S61  Parallel session
Plant Biomechanics

08-Jul-2015 15:20 17:00
 
 
Abstract: O 291
THE MECHANISM OF SESAME CAPSULE OPENING – A GRADATED BILAYER

The mechanism of sesame capsule opening – a gradated bilayer

Introduction: Hygroscopic movements occur in desiccating dead plant organs, promoting seed dispersal (Elbaum and Abraham, 2014). The movement is governed by the arrangement of cellulose microfibrils within cell walls and the distribution of the cells within the tissue. Hygroscopic movement is well studies in long and narrow plant awns - 1D constructs (Elbaum et al., 2007) and flat pods - 2D constructs (Armon et al., 2011). However, similar movements in capsules (3D constructs) are not studied. Sesame (Sesamum indicum L.) capsules consist of 4 locules with a simple exocarp, a parenchymatous, multi-layered mesocarp, and a multi-layered fibrous endocarp (Fig.1A). Dry capsules shatter at the false septum region and disperse the seeds. When opening, the capsule walls curve away from the false septum. We suggest that this hygroscopic movement is actuated by the fibrous endocarp - the only pericarp tissue that exhibits lignified cell walls.

Methods: The capsules were embedded in paraplast and sectioned at 12 µm thickness to investigate microscopic structure and birefringence. Sample birefringence was investigated using a LC-PolScope image processing system and the image analysis was done using Abrio 2.2 software. Directional cuts through rehydrated capsules were made to study stress forces within the capsule during drying.

Results: Microscopic observation showed that the endocarp was built of a bilayer of radial and longitudinal fibre cells with the layers positioned in a semi-circle one inside the other (Fig1B). Near the false septum the bilayer consisted only of longitudinal cells, while towards the carpel septum it was gradually reduced to a layer of radial cells. Endocarp sections examined by polarised-light microscopy showed that the cellulose microfibrils lie almost parallel to the fibre long axis (Fig.2). Upon drying, the fibres change their width, allowing bending parallel to fibre axis. Further complexity was added by pectin-rich cells, surrounding the endocarp fibres, which probably slow down the desiccation of the active tissue. Residual stresses within the capsule supported a bilayer twisting mechanism.

Discussion: Upon drying, the fibres shrink transversely and pull the false septum apart. We suggest that the endocarp contains residual stress because of the perpendicular direction of the fibre layers in the bilayer. The combination of humidity gradient caused by the pectin and changing proportions of longitudinal/radial fibres around the locule is suggested to cause the observed complex movement. Hygroscopic movements in capsules of other plant species and families may follow similar mechanistic model.

Figures


Figure 1: Sesame capsule in cross-section (A) and a schematic representation of the bilayer structure (B). Ep – epicarp, M- mesocarp, En – endocarp, FS - false septum, CS – carpel septum, L – locule.


Figure 2: LC-PolScope image of microfibril orientation in the fibre bilayer. A- visible light image, B- orientation image. En – endocarp, L – locule, EnE- inner epidermis of the endocarp, C – seed coat crystals.

References

Armon et al., Science, 333:1726-1730.

Elbaum and Abraham, Plant Sci, 223:124-133, 2014.

Elbaum et al., Science, 316:884-886, 2007.

 
Co-authors
I. Shtein1, B. Bar-On2, R. Elbaum1.
1The Hebrew University, The Faculty of Agriculture, Rehovot, Israel.
2Ben Gurion University of The Negev, Department of Mechanical Engineering, Beer Sheba, Israel.

 

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