Functional principles of baobab fruit pedicels - anatomy and biomechanics

Abstract

Background and aims: Fruit pedicels have to deal with increasing loads after pollination due to continuous growth of the fruits. Thus, they represent interesting tissues from a mechanical as well as a developmental point of view. However, only a few studies exist on fruit pedicels. In this study, we unravel the anatomy and structural-mechanical relationships of the pedicel of Adansonia digitata, reaching up to 90 cm in length.

Methods: Morphological and anatomical analyses included examination of stained cross-sections from various positions along the stalk as well as X-ray microtomography and scanning electron microscopy. For mechanical testing, fibre bundles derived from the mature pedicels were examined via tension tests. For establishing the structural-mechanical relationships, the density of the fibre bundles as well as their cellulose microfibril distribution and chemical composition were analysed.

Key results: While in the peduncle the vascular tissue and the fibres are arranged in a concentric ring-like way, this organization shifts to the polystelic structure of separate fibre bundles in the pedicel. The polystelic pedicel possesses five vascular strands that consist of strong bast fibre bundles. The fibre bundles have a Young's modulus of up to 5 GPa, a tensile strength of up to 400 MPa, a high density (>1 g cm-3) and a high microfibril angle of around 20°.

Conclusions: The structural arrangement as well as the combination of high density and high microfibril angle of the bast fibre bundles are probably optimized for bearing considerable strain in torsion and bending while at the same time allowing for carrying high-tension loads.

Keywords: Adansonia digitata L; Angola; biomechanics; composite material; density; fibre characteristics; mechanical stresses; pedicel; specific Young’s modulus; strengthening tissue.

Fig. 1.

(A) Freshly cut Adansonia digitata fruit stalk from Angola; (B) arrangements of cross-sections for anatomical studies; (C) A. digitata tree during dry season in the Kissama National Park, Angola.

Fig. 2.

Morphological characteristics of Adansonia digitata inflorescence: (A) young flower bud with bracteoles arising on the pedicel; (B) scars of the caducous bracteoles near the flower bud; (C) the same but on a mature pedicel and cross-section of dry mature pedicel near fruit; (D) junction between peduncle and pedicel from a young inflorescence; (E) junction between peduncle and pedicel from a mature inflorescence; (F) flower bud pending from the tree on its long peduncle/pedicel. Abbreviations: sc = scar of bracteole, br = bracteole, ju = junction.

Fig. 3.

Cross-section of a mature Adansonia digitata pedicel: (A) Near the branch; (B) near the fruit. Abbreviations: s: sclerenchyma cap, b: bast, c: cambium, x: xylem, e: epidermis, p: cortical parenchyma, m: mucilage ducts. Scale bar = 1 mm.

Fig. 4.

Scanning electron micrograph of the bast region. Abbreviations: f: fibre bundles, p: phloem, r: ray. Scale bar = 100 µm.

Fig. 5.

Cross-section of Adansonia digitata covering the transition zone from peduncle (A and B) to pedicel (C–F; 1 cm from a to f; using light microscopy, stained with Astrablue–Safranin. Scale bar = 2 mm.

Fig. 6.

Orthoslice reconstruction of an X-ray microtomography scan through a basal (A) and apical (B) part of a baobab peduncle/pedicel transition zone. (A) Course of longitudinal phloem tissue during the beginning of stelar reorientation. (B) Course of longitudinal phloem and xylem tissue during division of vascular bundles. Greyscale has been inverted for better visualization of tissues.