How can I know the nectar content in a flower? Give methodology for studying about its presence in a flower.

Lalitkumar V Ghetiya @Lalitkumar-Ghetiya

01 January 1970 19 10K Report

I'm interested in study of various flora visited by honey bees, and I want to know whether bees visited flower for collection of nectar, pollen or both. I want to study about the availability of nectar in the flower, how can I know about nectar's presence in a flower?

Sarah Alexandra Corbet Popular answer

Maybe this will help?

Apidologie 34 (2003) 1–10

DOI: 10.1051/apido:2002049

Review article

Nectar sugar content: estimating standing crop

and secretion rate in the field

Sarah A. CORBET*

Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK

(Received 13 May 2002; revised 25 July 2002; accepted 9 August 2002)

Abstract – Field techniques for sampling and measuring the standing crop and secretion rate of nectar are

described, in order to clarify some discrepancies and omissions in existing reviews of nectar measuring

techniques. Slender microcapillary tubes (a fresh one for each sample) are recommended for withdrawing

nectar, and a hand held sucrose refractometer, capable of operating with very small fluid volumes, is used

for measuring concentration. Potential errors due to the presence of solutes other than sucrose, or to

temperatures other than the calibration temperature, are discussed. I consider how measurements of

secretion rate are affected by reabsorption and by the nature of the bags used to exclude nectarivores.

standing crop / nectar concentration / secretion rate / microcap / refractometer / sucrose / glucose /

fructose / amino acids / nectarivore

1. INTRODUCTION

Floral nectar consists largely of sugars

(chiefly sucrose, glucose and fructose) and

water. Insects, birds and mammals take nectar,

and its sugars provide energy that fuels activity

or provisions the larvae. Although the

water content of nectar can be important to

plants (Galen et al., 1999) and to nectarivores

(Willmer, 1986; Lotz and Nicolson, 1999), it

is the sugar content of nectar that is usually of

primary interest, because energy is the currency

usually considered by, for instance,

zoologists exploring the extent to which foragers

maximise the net rate of energy gain (or

efficiency, the ratio of energetic gain to energetic

cost (Schmid-Hempel et al., 1985)), or

botanists examining the costs and benefits of

allocation of resources to pollinator attraction.

Zimmerman (1988) and Kearns and Inouye

(1993) review the ecological and evolutionary

context in which measurements of the quantity

and dynamics of nectar secretion are useful.

In the field, the sugar content of nectar can

be estimated from measurements of nectar

volume and solute concentration, measured

with a sucrose refractometer. Publications that

deal with techniques for exploring and quantifying

nectar solutes include Beutler (1953),

Cruden and Hermann (1983), Dafni (1992)

and Kearns and Inouye (1993). Some omissions

and discrepancies in these reviews make

it difficult for a neophyte to assemble suitable

equipment and bring the techniques into operation

without preliminary trials. Bee-pollinated

flowers often contain very small quantities of

nectar, for which micropipette diameter and

refractometer capacity are critically important,

but these reviews do not mention micropipette

diameter and the refractometer type recommended

in some of them is no longer in production

(see below). Sucrose refractometers

* Correspondence and reprints

E-mail: [email protected]

Present address: 1 St Loy Cottages, St Buryan, Penzance TR19 6DH, UK.

2 S.A. Corbet

are variously said to give percentage readings

in weight of sugar per unit volume of solution

(Kearns and Inouye, 1993, p. 170, presumably

a misprint) or weight of sugar per unit weight

of water (Cruden and Hermann, 1983, p. 235),

whereas in fact the usual units are g sucrose

per 100 g solution (Bolten et al., 1979; Dafni,

1992).

In this paper I consider micropipette diameter

and refractometer capacity and recommend

suitable instruments, and try to resolve

discrepancies about the units of measurement

by refractometers. I focus in more detail on

field methods for estimating standing crop and

secretion rate, and highlight some hints and

problems arising from experience over a 25-

year period.

The quantity of nectar sugar in a flower

fluctuates through time as nectar is supplied by

secretion or depleted by foraging animals or

by reabsorption. These are the only avenues of

transport for sugar; but water has additional

routes. It can be supplied by condensation

from humid air, or by precipitation; and it can

be lost by evaporation.

To interpret the foraging behaviour of nectarivores,

we need to know both the standing

crop and the secretion rate of nectar. The

standing crop, the quantity of nectar in a

flower at a given time, is usually expressed in

terms of mass of sugar per flower. It depends

on the quantity secreted, less the quantity reabsorbed

or removed, since secretion began.

