Floral nectar secretion and ploidy in Brassica rapa and B napus (Brassicaceae). I. Nectary size and nectar carbohydrate production and composition

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Floral nectar secretion and ploidy in Brassica rapa and

B napus (Brassicaceae). I. Nectary size and nectar

carbohydrate production and composition

Ar Davis, Vk Sawhney, Lc Fowke, N H Low

To cite this version:

Original

article

Floral

nectar

secretion and

_ploidy

in Brassica

_rapa

and B

_napus

_{(Brassicaceae). I.}

_Nectary

size and

nectar

_{carbohydrate production}

and

_composition

*

AR Davis

VK

_Sawhney

LC Fowke

N H Low

1

Department

of

_Biology;

2

_{Department of Applied Microbiology}

_{and Food Science,}

University

of Saskatchewan, Saskatoon, Saskatchewan, S7N 0W0 Canada

(Received

9

May

1994;

accepted

3

_{August 1994)}

Summary — Haploid (n

= _{10), diploid (2n = 20)} and

_{tetraploid (4n}

=

₄₀₎

lines of _{Brassica rapa}

_(syn

campestris),

and a line of

allotetraploid (4n = 38)

B napus, were examined to determine whether

ploidy

can influence nectar

production.

Flowers of all lines

developed

functional nectaries. Overall, nectar

car-bohydrates

consisted almost

_exclusively

of

_glucose

and fructose, present in

_{quantities slightly}

in favour of the former. Sucrose was detected in

_only

15% of _{samples, usually} in trace amounts. For all levels of

ploidy,

95% of total nectar

_carbohydrate

_{per flower} was

_expelled

from the lateral

_{(inner) pair}

of

_glands.

These

glands

were

_{directly supplied}

with

phloem

alone, whereas the median

_{(outer) glands,}

which were poor

nec-tar

yielders, usually

did not receive any vascular

_{supply. Haploids only produced}

30% as much nectar

car-bohydrate

as 2n and 4n lines of B rapa, which in tum exuded _only 44-50% of the average

quantity

of

nec-tar

_carbohydrate

released

_by

B _napus. A linear

regression (r= 0.803)

of mean

_{lateral-nectary}

volume on

average total

nectar-carbohydrate

per flower was determined for all

plants

of B rapa, but this was mod-ified

(r= 0.445)

when data for B napus were included. In all lines,

opportunity

exists for selection for

high nectar-carbohydrate production.

Plants

_yielding

the most floral nectar

_carbohydrate

had

_high

fre-quencies (80-95%)

of lateral

_glands

that were

_symmetrical

and of uniform size within a flower. Brassica rapa / Brassica napus / floral nectar / nectar

_{carbohydrates}

/ nectary /

ploidy

INTRODUCTION

Due

_primarily

to the

_pioneering

efforts of the late Dr A

Maurizio,

the influence of

ploidy

on floral nectar

_{production by}

bee-visited

_plants

has

_already

been

investi-gated

in 4

_plant

families. She determined

nectar

_yields

and

_nectar-sugar

concen-trations for up to 4

_species

each in the

Campanulaceae,

Fabaceae,

Lamiaceae and Solanaceae

_(Maurizio,

1954,

1956).

Except

for recent

_{investigations}

on Nico-tiana spp

(Cocucci

and

_{Galetto, 1992),}

other studies on

_ploidy

and nectar

secre-tion have centred on 2

_species,

Trifolium *

hybridum (Skirde, 1963)

and

_especially

T

pratense

(Paatela,

1962;

Valle et

al, 1962;

Skirde,

1963; Bond, 1968; Eriksson, 1979;

Fussell, 1992).

Most of these studies have

compared

nectar

_production

of

_plants

which are

_{naturally diploid}

with those which

are

_polyploid

_(triploid,

_tetraploid

and

octa-ploid); haploids

have not been considered. Nectar

_{produced by}

flowers of Brassica spp is a well-known attractant to

_potential

insect

_{pollinators (Eisikowitch,}

1981;

Murrell and

Nash,

1981;

Fries and

Stark,

1983;

Williams, 1985;

Mohr and

_{Jay, 1988)}

and

represents

a

_large

source of

_honey

world-wide

_(Crane,

_1975).

This

_study

was

con-ducted to determine any effect that

differ-ences in

_ploidy

level

_might

have on

_nectary

size and

_{nectar-carbohydrate production,}

concentration and

_composition,

in the Bras-sicaceae.

