Anatomical study of the seed of andiroba (Carapa guianensis) through stereo and scanning electron microscopy

Abstract

The points of susceptibility of seeds to deterioration are important factors, especially when stored in hot and humid tropical conditions. The seeds of andiroba (Carapa guianensis) are an important resource for Amazonian traditional communities, as the oil extracted from the seeds is widely used in popular medicine and the cosmetic industry. Yet andiroba seeds are subject to fungal spoilage, which affects the quality of the oil. We analyzed the morpho-histological characteristics of whole andiroba seeds by stereo and scanning electron microscopy to identify susceptibility points to fungi. The shell (tegument), a thick lignin-rich, protective wall structure, varied in the type and number of tissue layers, with polygonal and long shaped cells presenting pores (plasmodesmata). The junction of the three faces of the seeds at the top formed a relatively tight small cavity, the micropyle, rich in vascular bundles of helically and tracheid-shaped vessels. An interchange channel from the pod through the micropyle to the inner seed is formed between the shell and a thin brown skin towards the cotyledons, with its surface covered with adhered residues of dry placental tissue. The seed cotyledons (cross and longitudinal sections), presented different cell layers containing randomly distributed lipid droplets. We concluded that the highly irregular surface of the micropyle and channel at the seed top, which forms deep recesses and accumulates tissue residues, presents the righest risk for fungi conidia deposition and/or moisture absorption, which leads to spoilage.