The Physiology of Flowering Plants - KHAM PHA MOI

TROPISMS 327ABUniform illumination of shootGrowthRoot parallel to direction of gravityGrowthUnilateral illumination of shootGrowthLightRoot at an angle to direction of gravityGrowthFig: 12:7 Asymmetrical auxingradients are thought to lead totropic curvature. (A) In a shoot (orcoleoptile) illuminated from above,auxin is transported from the tipand distributed uniformly. Thegrowth rate of both sides of theshoot is the same and therefore itgrows straight upwards.Illumination from one side leads to aredistribution of auxin within theshoot. Growth is stimulated on theshaded side and inhibited on theilluminated side. (B) In the root,auxin is redistributed in the rootcap. When the root is placed at anangle to gravity, more auxin istransported to the lower side andless to the upper side. As auxininhibits root growth, the root bendsdownwards.Gravitythen moves back up the root in the epidermal layers. Removal of theroot cap abolishes gravitropic responses and stimulates root growth.Upon gravistimulation, an asymmetrical distribution of auxin develops.More auxin is delivered to the lower side, where it inhibits growth,whilst the reduced auxin content of the upper side stimulatesgrowth. This differential growth causes the root to bend downwards(Fig. 12.7B). In this case the negative effect of auxin on root growthcauses the directional growth response. The actual responsesobserved within roots are often more complex than a simple stimulationof growth on one side and inhibition on the other. Differentparts of the root may respond at different times and the rate ofelongation varies in a complex manner.Predictions and problemsThe simplicity of this model, and its agreement with many experimentalobservations, explains its durability. However, the strength ofany model lies in its ability to generate testable hypotheses and, inthe case of the Cholodny–Went model, not all of the predictions havebeen fulfilled. This has led to the model undergoing numerous modifications(although often retaining the same name). Some of these arediscussed below but the reader is referred to Firn et al. (2000) for anexcellent account of the development of the model and controversiesand Yamamoto (2003) for a recent update.