Intracellular localization of calcium, phosphorus and nitrogen in common bean seeds (Phaseolus vulgaris L. cv. Borlotto) by SEM, ESI and EELS techniques

Abstract

ABSTRACT Scanning, transmission electron microscopy, electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS) were used to localize calcium, phosphorus and nitrogen in seeds of Phaseolus vulgaris. Prismatic, straight and kinked twin calcium oxalate crystals were detected in the crystalligen layer of seed teguments. Cytoplasmic calcium and phosphorus-rich particles, probably of phytin, were detected in cotyledon parenchyma cells by ESI and EELS. Moreover, higher levels of calcium were mapped on osmiophilic droplets close to cell walls. Crystals, cytoplasmic phytin particles, osmiophilic droplets and calcium in cell walls disappeared after treatment with critic acid. By chemical assays, 1.1 g/100 g of phytic acid was detected in control seeds, and an 81% decline was observed in citric acid-treated samples. These data suggest that crystals and phytin complexes could play a role in determining the low calcium availability in common beans. Also, nitrogen found in protein bodies of cotyledons could represent an important site of protein reserves.