_{Haploid, diploid}

and

_tetraploid

plants

of B rapa

(syn campestris),

and

allotetraploid plants

of the

_genetically

related B _napus, were

_compared.

MATERIALS AND METHODS

Plant material

Haploid

(n

=

_{10), diploid (2n}

=

₂₀₎

_and

_tetraploid

(4n

=

40) plants

of B _rapa, and

_plants

of B _napus

(4n

= 38, 2n + 2n = 20 from B _rapa + 18 from B oleracea =

_38),

were grown under controlled

climatic conditions

(16

h

light/8

h dark,

150

μmol/m

2

s,

20-22°C)

in a

_greenhouse

(hap-loids)

or in

_growth

chambers. Except for the

hap-loid lines, all

_plants

were derived from seed of the

’rapid-cycling’

forms of Brassica

_(Williams

and Hill,

1986),

characterised

by

a shorter time to

flow-ering

than conventional lines. Plants of different

ploidy

were not from the same

genetic

back-ground.

Potential insect _pollinators were absent.

Nectar collection

Eighteen

to 22 flowers were _sampled for nectar

from each of 4

plants (numbered 1-4)

per

ploidy

level. The

_haploids

were an

_{exception; only}

3

plants (5-49,

5-63, 5-93) were available for

nec-tar studies. Over several

_{days, fully-open}

flowers,

24 h after anthesis, were excised and their nectar

immediately

collected

using filter-paper

wicks

(McKenna

and Thomson,

1988).

To enhance

col-lection of all of the viscous nectar _{present, tips} of

wicks were

_{initially dipped}

in _pure distilled water and the process of

withdrawing

nectar at the flower

base

_using

the wicks was viewed

through

a

dis-secting microscope. Commonly,

more than one

wick was _{required per nectar droplet.}

Nectar from each of the

_{heavily secreting}

lat-eral

_{glands (Davis}

et al,

1986)

of 5 additional,

identical flowers from these

_plants

was collected

using

Drummond

Microcaps (1 μl)

and its solute concentration

immediately

assessed

using

a

hand-held refractometer

(40-80%, Bellingham

and

Stanley, Tunbridge

Wells, UK). Even

_though

this instrument had been modified

_by

the

manu-facturer to accomodate minute volumes, the

extremely

low

quantities

of viscous nectar

avail-able from lateral

_glands

of the haploid plants,

unfortunately,

could not be

_analysed

for

concen-tration. After temperature correction to _20.0°C, concentrations of nectar solutes were calculated

as

_g/ml

from

g/g using

the formula for

conver-sion of refractometer

_{readings (Bolten}

et al, 1979;

Bbrquez

and Corbet, 1991).

Nectar

_{carbohydrate analysis}

From the

_filter-paper

wick collections of nectar

(above),

a

_subsample

of 16 floral exudates per

plant was selected at random. After elution from

the stored, dry wicks

_using

0.5-1.0 ml distilled water

_(3°C),

each nectar

_sample

was filtered,

diluted as

_required

and then

_{analysed separately}

on a Waters 625 metal-free

_{gradient high}

perfor-mance

_{liquid chromatograph. Carbohydrates}

were

separated using a Carbo Pac PA1

(Dionex)

pel-licular anion

_exchange

column

₍₄

x 250

_mm).

A Waters 712

Wisp autosampler

was utilized for

analysis;

100

μl

of each

sample

was

_injected.

Elution of the nectar

_{carbohydrates}

was

accom-plished

with a mobile

_phase

of 80 mM NaOH at a

flow rate of 1.0 ml/min. The

carbohydrates

were

detected

by

a

_{pulsed amperometric}

detector

(PAD;

Waters Model

464)

with a dual

gold

elec-trode and

_{triple pulsed}

_amperometry at a

sensi-tivity

of 50 mA. The electrode was maintained at the

_{following potentials}

and durations:

E

1

= _{0.05 V}

(T

1

= _{0.299 s);}

_E

₂

= _{0.60 V}

(T

- 0.80 V

_(T

₃

₌

= 0.499

s).

The

_{carbohydrates eluting}

from the column were

_{plotted by}

a Maxima 820

chromatography

work station

_(Millipore).

Peak

areas were

_compared

to known standards. Where individual nectar

_droplets

_per flower were

anal-ysed separately,

the total nectar

_carbohydrate

per flower was calculated

_by

summation of those

samples.

