Development of ovule, fruit and seed ofXyris(Xyridaceae, Poales) and taxonomic considerations

Abstract

The development of the ovule, fruit and seed of Xyris spp. was studied to assess the embryological characteristics of potential taxonomic usefulness. All of the studied species have (1) orthotropous, bitegmic and tenuinucellate ovules, with a micropyle formed by both the endostoma and exostoma; (2) a cuticle in the ovules and seeds between the nucellus/endosperm and the inner integument and between the inner and outer integuments; (3) helobial, starchy endosperm; (4) a reduced, campanulate and undifferentiated embryo; (5) a seed coat formed by a tanniferous endotegmen, endotesta with thick-walled cells and exotesta with thin-walled cells; and (6) a micropylar operculum formed from inner and outer integuments. The pericarp is composed of a mesocarp with cells containing starch grains and an endocarp and exocarp formed by cells with U-shaped thickened walls. The studied species differ in the embryo sac development, which can be of the Polygonum or Allium type, and in the pericarp, which can have larger cells in either endocarp or exocarp. The Allium-type embryo sac development was observed only in Xyris spp. within Xyridaceae. Xyris also differs from the other genera of Xyridaceae by the presence of orthotropous ovules and a seed coat formed by endotegmen, endotesta and exotesta, in agreement with the division of the family into Xyridoideae and Abolbodoideae.

Seed micromorphology and its taxonomic significance to Xyris (Xyridaceae, Poales)

“…The term ''reticules'' used by Silva (2010) and ''crosslines'' used by Mota et al (2015) correspond to the ''longitudinal and transversal striations'' reported in the present study and the ''reduced or evident intercostal spaces'' (Silva 2010) are herein described by the number of cell rows between the ridges. According to our results, the terminology proposed here is the most suitable since it is based on the ontogeny of the seed structures (Nardi et al 2015) and refers to characteristics that are easily observed.…”

“…It should be emphasized that the longitudinal ridges observed in the seeds of the studied species were also reported for all species of Xyris analyzed in previous studies (Rudall and Sajo 1999;Silva 2010) and are thus a pattern for the genus. An anatomical study of the ovule and seed development of Xyris asperula Mart., X. cipoensis L.B.Sm and Downs, X. nubigena Kunth, X. pterygoblephara Steud., X. pilosa Kunth., and X. subsetigera Malme (Nardi et al 2015) has demonstrated that the cells of the exotegmen degenerate, while some cells of the inner layer of inner integument (endotegmen) elongate radially, forming ridges in the seeds, termed here as endotegmic ridges. In Abolboda, the seed coat ornamentation is given by the cells of the exotegmen, which do not degenerate, forming exotegmic ridges (Carlquist 1960;Oriani and Scatena 2014).…”

“…The anatomy of the seeds was studied for species of Abolboda, Orectanthe (Carlquist 1960;Oriani and Scatena 2014) and Xyris (Weinzieher 1914;Rudall and Sajo 1999;Nardi et al 2015) and proved to be useful taxonomically. The importance of seed coat ornamentation for the taxonomy of Xyris has already been demonstrated in the literature (Wanderley 1992;Silva 2010;Mota et al 2015).…”

“…It is worth pointing out that the knowledge of the ontogeny of the seed is very important for the analysis of micromorphology because it facilitates the understanding of the origin of the tissues responsible for seed coat ornamentation. Nardi et al (2015) studied the ontogeny of seed coat of six species of Xyris and demonstrated that the mature seeds present exotesta, endotesta, and endotegmen, and that the endotegmen undergoes radial elongation, forming ridges in the seed surface.…”

“…In light of the large number of species of Xyris and the difficulty in identification because of the morphological similarity (Wanderley 2011), this study aimed to identify seed micromorphological characteristics with taxonomic importance to identify species in the genus. Based on a previous study of the seed development in Xyris (Nardi et al 2015), a standard terminology is proposed to describe the seed coat in species of this genus.…”