Influence of floral visitation on nectar-sugar composition and nectary surface changes in Eucalyptus

Floral nectaries and their production of major nectar carbohydrates were studied in three species of Eucalyptus in Australia. In E cosmophylla, E grandis and E pulverulenta, the nectary is located on the inner surface of the hypanthium, below the stamen filaments. Nectary surfaces possessed hundreds of modified stomata that were solitary, distributed uniformly, asynchronous in development, and served as exits for nectar flow. Nectar yields per bagged flower were greatest in E cosmophylla and least in E grandis, correlating with flower size but not nectary stomatal density. The nectar of E pulverulenta was sucrose-rich, but hexose-rich for the others. Few changes in nectar carbohydrate composition were detected between flowers whether protected or continually exposed to visitors leg, honeybees), and whether young or old, indicating an overall constancy in composition for the long period of nectar availability.