The standing crop increases when the secretion

rate exceeds the rate of reabsorption or

removal (as often happens in the early morning

before most insect nectarivores are active)

and it falls when rates of reabsorption and

removal exceed secretion rate (as often happens

at times when foragers are numerous).

Hence the standing crop shows variation from

hour to hour and from day to day, as well as

variation associated with weather- and flowerage-

related changes in rates of secretion and

reabsorption. It also varies from flower to

flower; rates of secretion may show intrinsic

plant-to-plant and flower-to-flower variation

(e.g. Gilbert et al., 1991; Feinsinger, 1978), and

may vary with the microclimate surrounding

individual flowers; and rates of removal will

depend on the frequency of foraging visits,

which may depend in part on position (e.g. sun

or shade; centre or margin of a bush; within or

outside the defended territory of certain species

of bee or bird).

2. SAMPLING NECTAR

AND MEASURING VOLUME

The sugar content of nectar is calculated

from the volume and the solute concentration

of the nectar sample from each flower. If nectar

is withdrawn from the flower into a tube of

uniform bore, such as a microcapillary pipette

(a microcap), the volume can be measured (as

the length of the column of liquid) before the

nectar is deposited on a refractometer prism

for measurement of solute concentration. If the

nectar sample is too viscous or too small to be

sampled in a microcapillary, sugar content

must be quantified in some other way (see

below), and water content may not be quantifiable

at all.

After initial exploration of the structure of

the flower to locate the nectar, preferably

under a stereomicroscope, the microcap is

touched gently against the nectar surface until

repeated probing yields no further nectar. Nectar

uptake can sometimes be speeded up by

tilting the flower so that the nectar flows

downwards into the microcap. At least initially

it is wise to tear open the drained flower after

sampling to check that all nectar has been

removed, because any clogging of the microcap

due to pollen, thrips, damaged floral tissues

or air bubbles may prevent capillary flow.

It is for this reason that unforced capillary flow

is preferable to aspiration, because application

of suction can draw bubbles into the capillary

(Pleasants, 1983). For this reason, too, a new

microcap should be used for every sample,

because a microcap that has contained nectar

is likely to contain a liquid meniscus that will

impede capillary flow. If it is necessary for

economic reasons to re-use microcaps, they

should be deposited in a screw-top tube of

absolute ethanol or acetone immediately after

use, and later drained, dried and checked visually

before re-use. Used microcaps rinsed with

water alone often retain a meniscus that blocks

capillary flow (Cruden and Hermann, 1983).

The size of the microcap must be appropriate

for the flower. If the external diameter is

too great it may be impossible to achieve contact

between the end of the lumen and the

Measuring nectar 3

nectar surface, and the microcap may be too

thick to reach the nectar without distorting the

corolla. Drummond Microcaps® (Drummond

Scientific Co., Broomall, Pa., USA; http://

www.dru-mmondsci.com) are slender (0.2 microlitre

microcaps have an external diameter of

0.5 mm, 1 microlitre microcaps 0.64 mm,

5 microlitre microcaps 0.92 mm), but many

graduated micropipettes are thicker-walled and

have a much greater external diameter.

The volumetric capacity of the microcap

should also be appropriate. If the microcap

holds less nectar than a flower, repeated fillings

may be necessary for a single sample,

with the risk that a meniscus will block the

lumen. If the microcap is too large, its greater

diameter will make nectar extraction difficult

and measurement of the column length inaccurate.

Drummond microcaps are available in

a range of volumes from 0.1 microlitres

upwards. The holder supplied with microcaps

can be used to discharge the contents onto the

centre of a refractometer prism by blocking the

pinhole with a finger and squeezing the rubber

bulb or, for better control, by blowing via a

length of flexible tubing.

If the corolla tube is very slender even the

smallest microcap may fail to drain the nectar.

Nectar can be removed from the very slender

corolla tubes of some Asteraceae by pulling

the corolla tube off the ovary and gently

squeezing so that the nectar emerges at the

base as a droplet. This can be deposited

directly onto a refractometer prism or first

taken up into a microcap for volume measurement.

Such nectar may be contaminated with

tissue fluids.

Alternatively, a microcap can be drawn out

into a fine hair-like point by melting the centre

in a flame and pulling the two ends apart. The

resulting tapered microcap is broken off at a

suitable diameter and used to take up nectar.

Its broken end will be sharp, and the probing

should be gentle to avoid piercing the floral

tissue and clogging the lumen. Volume measurement

in these tapered tubes is not straightforward.

