Nectar secretion and nectaries in basal angiosperms, magnoliids and non-core eudicots and a comparison with core eudicots

摘要

This comparative study combines original results with a review of the literature summarizing our present knowledge as regards nectaries in angiosperm flowers. The focus is mainly on the basal angiosperms (ANITA or ANA grade and magnoliids) which are evaluated at the family (and genus) level. In most orders at least one genus has been studied for the first time or was re-investigated. In the second part a comparison is made with members of the core eudicots (at the order level). In basal angiosperms, in a number of families and genera nectar is offered, though not in great amounts and in addition to pollen as a reward. Nectary sites are the undifferentiated perianth, staminodes ("petals"), stamens, and carpels. Staminodial nectaries dominate. Receptacular disc nectaries are absent. As regards histology, all types of differentiation - epithelial, mesophyllary and trichomatic nectaries - can be found, the epithelial ones being the most common type of nectar secretion. The diverse structures, the rare occurrence, and scattered distribution of nectaries in the basal groups indicate convergent evolution. What is the selective advantage of nectar secretion in pollen flowers? It has been proposed (Nepi et al. 2009) that nectar provision is an adaptation to insects which were already accustomed to sugary exudates (such as pollination drops of gymnosperms). An alternative (or supplementary) approach results from the observation that nectar production is limited mainly to the female phase of the proterogynous flowers. It may be concluded (see Erbar & Leins 2013) that the nectar assures the attractiveness of the flower in the non-pollen presenting phase. In addition, during searching for the sparse nectar the insect may come into contact with each stigma thus compensating the economic disadvantage of a choricarpous compared to a coenocarpous gynoecium.