The Responses of Field-grown Sunflower and Maize to Mechanical Support

The effects of mechanical support on two contrasting species of herbaceous annual, the dicot sunflower ( Helianthus annuus L.) and the monocot maize ( Zea mays L.), were investigated by comparing the growth and mechanical properties of supported plants and those which were left to sway freely in the wind. Providing support had its greatest effect on the more highly-stressed basal areas of the plants, such as the lower stem and the base of the lateral roots. The diameter of the stem bases of both species was approx. 10% lower in supported plants, but there was no difference between treatments in the diameter of the stem above 50 cm. Roots of both species also showed a reduction in rigidity and bending strength of 40–50% in the supported plants compared with freely swaying plants. There was a significant reduction in the partitioning of biomass to the root systems of supported plants of both species. There were differences in the way in which sunflower and maize responded to the provision of support; in sunflower, the reduction in lateral diameter was about twice that in maize, whereas in maize the decrease in the number of first-order laterals was twice that of sunflower. This study suggests that thigmomorphogenesis may be a localized response, but that different species can respond in different ways to mechanical stimulation.