Working under a stereoscopic microscope,

set up in the field if necessary, it is

possible to insert the tapered tip, and discharge

the nectar, into the lumen of a slightly larger

intact microcap, in which the length of the column

of nectar can be measured before the

droplet is deposited on the refractometer prism.

Alternatively, the drop of nectar might be discharged

into a dish of liquid paraffin, where its

diameter can be measured under a stereoscopic

microscope. If the drop is discharged

onto filter paper, the diameter of the wet area

can be measured as an index of volume (Dafni,

1992; Kearns and Inouye, 1993, p. 173), but

laboratory procedures (reviewed by Dafni

(1992) and Kearns and Inouye (1993)) are then

required for the estimation of sugar content.

A hydrophilic surface, such as clean glass,

is necessary for the capillary uptake of nectar.

Equally hydrophilic fine plastic or polythene

capillary tubing might have advantages over

glass for nectar sampling. It would be less

fragile and more easily handled, and could be

cut to lengths appropriate to each sample. It

would be softer, and so less likely to cause floral

tissue damage, and its flexibility would

make it easier to probe curved corolla tubes

and to implant tubing to monitor secretion rate

(see below). Portex autoclavable nylon tubing

with an internal diameter of 0.5 mm is suitable

for some purposes (Búrquez and Corbet, 1991).

Sometimes the consistency of the nectar or

the shape of the nectar-bearing surface preclude

the use of microcaps. If measurements

of volume and concentration are not needed,

nectar can be blotted up onto small triangles of

filter paper. These can be organised in the field

by pinning them to a sampling scheme outlined

on a sheet of ruled paper clipped to a

block of plastic foam (McKenna and Thomson,

1988). They are stored dry, and the nectar is

later redissolved in distilled water for sugar

analysis. Alternatively, nectar can be extracted

by centrifuging groups of flowers (Dafni,

1992; Kearns and Inouye, 1993). Nectar that is

intractably viscous or crystalline can be rinsed

out of flowers into a known volume of distilled

water (e.g. Mallick, 2000). Either the flowers

are shaken in stoppered tubes of water (e.g.

Käpylä, 1978), or known volumes of water are

discharged onto the nectary, if necessary left

until the sugar has gone into solution, and then

withdrawn (Corbet et al., 1979a). Successive

rinses yield progressively less sugar, and it is

not clear how much of this would have been

available to insect visitors. In Crataegus laevigata,

for example, the quantity of sugar in

solution rises at a diminishing rate over a

period of about 30 min (Corbet et al., 1979a).

The extent to which flies and other insects

4 S.A. Corbet

mimic this technique, perhaps by spitting

saliva onto the nectary and then reclaiming it,

remains unknown.

3. MEASURING SOLUTE

CONCENTRATION

The solute concentration in a flower

changes with time as a result of (a) equilibration

with the ambient humidity (Corbet et al.,

1979b), (b) selective reabsorption of solutes or

water (Nicolson, 1995), and perhaps (c)

changes in the concentration at which nectar is

secreted. Generally, in day-flowering species

the concentration of accumulated nectar is low

at night (when the relative humidity is high),

and increases during the morning to reach high

values when active depletion leaves very small

standing crops and relative humidity is low

around midday (Corbet et al., 1979a; Corbet

and Delfosse, 1984; Corbet et al., 1995). At a

given relative humidity, the rate at which

evaporation elevates the solute concentration

is inversely related to the size of the drop.

A small droplet of nectar has a relatively large

surface area and is quickly concentrated by

evaporation. A large volume of nectar, as in

the tubular corolla of a hummingbird flower,

has a smaller surface volume ratio, and evaporation

changes the concentration of the mass of

nectar slowly, if at all. The degree of microclimatic

protection offered by the corolla affects

the rate of evaporative water loss (Corbet

et al., 1979b; Plowright, 1987). In relatively

open flowers exposed to low relative humidities

evaporative concentration causes rapid

changes of concentration through the day, and

variation from flower to flower is exaggerated

because the traces of nectar in recently-visited

flowers become concentrated much faster than

the larger volumes in unvisited flowers. It may

sometimes be reasonable to assume that concentration

is constant, and to track standing

crop by measuring volume alone, in deep

flowers with abundant nectar; but in more

open flowers containing the smaller volumes

of nectar characteristic of insect pollination,

concentration can fluctuate rapidly and studies

of sugar content must be based on measurements

of concentration, as well as volume, in

individual flowers.

