The mechanics of explosive seed dispersal in orange jewelweed (Impatiens capensis)

Abstract

Explosive dehiscence ballistically disperses seeds in a number of plant species. During dehiscence, mechanical energy stored in specialized tissues is transferred to the seeds to increase their kinetic and potential energies. The resulting seed dispersal patterns have been investigated in some ballistic dispersers, but the mechanical performance of a launch mechanism of this type has not been measured. The properties of the energy storage tissue and the energy transfer efficiency of the launch mechanism were quantified in Impatiens capensis. In this species the valves forming the seed pod wall store mechanical energy. Their mass specific energy storage capacity (124 J kg(-1)) was comparable with that of elastin and spring steel. The energy storage capacity of the pod tissues was determined by their level of hydration, suggesting a role for turgor pressure in the energy storage mechanism. During dehiscence the valves coiled inwards, collapsing the pod and ejecting the seeds. Dehiscence took 4.2+/-0.4 ms (mean +/-SEM, n=13). The estimated efficiency with which energy was transferred to the seeds was low (0.51+/-0.26%, mean +/-SEM, n=13). The mean seed launch angle (17.4+/-5.2, mean +/-SEM, n=45) fell within the range predicted by a ballistic model to maximize dispersal distance. Low ballistic dispersal efficiency or effectiveness may be characteristic of species that also utilize secondary seed dispersal mechanisms.

Fig. 1.

Sequential video tracings of an I. capensis seed pod before and during dehiscence. The numbers show the time in milliseconds from the start of dehiscence.

Fig. 2.

Frequency distribution of predicted I. capensis seed dispersal distances. Dispersal distances were calculated from measured launch trajectories using a ballistics model (Vogel, 1988) (n=45).

Fig. 3.

Optimal and measured seed launch angles. (A) Relationships between predicted seed dispersal distance and launch angle at the maximum (4.08 m s⁻¹) and median (1.02 m s⁻¹) measured launch speeds. Vertical dotted lines indicate the launch angles giving 80% of maximum dispersal distance for each curve. (B) Frequency distribution of measured seed launch angles (n=45).

Fig. 4.

Representative force–extension curve for an I. capensis valve coil. Length is shown relative to the in situ coil length (14.0 mm).

Fig. 5.

Relationship between stored energy and I. capensis valve coil mass. Filled symbols, work storage measured during a progressive decrease in mass due to dehydration. Open symbol, work stored after rehydration to initial mass. The horizontal line shows a homogenous subset as established by Scheffé’s post hoc test (P > 0.05). Data are shown as mean ±SEM (n=6).

Fig. 6.

Representative seed dispersal distributions for plants with ballistic dispersal mechanisms of different efficiencies and/or reliabilities.