Determination of

_nectary

volume

Following

nectar collection, 10 of these flowers per

plant,

chosen randomly, were fixed and pro-cessed for

_scanning

electron

_{microscopy (SEM)}

(Davis

et al,

1986).

The lateral nectaries

(fig 1)

were found to _produce almost all of a flower’s

total nectar

_{carbohydrates (see Results).}

Accord-ingly,

to determine whether a

_relationship

existed between

_{nectar-carbohydrate production}

_per flower and nectary size, volumes of lateral _glands

were determined from SEM

_{images (Davis et al,}

submitted).

Examination of sections

_{of nectary}

tissue

_{using light}

_microscopy

After their nectar had been collected, the

peri-anth was removed and several flowers per plant

were fixed and

_dehydrated

before

_embedding

in LR White resin

(Davis

and

Gunning, 1992).

Sec-tions

(1.5

μm

thickness)

were cut

_{using glass}

knives on a Reichert Ultramicrotome OmU3,

stained with toluidine blue O and then observed and

photographed

with a Zeiss

_Axioplan

Univer-sal light microscope.

RESULTS

Nectary

location and

_anatomy

Regardless

of

_ploidy

level,

all

_plants

had floral

nectaries,

which were

_generally

in the normal

positions

for the genus

(Norris,

1941;

Clemente Munoz and Hernandez

_Bermejo,

1978;

Davis

et al,

_1986).

In

_{B napus and}

B rapa, each flower bears 2

_pairs

of nectaries at the base of the 6 stamens. Each lateral

(inner) gland

is located internal to the fila-ment base of a short stamen

_(figs

1,

2),

whereas each median

_{(outer) gland}

is found external to the filament bases of the

_long

stamens

_(figs

_{1, 4,}

_5).

In terms of

vasculature,

a

_disparity

existed between lateral and median

_glands

within the same flowers. Lateral nectaries received vascular traces of

_phloem

sieve elements that entered the

_gland

bases and

were evident

_throughout

the

_gland

interior

(figs

2,

3).

Median

nectaries,

however,

nor-mally

lacked any vascular

supply

(figs

4,

5).

Nectar

_{carbohydrate production}

and concentration

collectable

_quantities

of nectar at both median

_positions

per flower. On the other

hand,

in the n

_plants

_{of B rapa,} 2 flowers of line 5-49 lacked nectar at 1 lateral

_gland,

and,

throughout

the n

_{plants, only}

45% of flowers

_produced

nectar at both median

nec-taries. In

fact,

in 36% of

flowers,

nectar was

absent

_altogether

from the median

_glands.

The latter

_phenomenon

was most

_prevalent

in line

5-63,

in which 15 of 18 flowers pro-duced nectar

_only

from the lateral nectaries. This situation did not

_adversely

affect the

mean

_{nectar-carbohydrate production}

per flower in line

5-63,

which

_yielded

at least

as _{much sugar} as the other n lines

_(fig

_6).

Plants of each

_ploidy

level and

species

exhibited variation in total

nectar-carbohy-drate

production

per

flower,

such that

sig-nificant differences were detected for each

(fig 6).

Overall,

haploid plants produced,

on

average,

only

28-32% as much nectar

car-bohydrate

as 2n and 4n

_plants

_{of B rapa}

(table

I). However,

total

_quantities

of nectar sugar between the 2 latter lines did not dif-fer

_{significantly (table I).}

The total amounts of nectar

_carbohydrate

_{exuded per flower}

by

2n and 4n

_plants

_{of B rapa} were

_only

44-50% of the average

quantities produced

by

B _napus

_{(table I).}

In the 2n

_plants

_{of B rapa, and in B} napus, these mean

_weights

of nectar

car-bohydrate

accounted for over half of the flower

_{dry weight (table I).}

In the n and 4n

plants

of B rapa, the

proportion

of flower

dry weight

attributable to nectar _sugar was

less

_{(table I).}

Despite

these overall differences in mean

nectar-carbohydrate production,

the

pro-portion

of total

_carbohydrate

_{per flower that}

escaped only

from the lateral nectaries was

high

and consistent for all combinations of

species

and

_ploidy,

about 95% for each

(table I).

Similarly,

the concentration of

nec-tar solutes collected at the lateral

_glands

was

_equal

for all

_{rapid-cycling}

lines of

Bras-sica,

and

_usually

_{very similar} at both lat-eral

positions

within a flower

_{(table II).}

How-ever, the 4n line of B rapa had

significantly

greater

variation in nectar-solute

concen-tration at lateral locations within a

flower,

than did 2n

_plants

of _{B rapa,} or _{B napus}

(table II).