Solute concentration is measured with a

hand held refractometer. The droplet of nectar

is discharged from the microcap onto the centre

of the prism of the refractometer, and the

reading is taken immediately, to minimise

evaporation of the drop.

The refractometer measures the refractive

index of the solution, which depends on the

nature of the solute, concentration and temperature.

For a sucrose solution at 20 oC, the concentration

(g solute per 100 g solution) corresponding

to a given refractive index can be

read from tables (Weast, 1986; Reiser et al.,

1995) but this is not necessary because the

sucrose refractometers usually used by pollination

ecologists are calibrated directly in g

sucrose per 100 g solution (previously known

as % Brix among food technologists). For the

calculation of sugar content, these mass/total

mass measurements are converted to mass/

volume directly or by multiplying by the density

of a sucrose solution at the observed concentration

(Bolten et al., 1979) using tables

(Weast, 1986; Dafni, 1992; Kearns and Inouye,

1993), an equation (Prys-Jones and Corbet,

1991; Dafni, 1992) or the web (Association

Andrew van Hook for the Advancement of the

Knowledge on Sugar, 2002, http://www.univreims.

fr/Externes/AVH/MementoSugar/

001.htm).

Sucrose is often the main solute in nectar,

but other sugars, notably the hexose sugars

glucose and fructose, are often present or even

predominant. Fortunately, the presence of hexose

sugars scarcely affects the relationship

between solute concentration and refractometer

reading (Weast, 1986). The refraction, r, of

a solution is 104 times the difference between

the refractive index of the solution and that of

pure solvent (here, water) at the same temperature.

The refraction per unit percent solute is

known as the refractivity, r/P, where P is the

percent solute by weight. Marov and Dowling

(1990) and Lescure (1995) give equations that

relate the reading on a sucrose refractometer to

total dissolved solids for solutions containing

various proportions of hexose sugars (glucose

and fructose), but the ecologist rarely knows

what proportion of the sugars in the solution

are hexose sugars. Although broadly characteristic

of species or higher taxonomic groups

(Baker and Baker, 1983), this proportion can

change with time in some species (Nepi et al.,

Measuring nectar 5

2001), if not in others (Bernardello et al.,

1994; Davis, 1997). Fortunately, the corrections

to the refractometer reading required to

allow for the presence of hexose sugars are

trivial in relation to the variance in concentration

usually found in nature. Even for a concentrated

solution whose solutes consist

entirely of glucose and fructose, a refractometer

calibrated in % sucrose gives readings that

are too low by not more than 2% as sucrose w/w.

The hexose sugars glucose and fructose are

similar to sucrose in the relation between density

and concentration weight/weight and in

the energy content per gram, so that the error

in energy calculations introduced by assuming

all solutes are sucrose, when in fact they are

largely glucose and/or fructose, is only equivalent

to about 3–4% as sucrose w/w (Weast,

1986; Kearns and Inouye, 1993).

Among other nectar solutes that can

interfere are amino acids (Baker and Baker,

1986), some of which have refractivities very

different from that of sucrose. Whereas the

refractivities of sucrose, glucose and fructose

are all given as 14 in Wolf (1966), those of

common amino acids range from 4.3 to 29.4

(Jones (1975) or Swiss Institute for

Bioinformatics (2002) http://www.expasy.ch/

tools/pscale/Refractivity.html). Amino acids

usually comprise a small proportion of the

total solutes, and the estimated error due to all

non-sugar components is unlikely to exceed

3.6% as sucrose w/w and is usually much less

(Inouye et al., 1980). If all refracting solutes

are treated as sucrose, the overestimation of

energy content is likely to be less than the

overestimation of sugar content, because some

amino acids and other non-sugar components

can be metabolised.

Hand held refractometers are not generally

temperature compensated, but tables (supplied

with the instrument, or in Reiser et al. (1995,

Tab. 8.12, p. 207) or Weast (1986)) show that

within the usual working temperature range of,

say, 15–30 oC, the maximum temperature correction

for sucrose at 20 oC is less than 1% as

sucrose w/w.

The calibration of the refractometer scale is

necessarily a compromise between range and

accuracy. The low volume hand held sucrose

refractometers from Bellingham & Stanley

Ltd, Tunbridge Wells, UK (http://www.bsltd.

com; e-mail [email protected] (UK) or

[email protected] (North America))

cover the range 0–50% and 45–80% as sucrose

w/w, so for routine work in temperate climates

two instruments are needed. When a small

droplet of nectar is exposed to the air evaporation

quickly changes its concentration, so if a

small sample proves to be outside the range of

one instrument it cannot be retrieved and

tested with the other. Anyone who doubts the

speed of evaporation should place a tiny droplet

(say, less than 0.5 L) of nectar or water

under a stereoscopic microscope and simply

watch as it shrinks by evaporation. When concentrations

on the borderline between the two

instruments are frequent, and samples are

large enough, it may be possible to retain a little

of each sample in the microcap in case a

different range instrument needs to be used.

