Responses of the Root Systems of Sunflower and Maize to Unidirectional Stem Flexure

Plants of two contrasting species of herbaceous annuals, the dicot sunflower (Helianthus annuusL.) and the monocot maize (Zea maysL.), grown in the glasshouse were subjected to regular unidirectional stem flexure. Differences in morphology and mechanical properties of roots and shoots were then investigated. Root systems were divided into quadrants around the axis of stimulation and differences in root morphology and mechanics between the zones were investigated. There were considerable differences between roots in the leeward and windward zones compared with roots perpendicular to the axis of stimulation. First-order lateral roots in both species were thicker, more rigid and more numerous. These results suggest that plant roots respond locally to mechanical stimulation. There were, however, also differences in the responses of the two species. In sunflower, the tap root and stem base became elliptical in cross section with the major axis lying in the plane of stimulation. The lateral roots of flexed sunflowers in both the leeward and windward zones showed similar growth responses: roots were thicker, more numerous and weighed more than those in the perpendicular zones. However, only leeward roots showed significant differences in their mechanical properties; roots were more rigid, stronger and stiffer. In contrast, the leeward roots of maize were thicker and more numerous, with a greater biomass than the windward roots. However, only roots in the windward zone were stiffer than those in the perpendicular zone. These differences between sunflower and maize are related to their contrasting anchorage mechanics.