain geotropism.
Colodny -Went Theory of Geotropism
Cholodny and went independently suggested that geotropic responseof horizontally placed shoots is due to differential growth resulting from an unequal distribution of auxin (IAA) in the upper and lower halves of the shoot. More auxins accumulate on lower side that causes accelerated growth on lower side and results in stem to curve up. On other hand, a horizontally placed root moves down with auxin concentration of IAA stimulates cell elongation in stems but inhibits it in roots. The accumulation of auxin on the lower side of a horizontally placed root retards cell elongation on that side. This inhibition of cell elongation of roots by auxin may be due to synthesis of ethylene since its synthesis is initiated when the auxin accumulated.
Experimental data indicate an unequal distribution of IAA in horizontally placed coleoptile tips. The coleoptile tips from dark grown vertically oriented maize seedlings were excised and placed to be 40 units of IAA. When the tips were placed horizontally, the agar block received 40 units of IAA, an amount identical to that found with a vertical orientation. In another experiment the horizontally oriented coleoptile tip was split almost to tip and the upper and lower halves separated by a glass cover slip impermeable to IAA, more IAA diffused out of lower half (67%) of the split coleoptile than out of the upper half (33%). Similar results were obtained in experiments with dicot apical stem tips.
Statolith Theory of Geotropic Response
How could the stimulus of gravity trigger changes in growth in the upper or lower surfaces of stems and roots? Many explanations were provided but none could explain satisfactorily. Gotlieb Haberlandt (1902) and B. Nemec (1901) proposed that certain cells in the root tip contain amyloplasts with large, dense starch grains in them and these starch grains settle in the cells of the root cap in response to gravity and perceive the stimulus of gravity. The organelle perceiving the stimulus were called statolith and the cells containing statoliths were named statocytes. The statocytes are also found to be present in the tips of coleoptiles, the root endodermis, the sheath of vascular bundlesof hypocotyles, epicotyles and young foliage.
The proponents of statolith theory suggest that the statolith change their position in the cell when the plantbis moved from a vertical to a horizontal position. When the statoliths fall on to endoplasmic reticulum in a different part of the cell they bring about the release of calcium ions there. The raised concentration of calcium ions activate calmodulin, a small protein which is present in the cell and which is known to initiate the actions of several enzyme systems. These enzymes activate the growth of the cell. Activated calmodulin may switch on the cell membrane pumps for calcium ions and for IAA. Calcium ions and growth regulatory substance (IAA) would be passed across the tissue of the stem or root from cell to cell, extending the region of enhanced or inhibited growth;
The following facts support the statolith theory of geotropism response:
I. Maize mutants with smaller amyloplasts are less sensitive to the stimulus of gravity.
II. If roots or coleoptiles are treated with gibberellins and kinetin at high temperatures, all starch in the amyloplast disappears and they do not response to the stimulus of gravity. The sensitivity reappears as soon as starch grains reappear in destrached amyloplasts.
Inhibitor Theory of Geotropism
Current evidence shows that the explanation of geotropism in roots on the basis of differential auxin concentration (Cholodny-Went Theory) is no longer valid. Although IAA is present in root tips. Its transport in roots is highly polarized in the acropetal direction. Also their is clear evidence that geotropism in roots is regulated by root cap. When the root cap is removed, geotropism is inhibited. However, when the cap is regenerated, geotropism of root is restored. When half of the root cap of maize root tip is removed, the roots(placed horizontally or vertically) will develop curvature toward the side of the remaining half cap. Further the growth rate of maize roots increases after the cap is removed. Also, when the maize caps are placed on the tip of intact lentil roots, a decrease in root elongation results. These observations strongly suggest that a growth inhibitor, possibly abscissic acid, is produced by the cap cells which is than transported basipetally into the elongation zone and through the action of gravity (possibly via statoliths) may accumulate and inhibit cellular elongation on the lower side of the roots placed in horizontal position.