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The Role of Nectar Chemistry in Pollinator Health: A Comprehensive Guide
Study of Chemical Composition and Quality of Flower Nectar
Prepared by: Dr. Gholamali Halako
Researcher at the Golstan Agricultural and Natural Resources Research and Education Center
Nectar: A Gateway to Life
Nectar, this sweet and delicious liquid, has always been known as the primary attraction of flowers for pollinators, especially our beloved honeybees. For us beekeepers, nectar is not only the raw material for producing precious honey but also the vital fuel for the survival and activity of our colonies. But is the quality of nectar solely determined by its sugar content and energy?
Assuming that nectar quality is only about sugar is a significant oversimplification. The world of nectar is far more complex and fascinating than these assumptions. The incredible diversity in the composition and concentration of nectar indicates the presence of valuable non-sugar compounds that play vital roles in the nutrition and health of pollinators. Essential amino acids, secondary metabolites with amazing properties, and even the water content in nectar are all determining factors that we should not overlook. Many of these compounds have indirect effects on bees, from altering their feeding behavior to strengthening their immune system and protecting them against diseases. In this context, the water component of nectar, often overlooked due to evaporation and the difficulty of sampling small volumes, holds special importance.
In the following sections, we will take a closer look at each of these factors and discuss their role in the quality of nectar and, ultimately, the health and productivity of honeybee colonies. Join us as we open a new window into the fascinating world of nectar!
Nectar: A Fluid and Highly Variable Liquid
Nectar is a complex and dynamic liquid that has been described as highly variable due to constant changes in its composition and concentration. Imagine! The quantity and quality of nectar (both in terms of volume and chemical composition) can vary significantly. These differences in chemical composition also affect the health and lifespan of honeybees. These changes occur under the influence of various factors:
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Passage of Time After Initial Secretion: Freshly secreted nectar may undergo chemical changes over time.
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Pollinator Visits: The collection of nectar by insects and other pollinators naturally reduces its quantity.
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Microorganism Activity: Microbes can enter nectar and influence its composition.
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Weather Changes: Environmental conditions such as temperature and humidity can also affect the properties of nectar.
Table 1: Effects of Nectar Chemical Composition on Pollinator Health (Click on the row below to view the table)
Nectar Components | Effects on Pollinator Health |
---|---|
Sugars | Energy source for flight, thermoregulation, and growth, including endotherms with high metabolic rates |
Some birds (e.g., starlings) cannot digest sucrose | |
Glucose and fructose reduce oxidative damage in hawkmoths | |
Amino Acids | Non-essential amino acids often predominate, and ratios vary greatly; nutritional benefits are largely unknown |
May affect the taste of nectar | |
Amino acid preferences may influence sugar intake | |
Metabolized during flight | |
Pharmacological effects of non-protein amino acids may benefit the plant but not the pollinator | |
Proteins | Nectar preservatives |
Fatty Acids | Metabolized by hawkmoths |
Salts | Contribute to salt balance |
High K+ is deterrent (e.g., onion); may affect energy intake | |
Vitamin C (Ascorbic Acid) | Reduces oxidative damage |
Secondary Metabolites | Nectar preservatives |
Antioxidants, e.g., phenolics | |
Antiparasitic action on Crithidia in bumblebees or Nosema in honeybees | |
Pharmacological effects of caffeine and nicotine cause bees to overestimate nectar quality | |
Deterrents to nectar robbers or competing pollinators; preferred pollinator access is enhanced | |
Quercetin upregulates detoxification genes | |
Water | Excess water in dilute nectars must be removed |
Viscosity affects drinking rates | |
Water source in dry environments |

A Short Journey into the Chemical Complexity of Nectar
In the 1970s, the pioneering research of Herbert and Irene Baker opened a new window into the world of nectar chemistry. They drew researchers' attention to the diversity and abundance of non-sugar solutes in nectar. Prior to this, in many studies, the chemical composition of nectar was considered constant for a plant species, which facilitated the search for general patterns.
Numerous studies have examined the influence of various factors, including the evolutionary relationships of plants (phylogeny) and pollination type, on the composition of nectar sugars. Most of these studies are based on the analysis of one or a few nectar samples from each plant species or pooled samples (in cases of limited nectar volume). However, it is worth noting that pooling samples can obscure variations in nectar sugars at the species level.
