Concentration and volume of nectar

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Many previous studies have found that nectar concentration and volume of many angiosperm fluctuates diurnally and inter specifically (Bond and Fyfe 1968: Pham-Delegue et al. 1991). It has also been suggested that this in turn may influence the foraging strategy of many nectivores and potential pollinators. This study found that the nectar concentration and volume was influenced both dependently and independently by variables 1) plant species and 2) time of day. We found that volume and concentration varies significantly between the two angiosperm species in question Iris pseudocorus L and Rhododendron ponticum L. There is also significant diurnal variation in nectar concentration and volume for both species. The study also tested whether foraging frequency of 5 Bombus species varied diurnally. We found that the foraging frequency was higher for R.ponticum and that diurnal variation in visitation frequency was significant for both angiosperm species. Interestingly a trend between the visitation frequencies of B.terrestris to both angiosperm species corresponded to the time of day where nectar was highest for the respective flower. There was little notable correlation between time of day and visitation frequency for the other observed Bombus species.

Does the sucrose concentration and volume of nectar from the species Iris pseudocorus and Rhododendron ponticum vary diurnally? Does the frequency and diversity of pollinating Bombus species vary diurnally and inter-specifically?


Invasive species Iris pseudocorus L and Rhododendron ponticum L have achieved reproductive success by establishing mutualistic symbiosis with generalist native pollinators such as Bombus species (Arroyo et al. 2004). They offer nectar as a reward and a means of attracting potential pollinators. Bees gauge flower attractiveness in many ways, one being the value of the reward. If a flower does not offer a substantial reward its attractiveness is thus reduced. It has been suggested that nectar concentration and volume varies inter-specifically and diurnally (Bond and Fyfe 1968: Pham-Delegue et al. 1991). Does this affect the visitation frequency of Bombus species to the two flowers?

This study will consider diurnal variation in nectar concentration and volume of the two species I. pseudocorus and R. ponticum inhabiting the Isle of Cumbrae. This study will also look at whether visitation frequency and diversity of pollinator species from the genus Bombus varies diurnally, and if so whether this is correlated to any prevalent diurnal variation in nectar volume and concentration of the angiosperm species under question.

The aim of this study is to test the following null hypotheses:

Ho) - Nectar composition remains the same over the course of the day

  1. Secretion rate does not change
  2. Sugar content does not change

Ho ii) - There is no difference between flora species

  1. volume and rate is the same for both species
  2. sugar concentration is the same for both species
  3. dinural variation is the same

Ho iii) - The visitation frequency of Bombus species does not vary inter- specifically or diurnally


Study area

This study was carried out on The Great Isle of Cumbrae ((55°45'07?N 4°55'48?W? / ?55.752°N 4.930°W). Two flower species R.ponticum and I.pseudocorus were sampled for nectar at several sites over 2 days (The location of sites on the Island is described in Figure 1). Following nectar extraction, each site was observed for thirty minutes and the visitation frequency of Bombus species, landing on and drinking from any I. pseudocorus or R. ponticum flower was recorded. It should be reiterated that the bee had to be observed drinking from the flower in question. Bees were identified and recorded in a tally. Pseudo-replication may be a possible source of error in the results. An individual bee may have flown from one site to another or been observed on more than one occasion by different students involved in observation. All data was regarded as independent for purposes of analysis.

R. ponticum nectar was extracted from 5 flowers/inflorescence (cluster) from 5 different plants located on one large site. Care was taken to sample flowers from different positions on the inflorescence because upper flowers may contain more nectar than lower flowers (Cruden et al. 1983). Nectar was also sampled from inflorescences at different locations on the plant. Different levels of sun exposure affect both nectar concentration and volume. Flowers directly exposed to the sun will likely have higher concentrations and a lower nectar volume than those in the shade (Nepi, M and Pacini, E 2007a).

Iris nectar was collected from 15-20 individuals from 3 sites on day 1 and 3 different sites on day 2. Sample sizes from each site were increased from 3 to 5 individuals after day 1 to allow calculation of a more accurate mean.

I. pseudocorus nectar was sampled at six different sites over the 2 days because population size at one site was insufficient to obtain a large enough sample size. As a result sites were subject to different sun/wind exposure and possibly different soil types which may affect concentration, and secretion rate.

