The Control of Root Growth to Secure Anchorage

The knowledge from from recent studies on root biomechanics (see Section 1.3.2.2) is of particular interest when trying to understand the biomechanical adaptation of roots during plant growth. Several experiments have been carried out to quantify the root biomass investment according to the external loading on the plant [94,109]. Ennos and Fitter [92] showed that different root anchorage strategies can have an impact on this biomass allocation ratio between above and below ground parts during plant growth. Active biomechanical responses to external mechanical stresses have been studied and discussed by several authors, including the consequences of morphological modifications in root systems of plants regularly submitted to external forces, such as wind loading or shaking (see the review by Stokes and Guitard [159]). Such modifications include an increase in root branch number, cross-sectional area, and biomass along the axis of bending [73,90,109,159]. Responses may also occur at the cellular level, with consequences for wood mechanical properties [105]. In adult trees, uneven secondary growth occurs in the most highly stressed parts of individual roots, resulting in cross sections with shapes analogous to I or T beams, especially in the highly stressed region at the stem-root base. Such irregular radial growth results in an optimized root stiffness in the plane of bending for a given quantity of dry matter [103,160].

Although of importance for the anchorage of a plant, such responses of roots to external mechanical loading also have consequences for nutrient and water uptake. The optimal absorption of nutrients is significantly affected by root architecture and size [66,67], and a trade-off may exist between anchorage and nutrient uptake, particularly when the latter is limiting [159]. Nevertheless, it should be remembered that even though thigmomorphogenetic responses occur in the root system and can be even more pronounced than those in the stems of the same plants [90,159], such morphological changes are small when compared to the direct effects of differences in water and nutrient conditions on plant growth.

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