Patterns of nectar and pollen presentation influence the attractiveness of four raspberry and blackberry cultivars to pollinators

Floral rewards such as nectar and pollen have a major influence on successful pollination and fruit-set, even in self-fertile Rubus sp. (Rosaceae), which include most cultivated raspberries and blackberries. A 2-year study was designed to examine the diurnal dynamics of nectar secretion and pollen shedding in the raspberry cultivars 'Autumn Bliss' and 'Mailing Exploit', and in the blackberry cultivars 'Arapaho' and 'Hull', in order to establish the effect of these traits on floral attractiveness to various insect visitors. The volume of nectar secreted per flower was measured using calibrated capillaries, and the concentrations of sugars in the nectar were measured using a hand-held refractometer. The four Rubus cultivars differed in their patterns of nectar and pollen presentation. Volumes of nectar and concentrations of sugars were influenced by year, cultivar, age of the flower, and time of sampling. The four cultivars exhibited different patterns of nectar secretion over the 2 years. In year-1, an early morning peak in nectar volume was followed by a rapid decline, while the concentrations of sugars increased during the day. In year-2, both the volume of nectar and the concentrations of sugars increased from the morning to the afternoon. The patterns of pollen presentation were similar in both years of study. Anther dehiscence took only 3 - 8 h per flower in the poorest nectar producer, 'Arapaho'; while, in cultivars with higher nectar yields, pollen shedding lasted for 6 10 h. Pollen-shedding flowers secreted significantly higher volumes of nectar, with significantly lower concentrations of sugars, compared to pre-pollen-shedding flowers in year-1. The concentrations of sugars in the nectar were higher in both raspberry cultivars than in the blackberry cultivars, but both blackberry cultivars were as attractive to bees due to their more appealing nectar sugar composition. Bees were more attracted to flowers of cultivars which simultaneously offered ample nectar and some pollen (i.e., 'Autumn Bliss', 'Malting Exploit', and 'Hull') than to flowers with a rapid rate of pollen shedding, but a low nectar yield (i.e., 'Arapaho'). Higher rates of bee-mediated cross-pollination may provide economically important increases in fruit yields and better quality fruit.