Transient starch metabolism in ornamental tobacco floral nectaries regulates nectar composition and release

Enlargement of the floral nectary gland of ornamental tobacco during its development is accompanied by a major accumulation of starch granules in nectary amyloplasts. Quantification of starch in the nectary at various developmental stages showed little starch accumulation at early stages but increasing amounts of starch over the course of nectary development that reached a peak approximately 24 h prior to anthesis. After this point, the amount of starch declined dramatically, suggesting its conversion to sugars (sucrose, glucose, and fructose) for the nectar production that occurs at anthesis. Compositional and structural analyses of nectary starch showed that amylose content and degree of amylopectin branching also varied during nectary development. Increasingly complex starch structures were observed up to intermediate stages of nectary development, followed by decreased starch complexity and amount in the mature nectary. Although the total amount of carbohydrate stored in the nectary at mid-development is roughly equivalent to the carbohydrate in nectar sugars at anthesis, four- to five-times more sugar is secreted into nectar following anthesis. Radiolabeling of sugars prior to their transport into the flower bud, nectary, and nectar showed the flow of sugars into the nectary increased markedly after anthesis. The finding that the nectary is the strongest sink tissue of all floral organs suggests that two processes, starch degradation and rapid sugar influx, are determinants of sugar composition in floral nectar. A model is presented in which these two processes are coordinated for high-level nectar production and release.