Bellingham and Stanley no longer make the

metal-and-glass sucrose refractometers that

they could modify individually for very small

volumes of nectar as described by Dafni

(1992) and Kearns and Inouye (1993). These

have been replaced by Bellingham and Stanley

Eclipse hand held sucrose refractometers,

which are available in a low volume version

manufactured to accept small volumes of fluid

(code 45–81 for the range 0–50% and code

45–82 for 45–80% as sucrose). Although their

nominal minimum volume is 1 l, the instrument

I tested gave a faint but legible reading

with 0.2 l, and sometimes with even less.

This modification makes it possible to measure

volume and concentration on small samples

from individual flowers, which is desirable

because pooling samples from different

flowers is less informative, and potentially

misleading. The overall sugar concentration

based on pooled samples can differ by a few

% as sucrose w/w from both the mean and the

modal values based on measurements of individual

flowers. More importantly, the calculation

is based on the assumption that the entire

standing crop of nectar is withdrawn from

every flower, but that is unlikely to be

achieved – clogging of the pipette with tissue

or air bubbles becomes increasingly likely as

successive flowers are probed with the same

microcapillary, and the more concentrated,

viscous nectar of the emptier flowers is likely

to be incompletely sampled or incompletely

mixed in the micro-pipette.

6 S.A. Corbet

If only a low-range refractometer is available,

it is tempting to dilute the sample in the

microcap by adding a known volume of water.

Again, it is not clear that adequate mixing can

be achieved in the microcap, so this procedure

should be tested carefully before use. If a layer

of concentrated sugar solution adheres to the

walls of the microcap, the concentration of the

original solution will be underestimated.

4. STANDING CROP

The distribution of standing crop (the quantity

of nectar in a flower at a given time) within

a population of flowers may show some spatial

patterning (‘hot spots’ and ‘cold spots’ of

Pleasants and Zimmerman (1979); Kearns and

Inouye (1993)). Statistically it often departs

from a Poisson distribution (Brink, 1982). The

accumulated standing crop in some (unvisited)

flowers may be much larger than that in other

(recently-visited or poorly-secreting) flowers.

This distribution is the ‘bonanza-blank’

reward schedule of Brink (1982) and Feinsinger

(1978), who coined the term for hummingbird

flowers showing strong differences

in reward due to flower-to-flower differences

in 24-h sugar values (see below). There is evidence

that foragers selectively visit the fuller

flowers (e.g. Corbet et al., 1984). Under such

circumstances, the standing crop encountered

by bees foraging systematically (the encountered

crop) is likely to exceed the mean standing

crop measured by an unselective ecologist

(Possingham, 1989), and a high frequency of

forager visits is expected to result eventually

in a much more evenly distributed, low standing

crop. That situation is often found around

midday (Corbet et al., 1995).

Although the secretion rate of a population

of flowers in given microclimatic conditions

may be more or less characteristic of a given

plant species, the standing crop, because of its

high variability through time and space, is better

regarded as a feature of the recent and current

interaction between a population of flowers

and a population of foragers.

5. SECRETION RATE

To measure secretion rate, it is necessary to

eliminate other routes of gain or loss of water

and solutes, and then to measure the amount

by which the standing crop increases over a

known period of time. Usually, exchange of

water with the atmosphere is eliminated by

expressing rates in terms of mass of solutes,

and depletion by foraging animals is eliminated

by protecting flowers in a bag or cage

that excludes all but the smallest insects.

Bags used to protect flowers from insect

visits should be chosen with care to avoid

effects on the contained microclimate and

therefore on the concentration and production

of nectar (Corbet, 1990; Búrquez and Corbet,

1998). Wyatt et al. (1992) compared unbagged

flowers with flowers bagged in clear plastic

(polyethylene), brown paper, pellon (a soft,

white fabric of irregular mesh) and bridal veil

(nylon netting of mesh size 10 10 threads/

cm). Plastic bags caused marked elevation of

humidity and temperature, lowered the nectar

solute concentration and increased rates of

sugar secretion. Paper and pellon had lesser

effects, and bridal veil had very little effect,

either on the microclimate in the bag or on the

production and composition of nectar. Bridal

veil is recommended as the preferred material

for insect exclusion bags in studies of nectar

secretion.