Nectar-carbohydrate

composition

The nectar

_{carbohydrates}

of both

_species

of

Brassica,

regardless

of

_ploidy

level,

were

found to be

constant,

and consisted almost

entirely

of

_glucose

and fructose

_{(table III).}

The

_quantities

of

_{glucose usually slightly}

exceeded those of

fructose,

such that the

average ratio of the monosaccharides

ranged

from 1.02 to 1.13

_{(table III).}

_Only

in 2 cases,

_plants

2 and 4 of

_diploid

_{B rapa,}

was the mean ratio of

_{glucose/fructose}

below

_unity.

Volume of lateral nectaries

and

_{nectar-carbohydrate production}

Just as the

_{study plants displayed}

consid-erable variation in mean

nectar-carbohy-drate

_production

of their

flowers,

_interploidal

overlap

in size of their lateral nectaries was

also evident

_{(fig 7).}

Plant 4 of 4n B _{rapa had} lateral

_glands

over twice as voluminous as, and

_plant

2 of the 2n _{B rapa had}

_glands

larger

than,

any

plant

of B napus, on _average

(fig

7). Overall,

the average volume of each lateral

_gland

of the n

_plants

of B _rapa was

significantly

lowest,

ranging

from 25.0-38.3% of the mean size of nectaries from all other

ploidy types.

However,

despite

differences in flower

size,

the average volume of lateral nectaries did not differ

_{significantly}

between B napus and 2n _{and 4n B rapa}

_(Davis

et

_al,

submitted).

A

_regression

of mean

_{lateral-nectary}

(r= 0.803) (fig 7). The large

lateral nectaries of

_plant

4 of the 4n _{B rapa,}

however,

car-ried a

disproportionate weighting.

When the data for the 4

_plants

of B _napus were

combined with the _{B rapa}

data,

the new

regression

was still

_positive

and

linear,

but not as

_strong

_{(r= 0.445) (fig 7).}

Relationships

between volume of lateral nectaries and the total nectar

_carbohydrate

per flower within a

_ploidy

level were

_usually

strong (r> 0.84)

and inverse when

nectary

size within a flower was

_expressed

as a ratio

nec-taries were most constant in volume at

opposite,

lateral sides of the same flower

produced

the

_{greatest quantities}

of nectar sugar in n and 4n

_plants

_{of B rapa and} in

plants

of B napus

(fig

8).

In the former 2 cases, those

_plants

also had the

_largest

average

gland

sizes,

whereas in B _napus

a

_{plant (No 3)}

intermediate for

_gland

size had the most

_equal-sized

lateral

_glands

_(figs

7, 8).

A consistent feature of these 3

_plants

bearing

constant-sized nectaries in their

flowers was their

_{high frequencies}

_(B

_rapa:

n:

_85%,

4n:

80%;

B napus:

95%)

of

_gland

morphology

that resembled a

_symmetrical,

united

_outgrowth

above the short stamen

(Davis

et al,

submitted).

DISCUSSION

the lateral

_glands

was very similar for all the

_{rapid-cycling}

lines of Brassica. In 12

species

studied

_by

Maurizio

_{(1954, 1956),}

however,

nectar-carbohydrate

concentra-tions were

_considerably

lower than the

pre-sent results for Brassica.

Furthermore,

8 of 10

_species

which differed in concentration of nectar solutes had

_higher

concentrations in the 2n than in the 4n state. These

dis-crepancies

may be

explainable by

differ-ences in floral

_morphology.

In the

species

of

Campanulaceae,

Fabaceae,

Lamiaceae and Solanaceae that Maurizio

studied,

the base of the corolla is united and tubular.

Therefore,

nectar

_accumulating

at the base of those flowers is enclosed and

_relatively

protected

from the

_atmosphere,

such that

evaporation

of water from nectar would be reduced.

Furthermore,

in red

clover,

the corolla tube in 4n

_plants

is

_{commonly longer}

than in 2n

_plants

_(Paatela,

1962;

Valle et

al, 1962;

Skirde,

1963;

Bond,

1968),

and the solute concentration of nectar for

diploids

is similar

_(Skirde,

₁₉₆₃₎

or,

usu-ally, higher

(Maurizio,

1954;

Paatela,

1962;

Valle

et al, 1962;

Eriksson, 1979).

