Abstract
Numerous angiosperms rely on pollinators to ensure efficient flower fertilization, offering a reward consisting of nourishing nectars produced by specialized floral cells, known as nectaries. Nectar components are believed to derive from phloem sap that is enzymatically processed and transformed within nectaries. An increasing body of evidence suggests that nectar composition, mainly amino acids, may influence pollinator attraction and fidelity. This notwithstanding, little is known about the range of natural variability in nectar content for crop species. Sugar and amino acid composition of nectar harvested from field-grown plants at the 63–65 phenological stage was determined for a set of 44 winter genotypes of rapeseed, a bee-pollinated crop. Significant differences were found for solute concentrations, and an even higher variability was evident for nectar volumes, resulting in striking differences when results were expressed on a single flower basis. The comparison of nectar and phloem sap from a subset of eight varieties pointed out qualitative and quantitative diversities with respect to both sugars and amino acids. Notably, amino acid concentration in phloem sap was up to 100 times higher than in nectar. Phloem sap showed a much more uniform composition, suggesting that nectar variability depends mainly on nectary metabolism. A better understanding of the basis of nectar production would allow an improvement of seed set efficiency, as well as hive management and honey production.
Highlights
Many plants require pollinator visitation to obtain efficient seed set
Harvesting was carried out in the early to mid-afternoon, when the relative humidity was lower (Figure S3), to exclude contamination by dew and to allow the attainment of steady-state nectar production, which is dependent on the establishment of full photosynthetic rate and phloem loading
The results showed a highly significant (P = 0.000000) difference among cultivars with respect to nectar volume, which ranged from 20 to 750 nL flower−1 (Table 2)
Summary
Many plants require pollinator visitation to obtain efficient seed set. Dicotyledonous species often attract pollinators by offering them a reward of floral nectars. The nectar is a nutrient-rich aqueous solution of sugars, amino acids, organic acids, proteins, fats, vitamins, minerals, and other minor components, such as proteins with high antimicrobial activity (Nicolson and Thornburg, 2007). It is derived from the phloem sap and is produced by a group of specialized cells, the nectaries, usually present in the flower at the base of the petals (De La Barrera and Nobel, 2004). Its composition can vary greatly depending on plant species and environmental conditions (Herrera et al, 2006), as well
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