A study conducted by Herrera et al. (2006) on the species Helleborus foetidus clearly demonstrated this variability. In fact, phenotypic variations in nectar traits are common both among flowers of a single plant and among different populations of a species (Parachnowitsch et al., 2019). For example, research by Gijbels et al. (2014) revealed significant variations in sugar and amino acid levels among flowers of a single plant of the species Gymnadenia conopsea. In this study, 45% of the variation in nectar traits was observed among flowers of a single plant, and 20% among different populations of the species.
The diversity of nectar components, particularly sugars and amino acids, is of great importance for honeybees as key pollinators. Honeybees rely on nectar to meet their energy and protein needs. Variations in the ratio and concentration of sugars can influence the feeding preferences of honeybees and, consequently, their selection of nectar sources. Additionally, amino acids play a vital role in larval growth, protein production, and strengthening the immune system of honeybees.
This diversity, besides meeting the nutritional needs of honeybees, can also affect their nectar-collecting behavior, the process of converting nectar into honey, and their resistance to diseases and pests. Therefore, a deeper understanding of the factors influencing nectar diversity is essential for maintaining the health and productivity of honeybees and managing nectar resources in beekeeping areas.
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.
A study by Bertazzini and Forlandi in 2016 showed that the nectar of plants grown in greenhouses (compared to outdoor environments) differs in terms of solutes. According to Canto (2007), the greenhouse environment limits chemical variations in nectar, but it may not accurately represent nectar in natural environments.
Fortunately, recent advances in transcriptomics and metabolomics have enabled a more detailed examination of the mechanisms of nectar secretion. These studies have led to the discovery of new metabolites in nectar that were previously unknown (Chatt et al., 2021).
In the following sections, we will take a closer look at the role of various nectar compounds in the nutrition and health of pollinators. Stay tuned to Honey Hub to learn about the latest scientific findings on this sweet reward of nature!
Main Sugars in Nectar
The greatest nutritional value of nectar is attributed to the presence of its three simple sugars:
- Sucrose: A disaccharide composed of two monosaccharide units, glucose and fructose.
- Glucose: A six-carbon monosaccharide.
- Fructose: Another six-carbon monosaccharide.
In this section, the main sugars are explained in detail, followed by an examination of the effects and quality of sugars in nectar.
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).

2. Amino Acids: Secondary Nutrients with Vital Roles
Amino acids, despite their lower concentration compared to sugars, play crucial roles in flower nectar. The total concentration of amino acids typically ranges from micromolar to millimolar, while the concentration of sucrose in a 30% weight solution is approximately 1 molar. Studies have shown that the total concentration of amino acids in the nectar of 30 insect-pollinated plant species in the UK varied between 0.19 and 12.7 millimolar (Gardener and Gillman, 2001).
Simultaneous analysis of nectar and phloem sap in oilseed rape (Brassica napus) and other species has revealed that, despite similar total sugar concentrations in nectar and phloem sap, the total amino acid concentration in nectar is about two orders of magnitude lower than in phloem sap (Bertazzini and Forlani, 2016). This suggests that amino acids are retained in the nectary tissues during the nectar secretion process.
Amino acids in nectar play multiple roles in the nutrition and health of bees. These compounds, as building blocks of proteins, are essential for larval growth, royal jelly production, and strengthening the immune system of bees. A deficiency of amino acids in nectar can lead to reduced larval growth, weakened adult bees, and increased susceptibility to diseases.
Additionally, the ratio and type of amino acids in nectar can influence the feeding preferences of bees and, consequently, their selection of nectar sources. Some amino acids, such as proline, act as feeding stimulants for bees and can enhance nectar uptake.
Given the importance of amino acids in honeybee nutrition, a more detailed examination of amino acid composition in the nectar of various flowers and the impact of environmental factors on their concentration is essential for maintaining the health and productivity of honeybees.
More important than the total concentration of amino acids is their composition, which varies significantly in the nectar of different plant species (Petanidou et al., 2006). All 20 common amino acids found in proteins are present in nectar. Sometimes, the amino acid profile is heavily skewed toward non-essential amino acids, which are predominant in phloem sap. Four amino acids—glutamine, glutamate, asparagine, and aspartate—play a significant role in plant nitrogen metabolism (and have high nitrogen-to-carbon ratios) and are relatively abundant in nectar. In addition, alanine, serine, glycine, and proline, which are all non-essential amino acids, are also commonly found in nectar (Nicolson, 2007).