How-ever, for _{B rapa, because the corolla is} not fused into a

tube,

increases in

_{petal length}

associated with

_{higher ploidy (Davis}

et

al,

submitted)

have little influence on nectar

concentration;

the nectar

_droplets

still remain

_exposed

at the flower base.

_Indeed,

in this

_species,

nectar of mutant flowers

completely lacking petals

had the same

solute concentration as nectar from

_petalled

flowers

_(Brunel

et al,

1994).

That the concentration of nectar solutes at the lateral

_glands

was similar for all

rapid-cycling

lines of

Brassica,

and

_presumably

also for the

_{haploid plants,}

indicates that the

_great

_disparity

in total

_quantity

of

nec-tar

_{carbohydrates}

between B napus and the 3

_ploidy

lines of B rapa resulted from

larger

volumes of nectar

_being

secreted. This was

manifested in the

_larger

number of wicks

required

to

_gather

the nectar from flowers of B napus, and corroborates the

field-plot

study

of Szabo

₍₁₉₈₂₎

in

which,

on

aver-age, flowers of many lines of B napus

yielded

2.1 times as much nectar as _{B rapa}

(2n). Particularly noteworthy

is that this

dis-parity

in

_quantity

of nectar

_carbohydrate

is maintained

_despite

numerous

_competing

’sinks’

_(the

_expanding

_siliques)

located below

_{nectar-bearing}

_{flowers of B napus.}

On the

_{contrary, capsules}

of the self-unfruit-ful B rapa

rarely developed

in this

controlled,

pollinator-free, study

environment

_(Davis

et

al,

submitted).

In B rapa, 4n

_plants,

on _average,

pro-duced 1.13 times more nectar

_carbohydrate

than 2n

_plants,

but this difference was not

significant.

Because the solute concentra-tions of their collected nectars were

_identical,

the 2n and 4n

_{plants probably produced}

similar nectar volumes. This result

_disagrees

with every

previous study,

wherein nectar volumes in flowers of 4n

_plants

were

_usually

double or more, that of their 2n

_counterparts

(Maurizio,

1954, 1956; Paatela, 1962;

Valle

et al, 1962;

Skirde,

1963; Eriksson,

1979).

Again,

increases in

_length

of the corolla tubes of 4n over 2n

_plants

would

_probably

reduce

_{post-secretion}

_evaporation

of water from nectar, and therefore contribute to

higher

nectar volumes

_(and

lower

carbohy-drate

_{concentrations)}

in 4n

_plants.

However,

estimated total

_carbohydrate

in nectar

(prod-uct of volume and

_{concentration)}

in 4n

_plants

was

_always

found to

_outyield

the 2n

_plants,

as follows: Datura spp: 2.65

times;

Lobelia sp:

1.29;

Monarda sp:

2.33;

Nicotiana spp:

1.90;

Salvia _spp:

1.94;

Trifolium

_hybridum:

2.73;

T pratense:

1.98;

other

_{Trifolium spp:}

2.34.

_Although

4n

_plants

_{of B rapa did} not

outyield

the 2n

plants

significantly,

they

released 3.53 times more nectar

carbohy-drate than the n

_plants.

Whereas the median

_pair

of

_glands

was

usually

active,

though apparently

less so in

haploids,

it

_{produced only}

small

_quantities

of

nectar, such that 95% of the total nectar

carbohydrate

per flower from Brassica

plants

nectaries

(see

Búrguez

and

_{Corbet, 1991).}

These results are consistent with the

_great

disparity

in

_quantity

of

_phloem

_which

sup-plies

each

_{gland type}

in B arvensis

_(Frei,

1955)

and the

_species

studied

here,

B _napus

(Frei,

1955;

Davis

et al,

1986)

and B rapa

(present results).

The

_{heavily secreting}

lat-eral nectaries are

_{penetrated by}

numerous,

branching

sieve elements whereas the base of the

_larger,

_{median nectaries lacks any} vascular

_supply

or is

_barely

innervated

_by

phloem.

Therefore,

the

_large

differences in

nectar-carbohydrate yields

observed

over-all between B napus and the 3

ploidy

lev-els of B rapa are attributable to differences in

_{production capacity}

of the lateral

nec-taries.

Size of the lateral nectaries was not the

only

significant

factor

_responsible

for dif-ferences in

_{nectar-carbohydrate}

produc-tion.

