Nectary structure and ultrastructure in two floral morphs of Koelreuteria elegans subsp. formosana (Sapindaceae)

- Floral nectar is secreted through nectarostomata and the outer epidermal cell walls.
- The mixed mechanism would be correlated with the reduced number of nectarostomas.
- Nectar concentration was higher than 30% in both floral morphs.

The structure and ultrastructure of the floral nectary of staminate and pistillate flowers of Koelreuteria elegans subsp. formosana (Sapindaceae, Koelreuterieae) were studied. In both floral morphs, the floral nectary is extrastaminal, receptacular, thimble-shaped and persistent. The anatomical analysis revealed a differentiated secretory parenchyma and a less developed, inner non-secretory parenchyma. These anatomical features reveal that the nectary is structured, and is supplied by phloem traces derived from the central stele. In the middle of the floral nectary there are few modified stomata. The nectarostomata were of anomocytic type; they may occur at the level of or sunken below the adjacent epidermal cells. Statistically significant differences were observed between both floral morphs in relation to nectar volume and the different moments of day, whereas the sugar concentration showed no significant differences. Ultrastructural studies showed no difference between staminate and pistillate flowers. At anthesis, amyloplasts and lipid globules were observed in the secretory parenchyma cells, whereas at post-anthesis the secretory parenchyma cells were degenerated. Floral nectar is secreted through nectarostomata and the outer epidermal cell walls. These results are discussed in relation to other species belonging to different tribes within Sapindaceae.