Reabsorption is more difficult to eliminate.

It is usual to measure ‘apparent secretion rate’,

the rate of change of solute content of nectar in

an undisturbed, unvisited flower. In the many

species that show no evidence for reabsorption,

this probably represents the true secretion

rate. But in many other species there is strong

evidence, direct (Búrquez and Corbet, 1991;

Nicolson, 1995) or indirect (Búrquez and

Corbet, 1991), that reabsorption of nectar proceeds

in conjunction with secretion, and sometimes

continues after secretion has ended. The

effects of reabsorption can be minimised by

sampling a flower repeatedly at short intervals,

minimising the quantity of nectar available in

the flower for reabsorption. The cumulative

increase in solute content of such repeatedlysampled

flowers, the ‘gross secretion rate’,

often exceeds the apparent secretion rate

measured in undisturbed flowers over the

same total period of, say, 24 h. The difference

is the ‘apparent reabsorption rate’. Studies of

this kind sometimes reveal marked diel patterning

in the rates of both secretion and reabsorption

(Búrquez and Corbet, 1991). The

Measuring nectar 7

‘twenty-four hour sugar value’ of Beutler

(1953; Petanidou and Smets, 1995), the mass

of sugar accumulating in an undisturbed

bagged flower over 24 h, reflects the apparent

secretion rate. The gross secretion rate may be

much higher, and is probably a better (but less

easily measured) index of the quantity of sugar

supplied by a flower when forager visits are

frequent, and thus of its value as a honey

source. Further, if the balance between secretion

rate and reabsorption rate changes with

time, the 24-h sugar value will depend on the

time of day at which the sample is taken.

Sometimes the cumulative mass of sugar

secreted by repeatedly-sampled flowers is less

than the apparent secretion rate. Some authors

have attributed this to sampling damage to the

nectary (Búrquez and Corbet, 1991), but others

regard it as an adaptive feature: curtailed

secretion and a shortened flower lifetime after

a pollinator visit may reduce plant costs and

help promote xenogamy (Freitas and Sazima,

2001).

Some species begin to secrete nectar before

the flowers open (e.g. Pleasants, 1983). To

measure apparent secretion rate, it is common

practice to empty flowers of nectar, and then to

bag the emptied flowers and resample them

after a selected period. The initial nectar

removal must be done gently, as damage to the

flower may suppress secretion. Some authors

therefore use filter paper wicks for this initial

emptying. Preliminary sampling is necessary

in order to decide the interval over which

secretion is to be measured. To measure a

secretion rate that approaches the gross secretion

rate one should select an interval that is

long enough for measurable amounts of nectar

to accumulate, but not so long that reabsorption

becomes important.

If repeated resampling is expected to damage

the flowers, an alternative (but not statistically

equivalent) procedure is to use a different

set of ten (or more) flowers at each sampling

time. The selected flowers are emptied,

bagged, and then resampled for secretion rate

after a known interval. This procedure is

repeated at regular intervals from dawn until

dusk or, for nocturnal flowers, through the

night. A hand held refractometer can be operated

in the dark by looking through it at a

torch.

To monitor patterns of secretion and standing

crop through the life of a flower, ten or

more fresh flowers are selected at each sampling

time from a cohort of even-aged flowers

that were marked the previous evening, before

sampling began. (Coloured plastic drinking

straws, slit longitudinally and cut into short

lengths, make useful rings for marking the

stalks of small flowers.) If a pre-marked age

cohort is not used, recruitment of newlyopened

flowers during the day may cause an

apparent increase in mean standing crop at

times when secretion rate measurements are

not necessarily high. Cohorts opening at different

times of day may show different patterns

of secretion depending on the interaction

between flower age, weather and any circadian

periodicity of secretion and reabsorption.

On the other hand, if the aim is to monitor

patterns of secretion and standing crop in a

population, such as would be encountered by a

notional forager that is wholly unselective

with respect to flower age, a random sample of

flowers is taken at each sampling time.

Lengths of slender, flexible tubing can be

inserted into a flower, left in contact with the

nectary, and allowed to take up nectar as it is

secreted over a period of time; periodic marking

of the meniscus position allows the secretion

rate to be monitored. Bertsch (1983) used

graduated microcapillary pipettes for this purpose.

If the tubing or pipette is slender enough

to withdraw nectar as soon as it is secreted,

reabsorption may be prevented and this

method may measure gross secretion rate. If

the nectar is protected from evaporation from

the moment of secretion, the method can also

be used to examine the concentration at which

nectar is secreted (Bertsch, 1983).

