A Structural Investigation of Floral Nectaries in Disk and Ray Florets of Seven North American Aster Species (Asteraceae: Astereae)

This project examined nectary morphology and anatomy of several aster species belonging to the North American clade of Astereae. Inflorescences and foliage of Doellingeria umbellata (Miller) Nees, Eurybia divaricata (L.) G. L. Nesom, Solidago canadensis L., Solidago rugosa Mill., Symphyotrichum boreale (Torr. & A. Gray) Á. Löve & D. Löve, Symphyotrichum cordifolium (L.) G. L. Nesom, and Symphyotrichum oolentangiense (Riddell) G. L. Nesom were collected from southern Ontario. A species local to Saskatoon, Saskatchewan, Symphyotrichum laeve (L.) Á. Löve & D. Löve was also studied for its production of floral nectar. Nectaries in this family as a whole remain relatively understudied, despite Asteraceae being the largest family of flowering plants. This lack of information is possibly due to the diminutive size of nectaries in what are already small florets, found in species of this family. This study is unique in its investigation of nectary structure in both disk and ray florets; most other studies have tended to neglect ray florets when examining heterogamous species. \n Using scanning electron microscopy, each floret possessed an annular nectary surrounding the base of the style, atop the inferior ovary, which is typical in Asteraceae. The nectaries in disk florets were generally pentagonal, whereas nectaries in ray florets varied from pentagonal to mostly circular. Many nectarostomata were found on the surface of nectaries in disk florets of all species, but were absent on the nectaries of ray florets, except for D. umbellata. These nectarostomata are believed to be major points of nectar secretion; no other methods of nectar production (e.g., trichomes) or escape were found. The fact that all nectaries in ray florets were considerably reduced in size (height, diameter) and almost always lacked nectarostomata compared to their disk floret counterparts, strongly suggested that the nectaries in ray florets may not be functional (i.e., they do not produce nectar), even if they possess definite nectary tissue. \n Sectioning of resin-embedded florets not only allowed anatomical confirmation of the presence of nectariferous tissue (small, isodiametric parenchyma cells that stained densely) by light microscopy, but permitted assessment of the vascularization of nectaries in these taxa. The nectariferous tissue extended deeper than its junction with the corolla-staminal tube. Four species (Doellingeria umbellata, Eurybia divaricata, Solidago canadensis, Symphyotrichum boreale) lacked a direct vascular supply to their nectaries in both floret types. However, two species (Symphyotrichum cordifolium, S. oolentangiense) had phloem traces (consisting of sieve tube elements and companion cells) within nectaries of their disk florets, and the final species (Solidago rugosa) lacked vasculature in its disk florets, but was found to have phloem so close to its minute ray floret nectary that it is unclear whether the vascular tissue innervated the gland. For the two Symphyotrichum species with vascularized nectaries in disk florets, phloem in S. oolentangiense did not pass the mid-point of the glandular tissue (i.e., where the corolla attached to the nectary), whereas phloem originating from the vascular bundles of the adnate staminal filaments was found in multiple, vertically-directed strands throughout the disk floret nectary of S. cordifolium, often extending toward the guard cells at the nectary apex.\n For the nectar volume quantification of S. laeve, volume measurements were taken in the morning and afternoon for each floral stage. Measurements were taken for disk florets only, as rays did not produce enough nectar to measure volume properly. Nectar sugar concentrations could not be taken because the volume of nectar procured from the florets was too small to obtain a reading on the refractometers available. The floral nectar volume of florets in the staminate phase was 0.033 µl ± 0.010 (n=9), and 0.049 µl ± 0.013 (n=15) for florets in the pistillate phase. This small amount of nectar per floret suggests that foraging insects would need to visit many florets to obtain enough nectar to sustain them, and likely increases pollination effectiveness of foragers.

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