A comparative study of the response of the roots and shoots of sunflower and maize to mechanical stimulation

Despite numerous studies of the effects of mechanical stimulation on plant shoots, the response of roots to mechanical stimulation has largely been neglected. In this study the effects of shoot flexure on the morphology and mechanics of two contrasting species of herbaceous angiosperm, growing in a glasshouse were compared: maize (Zea mays), a monocot; and sunflower (Helianthus annuus L.) a dicot. Mechanical stimulation affected the root more than the shoot components. Root systems of mechanically-stressed sunflowers had a greater angle of spread and increased root number. As well as large morphological and weight effects, with increases over the control of 33% in the length of rigid root and 38% in the dry weight of lateral roots, in sunflowers, there were also mechanical effects. In both species roots of flexed plants were more rigid, stronger and composed of stiffer material and their root systems also provided greater anchorage strength. In contrast, there was only a small reduction in shoot weight and shoot height in flexed plants and no effects on mechanical properties. There were differences in behaviour between species; maize root morphology responded less than that of sunflowers to mechanical stimulation. The basal diameter of roots increased by only 8% compared with 16% in sunflowers, though the roots of both species showed similar increases in material stiffness. This difference is related to the lack of secondary thickening in the monocots compared with the dicot sunflowers.