Transcriptomics studies on the nectary tissues of cotton plants have revealed high expression of genes that use glutamate as a substrate for the biosynthesis of other amino acids, such as aspartate (Chatt et al., 2021). On the other hand, one or two essential amino acids may dominate the amino acid profile of nectar. For example, phenylalanine is abundant in the nectar of Mediterranean plants primarily pollinated by bees, constituting up to 47% of the total amino acids in the Lamiaceae family (Petanidou et al., 2006). The ratio of essential amino acids to total amino acids varies widely among different plant species, for example, ranging from 6% to 48% in 20 species of the genus Nicotiana.
Non-protein amino acids, such as taurine, gamma-aminobutyric acid (GABA), and beta-alanine, may also be present in high concentrations in some nectars and are sometimes classified as secondary metabolites (Nepi, 2014).
Pollen can be a potential source of amino acids in nectar, especially since it is significantly richer in these compounds (e.g., up to 1000 times more than nectar per flower (Descamps et al., 2021)). Contamination of nectar by pollen that falls into it can lead to high levels of proline and other amino acids (Descamps et al., 2021). While deliberate exposure of pollen to the nectar of Aloe marlothii did not increase its amino acid content (Nicolson, 2007), adding sunflower pollen to synthetic nectar caused amino acids to leach from the pollen into the nectar. Additionally, pollen contamination in visited flowers of Gentiana lutea enriched the amino acid profile of the nectar (Bogo et al., 2021).
Recent studies have shown that bacteria in nectar can induce pollen germination and bursting, thereby increasing protein (and ultimately amino acid) levels (Christensen et al., 2021). The risk of contamination is high for flowers with low nectar volumes, especially when the sampling method is highly sensitive for amino acid analysis (Power et al., 2021).
The functional importance of amino acids in nectar for pollinator health is not yet fully understood. It appears that their direct nutritional value is limited for most pollinator groups. Baker and Baker (1986) sought to identify a connection between pollinator type and nectar amino acids, suggesting that flowers pollinated by butterflies contain higher levels of amino acids because adult butterflies lack other nitrogen sources. However, no clear relationship between amino acid concentration or composition and pollination syndrome has been observed in the diverse genus Impatiens (including butterfly-pollinated species) (Vandelook et al., 2019).
Bees and hoverflies obtain their required amino acids from pollen, while nectar-feeding birds use arthropods and sometimes pollen as protein sources. It remains unclear why species of Erythrina pollinated by passerine birds have significantly higher amino acid concentrations compared to those pollinated by hummingbirds. Interestingly, nectar-feeding birds (Cinnyris talatala) do not prefer artificial nectar containing amino acids.
In addition to direct nutritional benefits, amino acids may contribute to the taste and attractiveness of nectar and influence the feeding choices of pollinators. This is a complex issue, as individual amino acids may have attractive or repellent effects that are masked in a complex mixture.
Interestingly, a newly identified taste receptor in honeybees responds to glutamate, aspartate, asparagine, and glutamine (the main nitrogen-transporting amino acids in plants, which are relatively abundant in nectar) [99]. Numerous studies have been conducted on the neural and behavioral responses of pollinators (especially bees) to specific amino acids or amino acid mixtures, which are beyond the scope of this discussion.
Non-protein amino acids can be surprisingly abundant in nectar and modulate insect behavior by acting as neurotransmitters, such as glutamate and glycine.
The case of proline is also noteworthy: this amino acid is commonly found in nectar, is the most abundant amino acid in honeybee hemolymph, and is used as an energy source during the early phases of flight in bumblebees and wasps [33, 100]. Hawkmoths also use amino acids as metabolic fuel .
The presence of amino acids in nectar can influence sugar preferences and the amount of nectar consumed by pollinators. For example, free-flying honeybees select lower sucrose concentrations when phenylalanine (known as a feeding stimulant) is present. They may also prefer higher sucrose concentrations to offset the deterrent effect of glycine [31].