It should be noted that actual sample size may vary from pre agreed sample size because some samples were lost or deemed invalid.

Data collection

Nectar was collected from both R. ponticum and I. pseudocorus by capillary extraction at 4 different time points over 2 days (10am, 12pm, 14pm, 16pm). 2 hours prior to nectar extraction, plants were covered with a plastic bag to prevent extraction by pollinators. This presumably allowed sufficient time for the respective flowers to produce and accumulate nectar. Plastic bags may generate microclimates that increase ambient temperature and humidity and may subsequently affect the rate of post secretory evaporation. It may have been more appropriate to use porous netting to alleviate some of the possible microclimatic influence, but unfortunately these were not available. This is an unavoidable error in the results but all results are equally biased.

Iris nectar was extracted by gently squeezing the base of the nectar bank at the apex of the stem. A capillary tube was used to pipette the nectar from the 6 nectar ducts on each flower (2 per petal). Care was taken not to damage the flower.

Once nectar was collected the volume was measured using the dimensions of the capillary tube (length of nectar x 35µl). It was then expelled onto the plate of a refractometer and the sugar concentration was measured. After measurement, the plate was thoroughly cleaned to avoid contamination of subsequent samples.

Data Analysis

Prior to statistical analysis, the normality of data was tested. Distribution of nectar concentration, for both species was tested for normality using the Kolmogorov-Smirnoff test. The test produced a p-value> 0.05 (p=0.15) showing that data does not significantly deviate from normal distribution therefore parametric testing can be used. Kolmogorov-Smirnoff test was used to test distribution normality of nectar volume. The test concluded that distribution of data did significantly deviate from normal. Despite efforts to transform data (log+1 and Analysis of variance), normality could not be achieved. 2 Way ANOVA (General Linear Model) was performed despite data for volume being skewed. Results for volume should be treated with caution. Pearson's chi-squared test was used to test whether visitation frequency of Bombus species varied diurnally.



A two way ANOVA indicated that plant species had a significant effect on nectar concentration (p=0.00). Time of day also significantly affected nectar concentration. (p=0.013). Because time of day and species can effect concentration dependently as well as independently, the significance of time of day*species was tested. This test produced a p-value=0.000 showing that independent variables time of day and plant species are dependent on one another. The null hypothesis stating that nectar concentration remains the same over the course of the day and that there is no difference between species can be rejected. Species type and time of day significantly influence nectar concentration both dependently and independently.


Two way ANOVA showed that plant species significantly affected volume (p=0.000), as did time of day (p=0.004). Statistical testing also confirmed that time of day and species type significantly affect nectar volume dependently as well as independently (p=0.000). The null hypothesis can therefore be rejected.

Bee species

Pearson's chi-squared test was used to test whether visitation frequency of the 5 observed bee species (unknown bee species excluded) to each of the plant species varied diurnally. For I. pseudocorus the difference was significant and a P=0.001 was produced. Results for R. ponticum were also statistically significant (P=0.002). The null hypothesis can therefore be rejected. The frequency of Bombus species does differ significantly diurnally.


Previous studies have found that secretion rate and concentration of nectar varies inter-specifically (Bond and Fyfe 1968: Pham-Delegue 1991) and this is concurrent with our findings. Species was a significant factor in determining both nectar concentration (p=0.000) and volume (p=0.000). R. ponticum has a higher mean concentration than I. pseudocorus for every time point throughout the day (Figure 4) . I. pseudocours had a higher mean volume than R. ponticum for every time point throughout the day (Figure 5). This data suggests that R. ponticum produces more concentrated nectar in smaller volumes.

While nectar secretion and concentration varies inter-specifically, there are many environmental variables that can influence secretion and concentration of nectar. Ambient temperature affects rate of photosynthesis and can contribute directly or indirectly to nectar production (Burquez & Corbet 1991 and 1998). In most species the rate of nectar secretion (quantified by nectar volume) is correlated to temperature (Huber 1956; Corbet et al. 1978). Ambient irradiance however seems to be the most significant influential factor (Petanidou& Smets 1996). Under low light conditions, nectar secretion may decrease considerably. Both temperature and irradiance fluctuate diurnally which may explaining why in our findings time of day is a significant factor in determining nectar concentration (p<0.013) and volume (p<0.004).

