Floral nectar secretion and ploidy in Brassica rapa and B napus (Brassicaceae). I. Nectary size and nectar carbohydrate production and composition

Haploid (n = 10), diploid (2n = 20) and tetraploid (4n = 40) lines of Brassica rapa (syn campestris), and a line of allotetraploid (4n = 38) B napus, were examined to determine whether ploidy can influence nectar production. Flowers of all lines developed functional nectaries. Overall, nectar carbohydrates consisted almost exclusively of glucose and fructose, present in quantities slightly in favour of the former. Sucrose was detected in only 15% of samples, usually in trace amounts. For all levels of ploidy, 95% of total nectar carbohydrate per flower was expelled from the lateral (inner) pair of glands. These glands were directly supplied with phloem alone, whereas the median (outer) glands, which were poor nectar yielders, usually did not receive any vascular supply. Haploids only produced 30% as much nectar carbohydrate as 2n and 4n lines of B rapa, which in tum exuded only 44-50% of the average quantity of nectar carbohydrate released by B napus. A linear regression (r = 0.803) of mean lateral-nectary volume on average total nectar-carbohydrate per flower was determined for all plants of B rapa, but this was modified (r = 0.445) when data for B napus were included. In all lines, opportunity exists for selection for high nectar-carbohydrate production. Plants yielding the most floral nectar carbohydrate had high frequencies (80-95%) of lateral glands that were symmetrical and of uniform size within a flower