page_title: Gravitropism
section_title: Compensation
hierarchical_section_title: Gravitropism / Compensation
caption_attribution_description: Compensation process in the bending mushroom stem (experimental data were collected on Coprynus). C - the compensating part. Created and uploaded by Audrius Meškauskas.
context_page_description: Gravitropism is a coordinated process of differential growth by a plant or fungus in response to gravity pulling on it. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems grow in the opposite direction. This behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing upwards. Hebaverns stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.
context_section_description: Bending mushroom stems follow some regularities that are not common in plants. After turning into horizontal the normal vertical orientation the apical part (region C in the figure below) starts to straighten. Finally this part gets straight again, and the curvature concentrates near the base of the mushroom. This effect is called compensation (or sometimes, autotropism). The exact reason of such behavior is unclear, and at least two hypotheses exist.
  The hypothesis of plagiogravitropic reaction supposes some mechanism that sets the optimal orientation angle other than 90 degrees (vertical). The actual optimal angle is a multi-parameter function, depending on time, the current reorientation angle and from the distance to the base of the fungi. The mathematical model, written following this suggestion, can simulate bending from the horizontal into vertical position but fails to imitate realistic behavior when bending from the arbitrary reorientation angle (with unchanged model parameters).
  The alternative model supposes some “straightening signal”, proportional to the local curvature. When the tip angle approaches 30° this signal overcomes the bending signal, caused by reorientation, straightening resulting.
  Both models fit the initial data well, but the latter was also able to predict bending from various reorientation angles. Compensation is less obvious in plants, but in some cases it can be observed combining exact measurements with mathematical models. The more sensitive roots are stimulated by lower levels of auxin; higher levels of auxin in lower halves stimulate less growth, resulting in downward curvature (positive gravitropism).
caption: The compensation reaction of the bending Coprinus stem. C - the compensating part of the stem.
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