Ontogenetic tissue modification in Malus fruit peduncles: the role of sclereids |
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Authors: | Melanie Horbens Alexander Feldner Monika H?fer Christoph Neinhuis |
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Affiliation: | 1.Institute of Botany, Technische Universität Dresden, Zellescher Weg 20b, D-01217 Dresden, Germany;2.Institute of Plant and Wood Chemistry, Technische Universität Dresden, Pienner Strasse 19, D-01737 Tharandt, Germany;3.Julius Kühn-Institute, Federal Research Centre for Cultivated Plants, Institute for Breeding Research on Horticultural and Fruit Crops, Pillnitzer Platz 3a, D-01326 Dresden, Germany |
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Abstract: | Background and AimsApple (Malus) fruit peduncles are highly modified stems with limited secondary growth because fruit ripening lasts only one season. They must reliably connect rather heavy fruits to the branch and cope with increasing fruit weight, which induces dynamic stresses under oscillating wind loads. This study focuses on tissue modification of these small, exposed structures during fruit development.MethodsA combination of microscopic, static and dynamic mechanical tests, as well as Raman spectroscopy, was used to study structure–function relationships in peduncles of one cultivar and 12 wild species, representatively chosen from all sections of the genus Malus. Tissue differentiation and ontogenetic changes in mechanical properties of Malus peduncles were observed throughout one growing season and after successive removal of tissues.Key ResultsUnlike in regular stems, the vascular cambium produces mainly phloem during secondary growth. Hence, in addition to a reduced xylem, all species developed a centrally arranged sclerenchyma ring composed of fibres and brachysclereids. Based on differences in cell-wall thickness, and proportions and arrangement of sclereids, two types of peduncle construction could be distinguished. Fibres provide an increased maximum tensile strength and contribute most to the overall axial rigidity of the peduncles. Sclereids contribute insignificantly to peduncle strength; however, despite being shown to have a lower elastic modulus than fibres, they are the most effective tissue in stiffening peduncles against bending.ConclusionsThe experimental data revealed that sclereids originating from cortical parenchyma act as ‘accessory’ cells to enhance proportions of sclerenchyma during secondary growth in peduncles. The mechanism can be interpreted as an adaptation to continuously increasing fruit loads. Under oscillating longitudinal stresses, sclereids may be regarded as regulating elements between maintenance of stiffness and viscous damping, the latter property being attributed to the cortical parenchyma. |
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Keywords: | Apple biomechanics fibres fruit peduncle fruit load functional anatomy Malus sclereids viscous damping |
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