In the hawkmoth Manduca sexta, preferences for sugar concentration in nectar are altered by the presence of an amino acid blend similar to natural nectar. Similarly, amino acids in nectar reduce the ability of bats to distinguish between different sugar concentrations. From the plant's perspective, this may help conserve nectar sugars and prevent overconsumption.
In previous sections, we examined sugars and amino acids, the two main components of nectar. However, nectar is not solely composed of these two groups of compounds. In this section, we will explore micronutrients, minor metabolites, and especially secondary metabolites, which play unique and important roles in plant-pollinator interactions.
Micronutrients and Minor Metabolites
Our knowledge of nectar chemistry, particularly regarding micronutrients and minor metabolites, is still in its early stages compared to sugars and amino acids. The low concentration of lipids, organic acids, minerals, and proteins in nectar, along with the lack of comprehensive data in this field, has created numerous challenges for researchers. Similar to amino acids, the origin of these non-sugar metabolites in nectar is not fully understood. However, recent advances in untargeted metabolomics have revealed a significant diversity of metabolites in floral and extrafloral nectar of plants such as cotton and squash.
Micronutrients and minor metabolites in nectar play multiple roles in the health and productivity of pollinators and honeybees. Despite their low concentrations, these compounds can have significant effects on the immune system, nectar-foraging behavior, the process of converting nectar into honey, and the resistance of bees to diseases and pests.
For example, some lipids and organic acids in nectar can serve as secondary energy sources for bees, aiding them in harsh environmental conditions. Minerals also play a vital role in the physiological processes of bees, and their deficiency can lead to impaired larval growth and weakened adult bees.
Additionally, minor metabolites in nectar can influence the quality and shelf life of nectar in the hive and prevent the growth of harmful microbes.
Given the importance of micronutrients and minor metabolites in the nutrition and health of honeybees, a more detailed examination of their composition in the nectar of various flowers and the impact of environmental factors on their concentration is essential for maintaining the health and productivity of honeybees.

The mineral ion content in nectar is often overlooked compared to pollen. It is assumed that pollinators meet their mineral needs by consuming arthropods or pollen. However, minerals in some nectars may help maintain salt balance in pollinators (Hiebert, 1993). Comprehensive chemical analyses of nectar from 20 species of the genus Nicotiana and 147 species of the Bromeliaceae family revealed that the average total concentration of mineral ions in millimolar units was higher than that of amino acids.
Unusually high potassium concentrations in the nectar of onion and avocado flowers are repellent to honeybees, leading to poor pollination. Interestingly, high levels of potassium and phosphate in feeding solutions deter honeybees but not native avocado pollinators in Mexico (Afik et al., 2014). This deterrent effect has also been confirmed using the proboscis extension response test in water-foraging honeybees (Lau and Nieh, 2014). Salt regulation in honeybees has been discussed in detail in (Kram et al., 2008).
Other components of nectar include proteins, lipids, and organic acids. Nectar proteins (called nectarins) play a protective role in floral and extrafloral nectar by preventing microbial degradation (Schmitt et al., 2021). In the abundant nectar of ornamental tobacco, nectarins help produce hydrogen peroxide at levels up to 4 millimolar through a redox cycle in the nectar (Carter and Thornburg, 2004). One of the nectar proteins in the flowers of Jacaranda mimosifolia is a lipase that hydrolyzes nectar lipids into free fatty acids. These fatty acids can accumulate to concentrations of 0.6 millimolar and may be attractive to bees (Lau and Nieh, 2016).
Fatty acids in nectar may also have a metabolic role; for example, hawkmoths have used palmitic acid in artificial nectar as fuel for resting metabolism (Levin et al., 2017). In species of Nicotiana, only one organic acid, malic acid, has been found in significant concentrations (up to 2 millimolar). Ascorbic acid (vitamin C) is also present in nectar and plays roles in the redox cycle, acting as a well-known antioxidant.
4. Secondary Metabolites: Guardians and Deceivers of Nectar
Secondary metabolites, including alkaloids, flavonoids, terpenoids, and phenolics, are found in the nectar of plants that use these compounds as defense mechanisms against herbivores. These compounds can act as both protectors and deterrents for honeybees.