6. CONCLUSIONS

Measurements of the standing crop and

secretion rate of nectar are often a valuable or

essential component of ecological studies of

flower-visiting animals (e.g. Waddington,

1983) or functional studies of floral biology

(e.g. Zimmerman, 1988). Such measurements

are therefore often required by biologists

whose primary interests and expertise lie elsewhere.

This paper is designed to facilitate their

8 S.A. Corbet

task, complementing the major reviews of relevant

techniques (Dafni, 1992; Kearns and

Inouye, 1993) by resolving some discrepancies

about units of measurement and explicitly

addressing some uncertainties about equipment

that have sometimes caused problems for

workers using these methods for the first time.

ACKNOWLEDGEMENTS

I am grateful to the many students and colleagues

who have tested and improved techniques

for measuring nectar. I thank Juliet Osborne for

comments on the manuscript, and Bellingham &

Stanley Ltd for lending a refractometer for volume

trials.

Résumé – La teneur en sucre du nectar : estimation

de la quantité de nectar disponible dans les

fleurs et du taux de sécrétion au champ. Cet article

décrit les méthodes pour échantillonner et mesurer

les taux de sécrétion et les quantités de nectar

disponibles dans les fleurs au champ. Il vise à résoudre

certaines omissions et certains désaccords présents

dans d’autres articles de synthèse sur les

techniques de mesure du nectar. Parce que la valeur

énergétique du nectar est importante pour les animaux

qui visitent les fleurs, la quantité de nectar est

souvent exprimée par la teneur en sucre (mg sucre

par fleur). Elle peut être calculée par les mesures au

champ du volume du soluté et de sa concentration

dans des fleurs prises individuellement. Pour

l’échantillonnage, je recommande les pipettes microcapillaires

en verre, suffisamment fines pour

échantillonner quantitativement les fleurs pollinisées

par les insectes sans les déchirer et je discute de

méthodes alternatives d’échantillonnage pour les

très petites quantités ou pour le nectar très visqueux.

J’indique un type de réfractomètre à main qui peut

mesurer la concentration du soluté (en g de saccharose

pour 100 g de solution) dans de très petits volumes

de nectar et je considère dans quelle mesure

les calculs de la valeur énergétique du nectar basée

sur les lectures du réfractomètre sont affectés par les

facteurs suivants : présence de sucres autres que le

saccharose ou de composés autres que les sucres,

température, regroupement des échantillons de nectar

provenant de plusieurs fleurs ou essai de dilution

d’un échantillon de nectar dans la micropipette. La

quantité de nectar est souvent distribuée irrégulièrement

parmi les fleurs d’une parcelle et la quantité

moyenne de sucre par fleur rencontrée par un insecte

qui butine systématiquement peut dépasser celle

échantillonnée par un écologiste non sélectif.

Le taux de sécrétion, communément exprimé en mg

de sucre par fleur et par heure, est mesuré par le taux

d’accumulation de nectar dans des fleurs vidées et

dont on a exclu les visiteurs en ensachant les fleurs.

Les sachets en voile de mariée ou en moustiquaire

agissent moins sur le microclimat, et donc sur la

concentration en nectar et le taux de sécrétion, que

les sachets en papier ou en polyéthylène. Certaines

espèces réabsorbent le nectar et cette réabsorption

par les fleurs peut réduire le taux apparent de

sécrétion. Pour minimiser cet effet, la durée pendant

laquelle la sécrétion de nectar est mesurée doit être

aussi brève qu’il est possible pour une mesure

précise. Un protocole pour suivre la quantité de

nectar disponible et le taux de sécrétion sur une

journée de l’aube au crépuscule est indiqué.

nectar / quantité disponible / taux de sécrétion /

concentration / réfractomètre / microcapillaire /

nectarivore

Zusammenfassung – Der Zuckergehalt im Nektar:

Schätzung der Nektarmenge und der Sekretionsrate

im Freiland. Diese Arbeit beschreibt

Methoden zur Sammlung von Proben und zur Messung

von Sekretionsraten von Nektar in Blüten im

Freiland. Das Ziel ist die Aufklärung von einigen

Auslassungen und Diskrepanzen in anderen Darstellungen

der Techniken zur Messung von Nektar.

Da der Energiegehalt des Nektars für die Blüten besuchenden

Tiere wichtig ist, wurde die Nektarmenge

(anstehende Ernte) häufig als Zuckergehalt (mg

Zucker pro Blüte) dargestellt. Dieser kann aus den

Messungen des Volumens und der Konzentration

der Lösung der einzelnen Blüten geschlossen werden.

