The mechanics of anchorage in seedlings of sunflower, Helianthus annuus L.

Forces applied to plants will subject many of the roots to tension, which must be transferred to the soil via shear if uprooting is to be prevented. The stress distribution will depend on the relative stiffnesses of the earth and root, and the mode of failure will depend on the relative strength of the soil and of the root soil bond. This study of the anchorage of sunflower radicles combined uprooting tests performed by a tensile testing machine with mechanical tests on the roots and soil.
The maximum extraction force increased with length to an asymptotic value and was reached at a very low displacement. Root hairs and soil particles covered the tapered top 20 mm of extracted root, but the lower cylindrical region was bare. The soil was stiffer than the root, so shear stress was initially concentrated at the top of the root, soil strength over the top 20 mm resisting uprooting. Lower regions of the root were stressed later, their sparser root hairs being sheared off, and resist uprooting only by friction. In a further lest upper and lower regions of radicles were uprooted separately. As predicted, the upper region generated much greater resistance to uprooting per unit length, and at much lower displacements than the lower region.
The top of the radicle is well adapted for anchorage, the profuse root hairs and mucigel it produces glueing the root to the soil. The lower regions are thus protected from damage.