Corolla length and differences in species flower morphology may also help explain some of the differences observed in the results. Many flowers have modified corolla length, reducing surface:volume ratios (Plowright 1987). Plowright (1987) demonstrated that nectar evaporates more rapidly from flowers with a shallow corolla. The flower I. pseudocorus has a relatively deep corolla. The proboscis of a potential pollinator must be 7mm to reach nectar and 15mm to extract it all (Sutherland 1990). The flower of R. ponticum is open, with more exposed nectaries, increasing its susceptibility to microclimatic effects. Beament (1979) found that nectar often undergoes post-secretory increases in concentration. This is largely due to evaporation caused by humidity gradients and high temperatures. "A 20% sucrose solution will lose water to all relative humidity's below 98%". This accounts for the common negative correlation between volume and concentration and is likely to be most prominent at the hottest part of the day.

Figure 2 shows that I. pseudocorus follows this inverse correlation. The mean volume is highest at 10am (3.75 µl) but the mean concentration is lower than at 14pm and 16pm. The mean concentration of nectar is highest at 14pm and 16pm when volume is much lower. It is possible that higher volumes in the morning could be explained by condensation/due accumulating in the nectar ducts. It is interesting that R. ponticum does not follow this pattern; instead volume is highest at 2pm (Figure 3). Rate of evaporation is also affected by the presence of waterproofing lipid monolayers on the nectar surfaces and sugar concentration gradients which would vary inter-specifically (Corbet, et al. 1979). Differences in flower morphology/corolla depth and their affect on post secretory evaporation in I. pseudocorus and R. ponticum may explain at least partly the inter-specific differences in nectar volume and concentration observed in the results. The results are represented graphically in figures 4 and 5.

Nectar concentration and volume are likely to influence the type of pollinator visiting the flower. Bees are more likely to feed and remain at flowers with a large nectar volume than those with a smaller volume (with equivalent concentration) (Harder 1986). As volume decreases, nectar becomes harder to reach; this is more notable in species with deep corollas (I. pseudocorus) and for bees with shorter proboscis (glossa). As a result species with deep corollas may compensate and produce higher volumes of nectar. Results suggest that this could be true for I. pseudocorus.

It has been suggested that bumblebees forage so that the net rate of energy uptake is maximised (Waddington 1983). When given the choice bees will usually opt for nectar that offers the greatest calorific reward (Marden J.H. 1984). Harder (1986) found that species like Bombus that feed by lapping, show preference to more concentrated nectars (50-65%). Energetic benefit increases with nectar concentration until the cost of a reduced rate of fluid intake resulting from very viscous nectar outweighs the reward benefit (Heyneman A.J 1983). This is agreeing with out findings. R. ponticum has a significantly higher total visitation frequency than I. pseudocorus (Total visitation frequency for R. ponticum N=489, total visitation frequency for I. pseudocorus N=195). This could be explained by the fact that R. ponticum has a higher mean nectar concentration and is more attractive in terms of calorific reward. It is also notable that the flowers are far more abundant and densely packed into inflorescences, perhaps creating a more appealing target. Bees reduce interflower travel in order to conserve energy and tend to move adjacently from flower to flower (J.C. Stout). R. ponticum may be more attractive in terms of energy preservation as well as calorific reward.

Our results show that time of day and plant species are factors that significantly affect the visitation frequency of Bombus species. Figures 6 and 7 show the visitation frequency of the 5 Bombus species visiting the two flower species. The visitation frequency of the 5 bee species is not highly correlated to time of day. However, in the case of B. terrestris (the most abundant species) we observed the highest visitation frequency to I. pseudocorus at 16pm and to R. ponticum at 10 and 12pm. Interestingly this increased foraging activity coincided with times where nectar concentration was highest for the respective flower species. While this could be coincidental, it is plausible that B. terrestris adapts its foraging strategy, throughout the day to optimise and forages more at each plant species when concentration is highest. This notion is certainly concurrent with previous findings and warrants further study.


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