Nectar production and nectar sugar composition of three oilseed rape (Brassica napus) cultivars in Hungary

In May 005, nectar production and sugar composition was studied in Hungary in three oilseed rape (Brassica napus L. var. napus) cultivars ('Baldur', 'Bekalb Catonic', and 'Colombo'). Nectar production was determined after isolation of flowers using a covering of tulle netting for 24 hours. Volume of nectar was measured with calibrated microcapillaries, and the sugar concentration in nectar was determined with a hand refractometer. Nectar sugar composition was analysed with thin layer chromatography and densitometry. In general, the young and pollen-shedding flowers were the best nectar producers, but in some cases, nectar was found in blossoms in the balloon stage, as well as in old flowers where all anthers had dehisced. In 113ekalb Catonic', both nectar volume and refraction increased in the order of balloon stage, young open flower, pollen-shedding flower. Pollen-shedding flowers, compared with young flowers, produced over twice as much nectar on 14 May under sunny and windy weather conditions and four times more nectar on 15 May 2005 under overcast conditions. Similar trends were observed in the other two cultivars, suggesting that weather conditions had a pronounced effect on nectar production/flower and nectar sugar concentration. Flowers were visited by bees only in sunny, dry weather, either because the sugar concentration of the nectar was more attractive for them or because weather conditions were more favourable for bee activity. Only hexoses could be detected in the nectar of the cultivars, and the amount of glucose corresponded roughly to that of fructose, with the exception of 'Baldur', which had a relatively high glucose content. The glucose: fructose ratio reached or exceeded 0.95 in all studied cultivars, indicating that resulting honeys would tend to granulate readily. Therefore, none of the rape cultivars produced nectar wih a low glucose: fructose ratio that would be suitable to produce less readily granulating rape honey.