The origin of nectar sugars is sucrose, which is transported through the phloem to nectar-producing tissues. In some plants, sucrose derived from photosynthesis may be temporarily stored as starch in these tissues and then broken down. This process allows the plant to secrete nectar more rapidly, as observed in squash flowers (Cucurbita pepo) with their high nectar production (Solhaug et al., 2019).

Analysis of phloem sap and nectar shows that invertase enzymes in the cell walls of nectar-producing tissues break down sucrose differently during nectar secretion. This breakdown regulates the ratio of the three main sugars and helps maintain the sucrose concentration gradient (Tiedge and Lohaus, 2018).

Water influx due to the higher osmolality of hexose solutions (glucose and fructose) leads to the production of very dilute and abundant nectars. These types of nectars, such as those in Aloe and Erythrina species, are often adapted to generalist pollinators like birds (Minami et al., 2021). A new model of nectar secretion shows how modulating the activity of cell wall invertase can explain differences in nectar volume and sugar composition (Minami et al., 2021).

Although sucrose hydrolysis should result in a 1:1 ratio of glucose and fructose, the imbalance observed in fresh nectar indicates the role of other biochemical pathways in nectar production. Additionally, the sugar profile may be altered by nectar microbes (we will discuss this in the next section). The reabsorption of sugars is also a mechanism that likely helps maintain homeostasis or recover the plant's investment in nectar production, but this process is not yet fully understood (Nepi et al., 2011).

From the perspective of honeybees, the conversion of sucrose to glucose and fructose by invertase enzymes is crucial for the digestibility and nutritional value of nectar. Honeybees, as key pollinators, rely on the simple sugars in nectar to meet their energy needs. The ratio of glucose and fructose in nectar can influence the feeding preferences of honeybees and, consequently, their selection of nectar sources. Additionally, the dilution or concentration of nectar affects the energy required by bees to collect and process it.

Furthermore, the presence of microbes in nectar and changes in the sugar profile can impact the quality and shelf life of nectar in the hive. The reabsorption of sugars by the plant can also reduce the nutritional value of nectar for bees. Therefore, a deeper understanding of the biochemical processes involved in nectar production is essential for maintaining the health and productivity of honeybees and managing nectar resources in beekeeping areas.

Compared to the three dominant sugars, other sugars in nectar are usually present in very small amounts. For example, maltose constituted only 2.5% of the sugars in the nectar of P. caeruleum and was even absent in some populations (Ryniewicz et al., 2020). However, the pentose sugar xylose can make up to 39% of the total sugars in the nectar of sister genera Protea and Faurea from the Proteaceae family [63]. Interestingly, xylose is metabolized by mammal pollinators (Nicolson and Wyk, 1998) and is abundant in the dilute nectars of Protea species pollinated by beetles (Jackson and Nicolson, 2002).

Sampling from field populations of the Greek valerian (Polemonium caeruleum) yielded results that differed from previous reports—based on a single population—which indicated high levels of sucrose and proline in the nectar (Ryniewicz et al., 2020). These findings suggest that generalizing results from one population to the entire species can be misleading.

Some argue that the observed association between nectar sugar composition and pollinator type is, in fact, a secondary result of flower morphology. Sucrose-rich nectars are more common in protected, tubular flowers visited by specialized pollinators (e.g., the Ericaceae family). In contrast, hexose-rich nectars dominate in open, exposed flowers visited by both generalist and specialist pollinators (e.g., the Asteraceae family) (Abrahamczyk et al., 2017).

This correlation, identified decades ago in a semi-quantitative study of nectar sugars in 900 plant species (Percival, 1961), can be explained by principles of physical chemistry. Hexose nectars have a much higher osmotic concentration compared to sucrose nectars of similar concentration; thus, they evaporate more slowly and are better balanced in dry air than sucrose nectars. This characteristic is important for flowers with long corollas that require nectar protection (Witt et al., 2013). Long-tongued pollinators, such as bees, butterflies, and moths, which can access nectar in protected flowers, appear to prefer sucrose-rich nectars.

The sugar composition of nectar in bird-pollinated flowers is linked to their feeding choices and digestive constraints. If sucrose in the nectar is not hydrolyzed within the flower, it must be broken down in the bird's gut before absorption. The invertase enzymes in plant nectaries are β-fructosidases, while animals use α-glucosidases, which are found in honeybees and nectar-feeding birds (Berenbaum et al., 2021). The activity of sucrase in the intestines of nectar-feeding birds matches the proportion of sucrose in the nectars they consume. This convergent coevolution is observed across continents in hummingbirds, nectar-feeding birds, and the flowers they pollinate (Berenbaum et al., 2021). Despite this association, these birds do not prefer sucrose over hexose nectars and even choose hexose nectars at low concentrations (McWhorter et al., 2021).

The methods used in preference tests are important because there are caloric and osmotic differences between sucrose and hexose: hexose solutions mixed on a weight percentage basis have about 95% the energy value of sucrose solutions (Fleming et al., 2004). Ultimately, the sugar composition of nectar may not be physiologically significant, as pollinators rapidly digest sucrose and efficiently absorb nectar sugars (Napier et al., 2013). A few exceptions include starlings and some acacia ants, which lack sucrase and avoid sucrose-containing floral and extrafloral nectars, respectively (McWhorter et al., 2021). Hummingbirds and bats use both fructose and glucose directly to support their high metabolic rates during hovering flight (Welch et al., 2018). In hawkmoths, glucose in nectar protects against oxidative stress caused by flight (Levin et al., 2017).