_{Haploid plants}

_{of B rapa had}

_glands

only

one-third of the

size,

and

_only

pro-duced 14% of the

_quantity

of nectar

carbo-hydrate

of

_plants

of B napus.

However,

the lateral nectaries of 2n and 4n

_plants

of B rapa were no smaller than the

correspond-ing glands

of B napus,

yet

yielded only

half

as much

_{carbohydrate.}

Therefore,

the abso-lute size of the

_glands

is not the

_prime

fac-tor. Future research on other

_nectary

fea-tures is warranted and

_might

assist selection for

_{high nectar-carbohydrate}

pro-duction,

as there can be considerable vari-ation in _sugar

_yield

between

_plants

within each

_ploidy

level.

However,

_plants

_having

a

_relatively

con-stant size of lateral nectaries at

_opposite

sides of the same flower did tend to pro-duce more nectar

_{carbohydrate.}

Attainable

nectary

size may be determined

by

the

symmetry

of the floral apex, and the size of other floral

parts, because,

in terms of floral

_ontogeny

in the

Brassicaceae,

devel-opment

of the nectaries occurs last

(Sat-tler,

1973; Davis,

1992).

Although

nectaries

were not

_investigated

in earlier studies of

ploidy, glands

of 4n

plants

of all other

species previously

studied occur on the

gynoecium

or between it and the

androe-cium,

and thus are

_probably

not

_spatially

restricted

_{by neighbouring}

_{floral organs} to the extent that occurs at the lateral

posi-tions in _{B rapa.} In the 4n

_plants

of _{B rapa,}

the lower

_percentage

of flower

_{dry weight}

accounted for

_by

nectar _{itself may indeed} reflect a subnormal increase in lateral

nec-tary

size

(and function)

relative to the size of

_{petals, sepals,}

etc,

compared

to the

diploids

of B rapa.

Maurizio

_{(1954, 1956)}

found

that,

within

a

_species,

the

_{carbohydrate composition}

in the nectar of

_{polyploid plants}

differed from 2n

_plants;

the ratio of sucrose to hexose

was

_usually

_higher

in the latter.

However,

sucrose was

_virtually

absent from the nectar of n, 2n and 4n

plants

of B rapa and from B napus, and the overall ratio of

glucose

to fructose

_(approx

1.1 to

₁₎

remained

con-stant,

regardless

of

_ploidy

level. This ratio accords with earlier

_{reports involving}

anal-ysis

of the floral nectar of these same

species (Low

et al, 1988;

Mesquida

et al,

1988;

Kevan

et al,

1991). Thus,

at least in nectaries of B rapa, there appear to be sim-ilar biochemical

_systems

in

_operation,

regardless

of

_ploidy.

ACKNOWLEDGMENTS

We thank A Ferrie for

_generously

allowing

access

to the

_{haploid plants}

of B rapa, and P Williams

for seed of the

_{rapid-cycling}

lines. W South and A

Shukla are thanked for

providing

a most

_helpful

introduction to HPLC. The careful assistance of S Stone with

critical-point drying

most of the tis-sues, and the technical

expertise

of Y Yano with

SEM, are

_{gratefully acknowledged.}

J Smith, J Sullivan and M Cowell cared for the

_plants.

ARD is

grateful

to the Natural Sciences and

Engi-neering

Research Council of Canada for a

Post-doctoral

_Fellowship.

napus

(Brassicaceae).

I. Taille des

nec-taires,

étude

_quantitative

et

_qualitative

des

_glucides

du nectar. On a

_comparé

des

_{lignées haploïdes (n}

=

10), diploïdes

(2n

=

20)

et

tétraploïdes

(4n

=

40)

de Bras-sica rapa

(syn

campestris)

et

_{allotétraploïdes}

(4n

=

38)

de B napus pour savoir si la

ploï-die

_pouvait

avoir une influence sur la sécré-tion nectarifère.

_Excepté

les

haploïdes, qui

étaient issues d’une culture de

_microspore,

toutes les

_lignées

provenaient

de

_graines

de formes de Brassica à

_cycle

_rapide.

Toutes les

_{plantes, indépendamment}

du niveau de

ploïdie, portaient

des nectaires

fonctionnels, chaque

fleur en

_possédant

2

paires. Chaque glande

latérale

_{(intérieure)}

est située au-dessus du

_point

d’insertion d’une étamine courte et _{entourée par la} base de 2 étamines

_longues

et de 2

_pétales

(fig

1,

centre). Chaque glande

médiane

(extérieure)

se trouve à la

_jonction

externe de 2 étamines

_{longues (figs}

1, 4,

5).

