The effects of air flow and stem flexure on the mechanical and hydraulic properties of the stems of sunflowers Helianthus annuus L.

Many studies have shown that wind affects plant development, causing them to develop shorter and usually stronger stems. Many of these effects have been shown to be due to a response to mechanical flexing of the stem which is known as thigmomorphogenesis. However, it is not known how wind affects the hydraulic properties of stems, nor have the effects of air flow past leaves been examined in isolation from mechanical flexing. This study, therefore, used a factorial experiment to distinguish between the effects of stem flexing and air flow, and examined the morphology, hydraulics and mechanics of developing sunflowers Helianthus annuus. It was found that flexure and air flow had opposite effects on several aspects of development; air flow increased plant height and length-specific stem hydraulic conductivity, k(h), and reduced stem rigidity and strength, while flexing did the reverse. There was also a clear trade-off between hydraulic and mechanical capability: as one increased the other decreased. A plant's response to wind must, therefore, be a complex response to at least two different stimuli and this might help explain why it varies with species and environment.