Phenotypic Plasticity of Grass Root Anatomy in Response to Light Intensity and Nutrient Supply

The phenotypic plasticity of axial root anatomy was investigated in response to the availability of above- and below-ground resources using eight grass species of the genera Bromus and Poa. In a 7-week garden experiment nutrients were varied by a factor of five and light treatments of 100, 30 and 5·5% daylight were applied. Both nutrients and light influenced root structure. The effect of nutrients was largely explained by plant size, but this was not the case for light. Shading to 30% daylight led to a higher proportion of stele, larger stelar cells and larger xylem vessels. This can be understood either as an increased need for high transport capacity in the shade, where leaf area is larger but root mass lower than in full daylight, or as an increased resistance against desiccation, which is more of a hazard in open sites. Under 5·5% daylight, tissue mass density was reduced due to a lower proportion of stele, though xylem characteristics were not influenced when a correction for the effect of root cross-sectional area was applied. This response may be interpreted as a mechanism to maintain root function with a lower investment in biomass when growth is limited by low irradiance. The results show that the response of a plant to resource limitation is not restricted to those organs responsible for the acquisition of that resource. Furthermore, the qualitative response to shading depends on the absolute level of irradiance. For this reason, care is needed when comparing the results of shading experiments conducted under different irradiances.