For honeybees, the diversity of secondary metabolites in nectar represents both a challenge and an opportunity. On one hand, these compounds can act as natural repellents, preventing bees from collecting nectar. For example, some alkaloids and terpenoids can give nectar a bitter or unpleasant taste that is unappealing to bees. On the other hand, some secondary metabolites, such as flavonoids, can act as antioxidants and immune boosters, helping bees combat diseases and environmental stress.
A comprehensive study by Palmer-Young et al. (2019), examining the chemistry of floral rewards in 31 plant species, revealed that chemical diversity is greater in pollen, and the concentration of defensive chemicals is also higher in pollen than in nectar. These findings suggest that honeybees encounter higher concentrations of secondary metabolites when collecting pollen and must have stronger defense mechanisms to cope with them.
Additionally, the presence of secondary metabolites in nectar can influence the process of converting nectar into honey and the final quality of honey. Some of these compounds can alter the flavor and aroma of honey and enhance its medicinal properties, while others can cause fermentation or spoilage of honey.
Given the diverse roles of secondary metabolites in plant-honeybee interactions, a more detailed examination of their composition in the nectar of various flowers and the impact of environmental factors on their concentration is essential for maintaining the health and productivity of honeybees.
Since the sensitivity of different pollinators to secondary metabolites varies, unpalatable compounds in nectar can act as taste filters, preventing inefficient pollinators and nectar robbers from accessing this valuable resource. For example, the dark-colored nectar of the South African species Aloe vryheidensis contains phenolic compounds that repel honeybees and nectar-feeding birds but have little effect on more generalist birds. Similarly, the unpalatable nectar of a milkweed species is preferentially consumed by spider wasps (Egan et al., 2022). Grayanotoxins in the nectar of the invasive plant Rhododendron ponticum in the UK are toxic to honeybees but not to native bumblebees (Bombus terrestris) (Tiedeken et al., 2014). Geographical variations in the filtering function of these compounds can have implications for the biology of invasive species (Egan et al., 2022). The deterrent effect of these compounds depends on the concentration of both sugar and toxin; for instance, honeybees and nectar-feeding birds show greater tolerance to nicotine in artificial nectars with higher concentrations (Köhler et al., 2012).
Low concentrations of secondary metabolites can also influence pollinator behavior. Caffeine, a well-known secondary metabolite, is present in the nectar of coffee (Coffea) and citrus (Citrus) flowers, which are highly attractive to bees. Studies have shown that honeybees fed caffeine at ecologically relevant concentrations (but below their taste threshold) develop better memory for the associated floral scent. It appears that caffeine causes bees to overestimate the quality of nectar. A field study confirmed this, showing that adding caffeine to sucrose solutions led to a significant increase in colony recruitment, even if it resulted in suboptimal foraging strategies (Couvillon et al., 2015). Similarly, bumblebees show greater tolerance to low-calorie solutions in the presence of nicotine (Wright et al., 2013). These pharmacological manipulations of pollinator behavior may benefit plants through improved pollen transfer but are not necessarily advantageous for the pollinators themselves.
In a completely different example of pollinator attraction through secondary metabolites, colored nectar is often associated with vertebrate pollinators, particularly on islands. Recently, it has been discovered that the blood-red color of nectar in flowers attractive to geckos is due to an alkaloid pigment called nesocodin (Roy et al., 2022).
Another important role of secondary metabolites in nectar is to protect pollinators against parasites and pathogens. The positive effects of consuming these compounds on the health of bumblebees (Bombus) and their gut parasites have been demonstrated in several studies (Stevenson et al., 2017). Although the precise mechanisms of this protection are not yet fully understood, a well-known example is callunene found in the nectar of Calluna vulgaris (Koch et al., 2019). This compound, identified during the investigation of honey extracts from plants important for bees in terms of their activity against the gut parasite Crithidia bombi, can detach the anchored flagellum of the parasite from the bumblebee's hindgut. In honeybees, caffeine consumption in food can reduce the spore load of the protozoan Nosema ceranae, but nicotine does not have such an effect (Bernklau et al., 2019). Whether secondary metabolites in nectar can completely eliminate infections or prevent their occurrence likely depends on changes induced by the pollinator's gut microbiome. Other health benefits of these compounds include the well-known antioxidant effects of phenolics in honey, which vary depending on the nectar source. The flavonoid quercetin, which is common in nectar and pollen, is preferentially selected by honeybees in choice tests and can regulate detoxification genes (Mao et al., 2013; Palmer-Young et al., 2019). However, the health benefits of secondary metabolites for most pollinators are still not fully understood.