Zur Probensammlung empfehle ich mikrokapillare

Glaspipetten, die dünn genug sind, um durch

Insekten bestäubte Blüten quantitativ ohne Verletzung

zu beproben. Außerdem diskutiere ich Sammlungsmethoden

für sehr kleine Mengen oder sehr

zähflüssigen Nektar. Ich setze mich für einen Typ

eines handlichen Refraktometers ein, der in sehr

kleinen Nektarvolumen Konzentration der Lösung

messen kann (in g Sucrose per 100 g Lösung) und

ich berücksichtige das Ausmaß des Vorkommens

von zusätzlich zu Sucrose gelösten Stoffen wie

nicht-Zucker Komponenten auf die auf den Ablesungen

des Refraktometers basierenden Berechnungen

des Energiegehalts. Auch der Einfluss von

Temperatur und von Sammelproben des Nektars

von mehr als einer Blüte oder der Versuch einer

Verdünnung der Nektarprobe in der Mikropipette

wurden einbezogen. Die Nektarmenge ist häufig

ungleich in den Blüten in einer Stelle verteilt und

die durchschnittliche Zuckermenge pro Blüte, die

von einem systematischen Sammler angetroffen

wird könnte über der Menge liegen, die von einem

unselektive Ökologen gesammelt wird.

Measuring nectar 9

Die Sekretionsrate, allgemein als Zucker pro Blüte

pro Stunde ausgedrückt, wird als die Rate der

Akkumulierung von Nektar nach einer Leerung der

durch Umhüllung vor Blütenbesuchern geschützten

Blüte gemessen. Die Hüllen aus Brautschleiern oder

Moskitonetzen haben einen geringeren Einfluss auf

das Mikroklima und damit auf die Stoffkonzentration

im Nektar und die Sekretionsrate als Hüllen aus

Papier oder Polythen. Einige Arten resorbieren

Nektar und diese Resorption durch die Blüten kann

die scheinbare Sekretionsrate vermindern. Um diesen

Effekt zu verringern, sollte das Intervall, in dem

die Nektarsekretion gemessen wird, so kurz sein

wie es für eine genaue Messung möglich ist. Ein

Protokoll für ein Monitoring der Nektarmenge und

der Sekretionsrate über einen Tag vom Morgengrauen

bis in die Abenddämmerung wäre ausgezeichnet.

Sekretionsraten / Nektarmenge / Konzentration /

Mikrokapillare / Refraktometer / Zucker /

Aminosäuren

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To access this journal online:

www.edpsciences.org

Arvind Singh

Please have a look at these links and PDF attachment.

http://aob.oxfordjournals.org/content/103/3/533.full

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2803636/

Masoud Latifian

Hello Dear

Washing is recommended for nectar collection from flowers with low nectar volumes in the field (with the understanding that one wash underestimates the amounts of sugars present in a flower), as is immediate analysis of sugar mass. In view of the great variation in results depending on nectar collection and storage methods, caution should be exercised in their choice, and their accuracy should be evaluated. The use of pulsed amperometric detection, more specific than refractive index detection, may improve the accuracy of nectar sugar analysis.

Mukesh Kumar Singh

we discussing on the above matter and got there is a system of mjcro pipette that can measure directly nectar content

Ekaterina Kozuharova

https://scholar.google.bg/scholar?hl=bg&q=corbet+nectar&btnG=

Dafni, A. (1992). Pollination ecology: a practical approach.

Lewis Maccarter

As implied by Mukesh Singh, many, if not most flowers produce nectar, pollen, and even odor only when the conditions are correct (especially temperature). For each species of flower watch for when, and at what temperature, lots of bees visit that kind of plant. The rest seems to be covered by the other answers here. Small 'nuclear' bee hives are useful for making such observations. Maybe hire a bee keeper to provide hives near your sample sites.

Mukesh Kumar Singh

Even different species of bees are preferring different flowers, its depend on shape size of flower and availability of variety of flowers, then it can be observed

Lewis Maccarter

There is another challenge; some flowers are adapted for different pollinators, even excluding domestic bees, which pollinate many row crops. Some are for humming birds (you would have to bird watch to see when the maximum nectar is present). Some white ones are for moths and the nectar is available some time in the night. Each orchid in its natural environment has a different pollinator. Then there are the flowers pollinated only by bats. Well, have fun!

Sarah Alexandra Corbet