Wist Tyler J, Davis Arthur R
Department of Biology, 112 Science Place, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2.
Ann Bot. 2006 Feb;97(2):177-93. doi: 10.1093/aob/mcj027. Epub 2005 Dec 9.
In spite of the impressive species diversity in the Asteraceae and their widespread appeal to many generalist pollinators, floral-nectary ultrastructure in the family has rarely been investigated. To redress this, a study using Echinacea purpurea, a plant of horticultural and nutraceutical value, was undertaken. Nectar secretion of disc florets was compared with floral nectary ultrastructure taking into account nectar's potential impact upon the reproductive success of this outcrossing species.
Micropipette collections of nectar in conjunction with refractometry were used to determine the volume and nectar-sugar quantities of disc florets throughout their phenology, from commencement of its production to cessation of secretion. Light, scanning-electron and transmission-electron microscopy were utilized to examine morphology, anatomy and ultrastructure of nectaries of the disc florets.
Florets were protandrous with nectar being secreted from anthesis until the third day of the pistillate phase. Nectar production per floret peaked on the first day of stigma receptivity, making the two innermost whorls of open florets most attractive to foraging visitors. Modified stomata were situated along the apical rim of the collar-like nectary, which surrounds the style base and sits on top of the inferior ovary. The floral nectary was supplied by phloem only, and both sieve elements and companion cells were found adjacent to the epidermis; the latter participated in the origin of some of the precursor cells that yielded these specialized cells of phloem. Companion cells possessed wall ingrowths (transfer cells). Lobed nuclei were a key feature of secretory parenchyma cells.
The abundance of mitochondria suggests an eccrine mechanism of secretion, although dictyosomal vesicles may contribute to a granulocrine process. Phloem sap evidently is the main contributor of nectar carbohydrates. From the sieve elements and companion cells, an apoplastic route via intercellular spaces and cell walls, leading to the pores of modified stomata, is available. A symplastic pathway, via plasmodesmata connecting sieve elements to companion, parenchyma and epidermal cells, is also feasible. Uncollected nectar was reabsorbed, and the direct innervation of the nectary by sieve tubes potentially serves a second important route for nectar-sugar reclamation. Microchannels in the outer cuticle may facilitate both secretion and reabsorption.
尽管菊科植物具有令人印象深刻的物种多样性，且对许多广食性传粉者具有广泛吸引力，但该科植物花蜜腺的超微结构却鲜有研究。为了弥补这一不足，我们以具有园艺和营养保健价值的紫锥菊为材料开展了一项研究。考虑到花蜜对这种异交物种繁殖成功的潜在影响，我们对管状小花的花蜜分泌情况与花蜜腺超微结构进行了比较。
使用微量移液器收集花蜜并结合折射测定法，以确定管状小花在整个物候期（从花蜜产生开始到分泌停止）的花蜜体积和花蜜糖分含量。利用光学显微镜、扫描电子显微镜和透射电子显微镜来检查管状小花蜜腺的形态、解剖结构和超微结构。
小花为雄蕊先熟，花蜜从开花期一直分泌到雌蕊期的第三天。每朵小花的花蜜分泌量在柱头可接受性的第一天达到峰值，使得最里面的两圈开放小花对觅食访客最具吸引力。改良的气孔沿着环绕花柱基部并位于下位子房顶部的领状蜜腺的顶端边缘分布。花蜜腺仅由韧皮部供应养分，筛管分子和伴胞都位于表皮附近；伴胞参与了一些产生这些特殊韧皮部细胞的前体细胞的起源。伴胞具有壁内突（传递细胞）。叶状细胞核是分泌薄壁细胞的一个关键特征。
线粒体丰富表明存在外分泌机制，尽管高尔基体小泡可能参与了颗粒分泌过程。韧皮部汁液显然是花蜜碳水化合物的主要来源。从筛管分子和伴胞出发，通过细胞间隙和细胞壁的质外体途径，通向改良气孔的孔口是可行的。通过胞间连丝连接筛管分子与伴胞、薄壁细胞和表皮细胞的共质体途径也是可行的。未被采集的花蜜会被重新吸收，筛管对蜜腺的直接支配作用可能是花蜜糖分回收的第二条重要途径。外表皮中的微通道可能有助于分泌和重新吸收。
