Characterisation of calcium crystals in Abelia spp. using X-ray diffraction and electron microscopy

Abstract

SummaryThe localisation, chemical composition, and morphology of calcium (Ca) oxalate crystals in the leaves and stems of Abelia mosanensis and of A. × grandiflora were analysed using a variable pressure-scanning electron microscope (VP-SEM) equipped with an X-ray diffraction system, a low temperature SEM (LT-SEM), and a transmission electron microscope (TEM). Foliar analyses of macro- and micro-elements were performed on the leaves and stems of A. mosanensis. A greater number of Ca oxalate crystals were observed in A. mosanensis than in A. × grandiflora. Three morphologically distinguishable types of crystals were observed: the prismatic crystals found inside the chloroplast, multifaceted star-like spherical and bladed aggregate crystals (druses) inside the vacuoles of the mesophyll cells, and small angular crystals (sand crystals) inside the cuticle. Semi-solid crystals that may drip and accumulate to become a solid Ca oxalate crystal were observed by LT-SEM, which indicated the growth of druses of Ca oxalate crystals. The growth of prismatic crystals and of druses were evident through the formation of crystalline lamellae. Micro-analysis indicated that the crystals were Ca oxalate and contained magnesium in A. mosanensis, or silicon in A. × grandiflora. Abelia stems with low foliar calcium concentrations showed no Ca oxalate crystal formation. This is the first report, to our knowledge, on the presence and possible growth of crystals of different morphologies and chemical compositions in Abelia.