Effects of Vibration on Mechanical Properties and Biomass Allocation Pattern ofCapsella bursa-pastoris(Cruciferae)

Abstract

The herbaceous dicot speciesCapsella bursa-pastoris(Cruciferae) was used to determine the influence of chronic mechanical perturbation on the biomass allocation pattern (i.e. dry weight distribution among roots, stems and reproductive structures) and the mechanical properties of roots and stems (i.e. tensile breaking stress and Young's modulus). It was hypothesized that mechanically stimulated plants would allocate more of their total biomass to root systems and less to shoots compared to control plants and that the breaking stress (a measure of strength) and Young's modulus (a measure of material stiffness) would increase for roots and decrease for stems because these responses would adaptively reduce the bending moment at the base of shoots and increase the anchorage strength of root systems. It was also hypothesized that mechanical perturbation would maladaptively reduce the relative fitness of individuals by reducing biomass allocation to their reproductive organs and the ability to broadcast seeds by means of elastic stem flexure. These hypotheses were tested by vibrating cultivated plants for 60 s every day during the course of growth to maturity and comparing their dry weight distributions and the mechanical properties of their body parts (measured in tension) to those of undisturbed control plants. Based on a total of 51 experimentally manipulated and 44 control plants for which mechanical properties were successfully tested, chronic organ flexure resulted in more massive root systems and less massive vegetative shoots, increased the magnitudes of root breaking stress and Young's modulus and had the reverse effect on stems, reduced the dry weight of reproductive structures at maturity, delayed the formation of the first mature flower and fruit, and accelerated the on-set of plant senescence compared to control plants. These responses to chronic organ flexure are interpreted to be vegetatively adaptive, since they reduce the probability of stem and root failure as a consequence of wind-pressure or foraging, and to be reproductively maladaptive, since they reduce reproductive effort and the ability to mechanically discharge seeds.