Sugar secretion from the nectary ofStrelitzia: an ultrastructural and physiological study

Summary

Flowers ofStrelitzia reginae grown at a constant 20°C have been shown to secrete nectar at a rate of up to 5.0 mg (d.w.) sugar h⁻¹ (mean rate 1.2±0.1 mg h⁻¹) for up to seven days. The nectar has a total concentration of about 25% during the early part of the secretory period but often falling to less than 10% towards the end of secretion.

Each flower has three septal nectaries, the cuticle-lined ducts of which open into a nectar basin formed by the fused bases of two petals on the top of the receptacle. The layer of epithelial cells which secretes the sugars is thrown into highly convoluted folds and the distal parts of these cells have profuse wall inpushings. Both of these modifications have the effect of increasing the surface area of the plasmalemma apparently available for unloading the nectar. The glandular epithelium of the three, 26 mm long, nectaries of a single flower would be lined by more than 17×10⁶ cells with a total plasmalemma surface area for unloading of at least 2,000 mm². There is little evidence to suggest that secretion is a granulocrine process inStrelitzia. While there is abundant, stacked endoplasmic reticulum, and numerous vesicles containing fibrillar material, these do not appear to be directly concerned with sugar secretion. Data from specific flowers suggest that transmembrane fluxes in the range of 1.0×10⁶ to 1.0×10⁻⁷ mol s⁻¹ m⁻² would be necessary to sustain the observed rates of secretion. While these are relatively high, when taken together with the structural information, they lead to the conclusion that secretion inStrelitzia is probably an eccrine process.