Le nectar a été

_prélevé

24 h

_après

l’anthèse dans 18 _{à 22 fleurs par}

plante.

Quatre

plantes

par niveau de

ploïdie

ont été

étu-diées,

seulement 3 pour les

haploïdes.

Dans les

_lignées

à

_{cycle rapide plus}

de 94% des

glandes

médianes ont

_produit

du nectar,

que l’on

pouvait

récolter à l’aide de bande-lettes en

_{papier filtre ;}

chez les

haploïdes,

ce

pourcentage

était inférieur de

_plus

de la moitié.

Néanmoins,

dans

l’ensemble,

les nectaires médians n’ont

_produit

que 5% de la

_quantité

totale des

_glucides

du nectar

flo-ral,

la

_paire

latérale en fournissant la

majeure partie (tableau I).

Cette

_disparité

dans la

_production

de

_glucides

a une

rai-son

_{anatomique :}

seuls les nectaires

laté-raux sont directement

_{approvisionnés}

en

phloème (figs

2 et

_3),

les

_{grandes glandes}

médianes étant

_dépourvues

de vaisseaux

(figs

4 et

_5).

Au sein de

_chaque

niveau de

ploïdie,

la

_quantité

totale de

_glucides

du nectar

_produite

variait d’une

_plante

à l’autre et l’occasion se

_présente

donc d’exercer

une sélection. Les

_haploïdes

n’ont

_produit

que 30% de la

quantité

de

_glucides

secrétée par fleur chez les

lignées

2n et 4n _{de B rapa,}

qui

eux-mêmes n’ont secrété que 44 à 50% de la

_quantité

moyenne

produite

par B napus

(tableau

I).

Parce que la concentra-tion moyenne du nectar _{secrété par les}

nec-taires latéraux est semblable chez toutes les

_lignées

à

_{cycle rapide (>}

1

_{g/ml ;}

tableau

II),

on attribue ces différences à la

_disparité

dans les volumes de nectar sécrétés. La taille des nectaires

latéraux,

qui produisent

95% des

_glucides

du nectar

_floral,

a été esti-mée sur 20

_{glandes (10 fleurs)}

_par

_plante

par

microscopie électronique

à

_balayage.

Une

_régression

linéaire

_{(r= 0,803)}

de la taille moyenne des nectaires latéraux sur

la teneur _{totale moyenne} en

_glucides

du nectar a été _{déterminée pour} toutes les

plantes

de B _{rapa, mais} sa valeur a

_changé

(r= 0,445) lorsque

les données de B napus ont été incluses. Au

total,

les

_plantes

four-nissant le

_plus

de

_glucides

du nectar ont un

pourcentage

élevé

_(80-95%)

de

_glandes

latérales

_uniformes,

_{symétriques (fig}

₁₎

et de taille constante _pour une même fleur

(fig

8).

Cette relation n’est pas valable pour les

_plantes

2n _{de B rapa}

_{(fig 8). L’analyse}

par

chromatographie liquide

à haute pres-sion a _{montré que les}

_glucides

du nectar floral étaient

_{principalement}

constitués de

glucose

et de fructose dans le

rapport

approximatif

de

1,1

à

1,

avec une très

_petite

quantité

de saccharose

_{(tableau III).}

La

com-position

en

_glucides

reste constante _pour toutes les combinaisons

_d’espèces

et de

ploïdie

(tableau

III).

Brassica

_{rapa/Brassica napus /}

nectar /

glucide

/ nectaire /

_ploïdie

Zusammenfassung —

Florale Nektarse-kretion und Ploidie bei Brassica rapa und B _napus

_{(Brassicaceae).}

II. Grösse der

Nektarien,

Produktion und

Zusammen-setzung

der

Nektar-Kohlenhydrate.

Zur

Untersuchung

des Einflusses der Ploidie auf die

_{Nektarproduktion}

wurden

_haploide

(n

=

_{10), diploide (2n}

=

20)

und

tetraploide

allotetraploide (4n

=

38)

Pflanzen von B napus

verglichen.

Außer der

_haploiden

Linie,

die aus einer

Mikrosporenkultur

stammte,

hatten alle Linien kurze

Entwicklungszei-ten. Alle Pflanzen wiesen

_unabhängig

von

ihrem

_Ploidiegrad

funktionale Nektarien auf. Im

_typischen

Fall hatte

_jede

Blüte zwei

Nek-tarienpaare.

