Morphology and Anatomy of Branch-Branch Junctions in Opuntia ficus-indica and Cylindropuntia bigelovii: A Comparative Study Supported by Mechanical Tissue Quantification.

Max D Mylo,Tom Masselter,Kathrin Drozella,Jochen Leupold,Linnea Hesse,Thomas Speck,Olga Speck

doi:10.3390/plants10112313

Abstract

The Opuntioideae include iconic cacti whose lateral branch–branch junctions are intriguing objects from a mechanical viewpoint. We have compared Opuntia ficus-indica, which has stable branch connections, with Cylindropuntia bigelovii, whose side branches abscise under slight mechanical stress. To determine the underlying structures and mechanical characteristics of these stable versus shedding cacti junctions, we conducted magnetic resonance imaging, morphometric and anatomical analyses of the branches and tensile tests of individual tissues. The comparison revealed differences in geometry, shape and material properties as follows: (i) a more pronounced tapering of the cross-sectional area towards the junctions supports the abscission of young branches of C. bigelovii. (ii) Older branches of O. ficus-indica form, initially around the branch–branch junctions, collar-shaped periderm tissue. This secondary coverage mechanically stiffens the dermal tissue, giving a threefold increase in strength and a tenfold increase in the elastic modulus compared with the epidermis. (iii) An approximately 200-fold higher elastic modulus of the vascular bundles of O. ficus-indica is a prerequisite for the stable junction of its young branches. Our results provide, for both biological and engineered materials systems, important insights into the geometric characteristics and mechanical properties of branching joints that are either stable or easily detachable.

Highlights

Licensee MDPI, Basel, Switzerland.The plant family of the Cactaceae with its succulent growth forms and adaptations to habitats with little or only seasonally available water have fascinated researchers for centuries
We have addressed the following three main aspects: (i) quantitative comparisons of the geometry, size and shape of the junctions and lateral branches; (ii) microscopic imaging and magnetic resonance imaging (MRI) scans of entire branch–branch sites to identify the tissues involved and to visualise their 3D spatial arrangement; and (iii) tensile tests on fresh dermal tissues and vascular bundles to characterise their mechanical properties
We analysed the junctions of O. ficus-indica and C. bigelovii by means of their morphology and anatomy

Summary

Introduction

Licensee MDPI, Basel, Switzerland.The plant family of the Cactaceae with its succulent growth forms and adaptations to habitats with little or only seasonally available water have fascinated researchers for centuries. Cacti differ greatly in their outer appearance; species can be found with unbranched to strongly branched columnar, tree-like, creeping, epiphytic and climbing habits [1]. They are united by xerophytic adaptations such as succulence, the absence or the early abscission of leaves (with the exception of the Pereskioideae) and extensive but shallow. Within the Cactaceae, the Opuntioideae represent the second largest subfamily with over 300 species [3]. A remarkable feature of the Opuntioideae is the outer geometry of their jointed branches, which is a characteristic and name-giving feature of their tribes. A thickened palisade cortex is typical, which, is not as distinct as in the related subfamily

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