Investigation of Selected Compounds in Nectar and the Antioxidant, Amylase and Glucosidase Inhibitory Capabilities of Canna Flower Nectar (Canna × generalis)

Floral nectar is not only an important nutritional source for pollinators but also contains numerous biologically valuable compounds. This study evaluated the nectar characteristics, phytochemical composition, and biological potential of four Canna × generalis cultivars (CG-1, CG-2, CG-3, and CG-4) at different times throughout the day. Nectar volume and soluble solids content (°Brix) were measured. Phytochemical constituents were screened using spectrophotometric methods, while total phenolic, flavonoid, and alkaloid contents were determined using the Folin–Ciocalteu, aluminum chloride, and bromocresol green methods, respectively. Antioxidant activity was evaluated using DPPH, ABTS•+, and FRAP assays. In addition, α-amylase and α-glucosidase inhibitory activities and in vitro cytotoxicity against human liver cancer cells (HepG2) and breast cancer cells (MCF-7) of the flower nectar were also determined. The results showed that floral nectar volume ranged from 40.76 to 92.39 µL per flower and reached the highest value in the morning, whereas soluble solids content ranged from 18.76 to 30.81 °Brix and gradually increased from midday to afternoon. Phytochemical analyses revealed the presence of various groups of secondary metabolites, including polyphenols, flavonoids, tannins, coumarins, carotenoids, saponins, quinones, and terpenoids. The nectar collected in the morning from the CG-4 cultivar exhibited the highest phenolic content (12.02 ± 0.06 µg GAE/mL) and alkaloid content (2.66 ± 0.00 µg AE/mL), while the highest flavonoid content was recorded in the nectar of CG-4 recorded in the afternoon (8.57 ± 0.09 µg QE/mL). All samples exhibited antioxidant activity and inhibitory activity against α-amylase and α-glucosidase. The nectar of CG-2 collected in the afternoon showed the strongest DPPH radical scavenging activity (IC50 = 19.60 ± 0.09 µg/mL), whereas CG-2N and CG-3M exhibited the strongest ABTS•+ radical scavenging activity, with no significant difference between them (CG-2N: IC50 = 13.04 ± 0.08 µg/mL; CG-3M: 13.91 ± 0.30 µg/mL). The nectar of CG-2M at morning showed the strongest α-glucosidase inhibitory activity (IC50 = 0.11 ± 0.00 µg/mL). In addition, CG-4M and CG-4A exhibited the strongest ferric reducing activity, which exceeded the linear range of the assay at the 50% concentration, while CG-4M showed the strongest α-amylase inhibitory activity (IC50 = 78.49 ± 0.14 µg/mL). CG-2 showed no significant cytotoxicity against HepG2 and MCF-7 cells (IC50 > 128 µg/mL). Overall, these findings warrant further investigation of Canna × generalis floral nectar as a potential source of bioactive compounds with in vitro antioxidant and digestive-enzyme inhibitory activities.