Die

_{lateralen, innenständigen}

Drüsen befanden sich oberhalb der

Anhef-tungsstelle

eines kurzen Staubblattes

_(Abb

1,

Mitte).

Sie wurden von den Basen zweier

langer

Staubblätter sowie zweier Blüten-blätter

_eingefasst.

Die

medianen,

außen-ständigen

Drüsen befanden sich an der äußeren

_{Verbindungsstelle}

von zwei lan-gen Staubblättern

(Abb

1, 4,

5).

Der Nek-tar wurde 24 Stunden nach

_Blühbeginn

aus

18-22 Blüten pro Pflanze

gesammelt.

Für

jeden Ploidiegrad

wurden von den

haploi-den

3,

von den anderen zumeist 4

Pflan-zen untersucht. In den Linien mit kurzen

Entwicklungszeiten produzierten

über 94% der medianen Drüsen

Nekar,

der mit

Filter-papierdochten eingesammelt

werden

konnte;

bei den

_{haploiden betrug}

der Pro-zentsatz

_weniger

als die Hälfte.

_Insgesamt

produzierten

die medianen Nektarien

durch-gehend

nur 5% des Nektarzuckers der

Blüte,

die

_{überwiegende Menge}

wurde von

dem lateralen Paar

_erzeugt

_{(Tabelle I).}

Eine unterschiedliche Produktion von

Nektar-kohlenhydraten

wurde durch anatomischen Befunde untermauert. Nur die lateralen Nek-tarien wurden direkt von Phloem

_versorgt

(Abb

2,

3),

während die grossen medianen Drüsen keine

_{Gefäßversorgung}

aufwiesen

(Abb

4,

5).

Innerhalb

_{jedes Ploidiegrades}

unterschieden sich die

_Gesamtmengen

der Kohlenwasserstoffe

_{(Abb 6),}

hierdurch bie-tet sich die

_Möglichkeit

zur Selektion.

Haploide

Pflanzen von B _rapa

_produzierten

im

_Vergleich

zu den 2n und 4n Linien nur

30% Nektarzucker pro Blüte. Die 2n und 4n Linien

_erzeugten

wiederum nur 44-50% der mittleren

_Zuckermenge

von _{B napus}

(Tabelle I).

Da die Konzentration des Nek-tars der seitlichen Nektarien bei allen rasch-entwickelnden Linien ähnlich war

_(höher

als

1

_g/ml;

Tabelle

_II),

werden die unterschied-lichen

_{Kohlenhydratmengen}

auf Unter-schiede in den

_{erzeugten Nektarmengen}

zurückgeführt.

Die Größe der lateralen

Nek-tarien,

die 95% der

_{Nektar-Kohlenhydrate}

produzieren,

wurde bei 20 Nektarien

₍₁₀

Blüten)

durch

Rasterelektronenmikrosko-pie

bestimmt. Eine lineare

_Regression

des Größendurchschnitts der lateralen Nekta-rien auf die mittleren

_{Kohlenhydratmengen}

pro Blüte wurde für alle Pflanzen von B rapa bestimmt

_(r=

0,803;

Abb

_7).

Diese änderte sich

_jedoch,

wenn die Daten von B _napus

eingeschlossen

wurden

_(r=

0,445;

Abb

_7).

Im ganzen

gesehen

hatten die Pflanzen mit der höchsten

Zuckerproduktion

einen hohen Prozentsatz

_(80-95%)

von

_{einheitlichen,}

symmetrischen (Abb 1)

und

_{gleichmäßig}

großen

lateralen Nektarien

_{(Abb 8).}

Diese

Beziehung

traf

_dagegen

nicht für die 2n Pflanzen von B rapa

(Abb 8)

zu. Die

_Analyse

mit

_{Hochdruckflüssigkeitschromatographie}

ergab,

daß die

_{Nektar-Kohlenhydrate}

über-wiegend

aus Glukose und Fruktose im Ver-hältnis von

1,1

zu 1 bestanden. Saccharose

war in

_{geringen Mengen}

vorhanden

_(Tabelle

III).

Diese

_{Zusammensetzung}

war bei allen

Ploidiegraden

und Arten

_{gleich (Tabelle III).}

Brassica rapa / Brassica napus / Nektar /

Kohlenhydrate

/ Nektarium / Ploidie

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