The microbial populations residing in nectar may also be influenced by secondary metabolites. The potential antimicrobial effects of these compounds have been investigated in the nectar of almonds, citrus, and tobacco. While the composition of bacterial communities varied among these nectars, their growth was only weakly inhibited by the respective secondary metabolites (amygdalin, caffeine, and nicotine). In contrast, microbes can also reduce the levels of certain secondary metabolites in nectar (Vannette and Fukami, 2016). The relative stability of sugar composition in some nectars suggests that secondary metabolites may play a preservative role, but further research is needed to fully understand this role and the synergistic effects between different compounds (Koch et al., 2019).
Continuing our exploration of the essential components of nectar, we have so far discussed sugars, amino acids, micronutrients, and secondary metabolites. In this section, we aim to focus on a part of nectar that is often overlooked: water, and examine its vital role in pollinator nutrition and their interactions with flowers.
5. Water: A Vital Nutrient and Regulator of Nectar
Unlike the soluble components of nectar previously discussed (such as proteins, non-protein amino acids, and secondary metabolites, which do not have a direct nutritional role), water is considered an essential nutrient for pollinators. However, the water component of nectar is rarely emphasized in nectar chemistry research. This lack of attention is partly due to the variability of water content in relation to environmental conditions. Additionally, the small volume of nectar in many insect-pollinated flowers requires specific collection methods, such as using wicks or washes, which do not provide precise information about the volume or water content of nectar.
Nectar concentration is heavily influenced by the local climatic conditions around the flower (microclimate). If not protected, nectar tends to equilibrate with the ambient humidity, leading to water evaporation in most conditions (except in highly humid environments). You might find it interesting to know that a 20% sucrose solution loses its water to the air at all humidity levels below 98% (Corbet, 1979).
The rate of water evaporation from nectar depends on various factors, including:
- Floral Morphology: The structure and shape of the flower can influence the rate of evaporation.
- Microclimatic Gradients: Differences in temperature and humidity inside and around the flower.
- Nectar Sugar Profile: Evaporation is slower in nectars containing hexoses compared to those containing sucrose.
- Nectar Volume: Evaporation occurs faster in small volumes of nectar, as the surface-to-volume ratio is higher in smaller droplets.
Continuous evaporation from open flowers, combined with intermittent nectar secretion, potential reabsorption of sugars by the plant, and periodic harvesting by pollinators, can lead to significant daily variations in the volume and concentration of nectar within a single species. These variations, in turn, can influence the attractiveness of the flower to different pollinators. However, nectar in open flowers is often more dilute and abundant than expected and may serve as an important water source for pollinators in dry environments (Willmer, 2011).
Interestingly, the nectar consumed by pollinators is often more dilute than the artificial nectars they choose in preference tests. Bats are a notable example in this regard. The classic bat pollination syndrome involves abundant, highly dilute hexose-rich nectar with an average concentration of 17% by weight (von Helversen and Winter, 2003). However, in choice experiments, bats prefer much higher concentrations. A proposed explanation for this discrepancy, based on experiments with free-flying bats in Costa Rica, is that competition for food drives bats to seek larger volumes while being less selective about concentration. However, this hypothesis remains debated.
In contrast, bumblebees in artificial flowers respond more to increases in nectar concentration than to volume. In these conditions (and likely in real flowers), concentration may be a more reliable and easier-to-assess cue for food source quality. Pyke et al. (2017) argue that evolution should lead to nectar concentrations that benefit individual plants more than pollinators, and plants use a combination of nectar traits (volume, concentration, and composition) to manipulate pollinator behavior.
Dilute nectars offer an important viscosity advantage. The viscosity of sucrose solutions increases exponentially with concentration, significantly affecting the ease of nectar consumption. Optimal nectar concentrations for different pollinator groups depend on their feeding techniques; suction feeders require lower concentrations than licking feeders (Kim, 2011).
The honeybee tongue functions as a hairy structure, and their licking frequency remains constant in solutions with equal viscosity but different concentrations. Surprisingly, individual bees can switch their feeding mode from licking to sucking at nectar concentrations below 30% by weight, leading to faster energy intake (Wei, 2020).
