Nectaries in Rosaceae.

The morphologically heterogeneous family Rosaceae is, among others, characterized by differently shaped floral axes through different growth of the same. A central conical proliferation bearing the choricarpous gynoecium (as in Fragaria, Geum, Potentilla or Rubus) may also be missing. Members of tribe Maleae (as Crataegus, Malus, Pyrus) have an inferior ovary, i.e. a gynoecial hypanthium. One character, however, distinguishes all representatives of Rosaceae: a perigynous hypanthium, on which upper rim perianth members and stamens are borne. Its shape varies from discto bowl- to tube- to urn-shaped. Studying the nectary in 19 genera (36 species) covering all presently accepted subfamilies and a large part of the tribes of Rosaceae (for six smaller tribes within Amygdaloideae reliable data were evaluated from the literature) we can conclude that if nectar is present in flowers of Rosaceae, it is secreted from a mesophyllary nectary located in or on the inner side of the perigynous hypanthium (receptacle). The differences of the hypanthial nectaries in the various genera/species lie only in the size of the nectary tissue, its exact location on the perigynous hypanthium, its degree of protrusion (flush or non-flush with the inner hypanthial wall), in the nectary slits (sunken between the epidermal cells or not), hairs around the nectary (present or absent on perigynous hypanthium or on/among carpels) and the striation of the nectary epidermal cells. We found no evidence of involvement of other tissues in the nectary. In many cases, the nectary extends from beneath the filament bases to the gynoecium. In some cases, filament bases interrupt the nectary and cause an undulating upper border of the nectary (e.g. in Crataegus, Fragaria, Geum). In Cydonia and Pyrus, a distinct proliferation of the upper part of the hypanthium bears part of the nectary and surrounds as annular bulge the styles, but stylar or ovarian tissue do not contribute to the nectary. Some genera can be characterized by a unique position of the nectary in correlation with the shape of the perigynous hypanthium. In Geum and Waldsteinia (Colurneae), the nectary is formed as a groove in front of the inner filaments along the upper edge of the perigynous hypanthium. The genus Rosa is characterized by an urn-shaped perigynous hypanthium, whose throat usually is almost closed by a thick proliferation of the floral axis that may be the site of only remnants of a nectary. The urn-shaped perigynous hypanthium of Rosa, unique in Rosaceae, forces the long styles of the individual carpels to "squeeze" through the narrow orifice. This achieves the advantageous close proximity of the stigmata, so that the capitate stigmas an outer compitum, a compensation of the disadvantage of a choricarpous gynoecium. What is the selective advantage of nectar secretion in pollen flowers? Nectar production In Rosaceae may be poor and limited mainly to the female phase of the mostly proterogynous flowers. In this way, the attractiveness of the flower is also assured in the non-pollen presenting phase. In addition, during searching for the sparse nectar the insect may come into contact with several stigmata thus compensating the economic disadvantage of a choricarpous compared to a coenocarpous gynoecium. [ABSTRACT FROM AUTHOR]