The behavioral flexibility of bees in response to varying nectar concentrations is also evident when they remove excess water from dilute nectar through repeated regurgitation and evaporation. This process may begin during nectar collection in honeybees; they return to the hive with sugar concentrations approximately double that of the initial nectar. This behavior seems highly logical given the significant cost of converting nectar into honey. Both social and solitary bees concentrate nectar on their tongues.
For nectar-feeding birds, processing the excess water in dilute nectar poses a physiological challenge. They either excrete the excess water (like hummingbirds) or simply avoid absorbing it (like nectar-feeding birds) (Nicolson and Fleming, 2014).

The Impact of Climate Change on Nectar: Challenges Ahead
Future research in nectar chemistry should examine the effects of climate change caused by human activities on both wild plants and crop plants dependent on pollinators. Here, only two studies are mentioned that have investigated the interactive effects of multiple abiotic factors on nectar traits. Global warming, increased atmospheric CO2, and nitrogen enrichment have complex (and sometimes contradictory) effects on the sugars and amino acids of squash nectar (Cucurbita maxima) (Hoover et al., 2012). Increased temperature and water shortage also have different impacts on the flowering resources of Borago officinalis plants: both types of stress reduce nectar volume and consequently the total sugars it contains, while both increase total amino acids in nectar and alter their composition. However, pollen is more affected by high temperature than by drought (Descamps et al., 2021). For a review of the metabolic changes occurring in flowers in response to climate change, refer to the article by Borghi et al. (2019). Finally, while increased atmospheric CO2 may reduce protein levels in pollen, the increased soluble carbohydrate content might make the production of nectar sugars more economical for the plant. There are still many aspects regarding the effects of abiotic stresses on the production and composition of nectar that require further investigation.
Summary
Throughout this post, we explored various dimensions of nectar, this valuable reward that flowers offer to pollinators. We found that sugar concentration is not the sole criterion for assessing nectar quality, and a variety of non-sugar compounds with functions beyond nutrition play crucial roles in plant-pollinator interactions (Table 1). Unfortunately, most studies have focused on individual nectar compounds, and the interactive effects of these compounds on pollinator responses, such as the simultaneous influence of phenolics and potassium in onion nectar, have received less attention.
Nectar exhibits significant variability due to genetic and environmental factors, harvesting by pollinators, and contamination with pollen and microbes. The health of pollinators, in addition to sufficient calorie intake, depends on their ability to cope with this chemically variable and fluctuating nectar composition. Furthermore, the value of nectar also depends on its quantity (volume per flower and floral density), and ultimately, the overall availability of nectar can overshadow considerations of quality.
The need for diversity in pollinators' diets, especially in light of bee population declines, has been emphasized repeatedly. Typically, the focus is on pollen quality and diversity as a protein source. Large monoculture plantations in agricultural landscapes lack dietary diversity, and supplementary food sources are essential. However, it is also possible to improve the quality of existing nectar resources in these landscapes in the short term.
Plant breeding has led to significant changes in nectar-related traits, although pollination is usually not a priority in breeding programs. Nectar and pollen of different genotypes of broad beans (Vicia faba) show significant differences, and based on bee preferences, breeding for higher nectar concentrations may be more desirable than larger volumes. In canola, another pollinated crop, different varieties cultivated in the field exhibit significant variability in nectar volume, while their sugar or amino acid composition remains relatively stable. For cultivated sunflowers, the floret size is crucial for the bees' easy access to nectar. Secondary metabolites may also change due to domestication; for example, nectar from different blueberry varieties has reduced levels of a caffeic acid ester, which can help protect bumblebees from infection by pathogens. Secondary metabolites in the nectar of pollination-dependent crop plants vary more significantly among varieties than pollen does. For instance, apples (Malus domestica) exhibit strong chemical diversity among their varieties. Selection and breeding for nectar-related traits hold the potential to benefit pollinators and enhance crop pollination rates.
Looking Ahead
A comprehensive understanding of the chemical complexity of nectar and the factors affecting it is vital for maintaining the health and diversity of pollinators in the face of challenges posed by environmental changes and modern agriculture. We hope this series of articles has broadened your perspective on nectar, this valuable food source, and its role in our ecosystems.
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