full text - Journal of Apicultural Science

Vol. 49 No. 2 2005
Journal of Apicultural Science
5
MORPHOLOGICAL FEATURES OF THE NECTARY
AND OF THE POLLEN GRAINS AND THE FORAGING
VALUE OF THE FLOWERS OF YELLOW AZALEA
(Rhododendron luteum Sweet)
El¿bieta
Weryszko-Chmielewska,
Miros³awa Chwil
Department of Botany, Agricultural University of Lublin, 20-950 Lublin, ul. Akademicka15.
Received 5 August 2005; accepted 14 October 2005
S u m m a r y
The study was concerned with the amounts of pollen supplied by the pollen, nectary
structures and pollen grain properties of Rhododendron luteum Sweet. The weight of nectar
produced during the flower’s lifetime and the weight of pollen released by the flowers was
determined. The size of pollen tetrads and of individual pollen grains was measured and pollen
viability was determined. The micromorphology and the anatomy of nectaries was examined
using scanning electron microscopy and light microscopy. The plants of Rh. luteum produced on
average 32.7 mg of sugars and 9.5 mg of pollen per 10 flowers. The pollen grains of Rh. luteum
were rated as medium-sized of flat spherical shape. The grains occur in tetrads that have little
degree of globularity forming a tetrahedral system. Viable pollen grains accounted for 74% of
the anther’s content. Tetrads with four viable grains were the most frequently occurring (40%)
whereas those with all sterile grains accounted for 4%. In pollen tetrads three categories of
pollen grains were observed: viable grains containing mostly protein substances, viable grains
filled abundantly with starch and unviable grains without protoplasts. The nectary gland is
situated at the base of the ovary and forms its external layer. In the epidermis of the nectary
stomata occur which have the capability to adjust the size of the opening. The secretion cells
have a smaller size than the adjacent parenchyma cells. In the ovary walls numerous conductive
bundles occur the ramifications of which penetrate into the glandular layers of the nectary.
Keywords: Rhododendron luteum, nectar secretion, pollen, nectary, structure.
INTRODUCTION
Many
species
of
the
genus
Rhododendron are native to the
mountainous areas of central Asia
(Maurizio and Grafl 1969, Podbielkowski 1991). A substantial sensitivity to
low temperatures is a problem in their
cultivation. Among the more resistant and
frequently planted species is Rh. luteum
Sweet. In nature it is found in scarce
locations in some European countries.
Breeding work is being conducted aimed
at developing azalea varieties with new
ornamental properties and improved
resistance to freezing (Kho and Baer
1970, Klein and Muras 1999). Rh.
luteum is often used in crosses as a
parental material along with other species
of the genus (Vainola 1994).
Azaleas are classified among forage
plants for insects due to their abundant
nectar and pollen production (Maurizio
and Grafl 1969, Bauer 1986, Piazza et
al. 1991, Szklanowska and Denisow
1994). Nectar secreted by the flowers of
different
species
of
the
genus
Rhododendron is harvested by bees,
bumblebees, butterflies and other insects
(Maurizio and Grafl 1969, Bauer 1986,
Sai-Chit et al. 2000). The pollen of
6
Rhododendron representatives has been
observed in honeybee honeys mainly from
the mountainous regions of Europe and
was referred to as principal or secondary.
Honeybees form small pollen loads
thereof, white or silvery grey (Maurizio
and Grafl 1969).
Azalea honey is nearly white or slightly
yellowish, very sweet and smooth without
distinct aroma (Maurizio and Grafl
1969). It shows very low electric
conductivity values and ash contents
(Piazza et al 1991). Fructose is a
prevalent sugar along which glucose and
sucrose were found. It was also established
that azalea honeys are abundant in
enzymes (Maurizio and Grafl 1969).
The objective of the study was to
examine the abundance of nectar secretion
and pollen yield of Rh. luteum, as well as
to examine the surface micromorphology
and anatomy of the nectary. In addition,
analyses related to the pollen morphology
of that species were made.
MATERIAL AND METHODS
A study bearing on the beekeeping
value of the flowers, nectary morphology
and anatomy, and pollen grain properties
of yellow azalea Rhododendron luteum
Sweet. (=Rh. flavum G.Don = Azalea
pontica L.), was carried out in the years
2004-2005. The plant material originated
from the Botanical Garden, Marie Curie-Sk³odowska University (UMCS) in
Lublin.
The weight of nectar produced in a
flower’s lifetime was examined using the
method as described by Jab³oñski and
Szklanowska (1979). Nectar was
collected twice in five samples. Each
sample was composed of nectar obtained
from 6-8 flowers. Concentrations of sugars
in the nectar was determined using a
refractometer and the weight of sugars per
10 flowers was calculated. In order to
assess the weight of pollen contained in the
flowers a method by Warakomska
(1972) was used. Four samples of 50
stamens each were collected and pollen
was washed out from the stamens. The
weight of pollen per 10 flowers and per
100 stamens was determined.
The measurements of pollen tetrads
(n=200) were made in conformance with
the recommendations by Oldfield (1959),
by determining the value of D and d
parameters and the D/d index that
describes the shape of a tetrad. In addition,
the size of individual pollen grains was
measured by determining the length of the
equatorial diameter (E) and that of the
polar axis (P). The shape index (P/E) was
determined. The viability of pollen grains
was examined at full blooming by staining
with acetocarmine (n=250). The presence
of starch in the sporomorphs was detected
following the treatment with Lugol’s
solution.
Micromorphology of the nectaries was
examined using a scanning electron
microscope (SEM) after the plant material
was prepared according to the method by
Weryszko-Chmielewska (2003). Nectary
anatomy was analysed based on
preparations of sections cut by hand and of
semithin sections 1 µm thick prepared
using
the
method
described
by
Weryszko-Chmielewska et al. (2003).
RESULTS
Beekeeping value of yellow azalea
flowers. The nectar of yellow azalea was
collected throughout flower lifetime. The
weight of nectar produced by 10 flowers
ranged from 36.7 to 93.3 mg averaging
84.0 mg. The concentration of sugars in
nectar reached an average of 39% with the
boundary values of 29 and 46%. It appears
from the calculations that the average
weight of sugars in nectar from 10 flowers
was 32.7 mg.
Vol. 49 No. 2 2005
Journal of Apicultural Science
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Table 1.
Average weight of pollen of Rhododendron luteum.
Weight of pollen from:
1 stamen
1 flower
10 flowers
100 stamens
0.19 mg
0.95 mg
9.54 mg
19.08 mg
Table 2.
Morphological traits of pollen of Rhododendron luteum.
Trait
Measurement data
equatorial (E)
Pollen
grain size
min.
31.0
max.
39.1
average
Length of axis
min.
polar (P)
(µm)
26.1
max.
35.9
average
31.1
P/E
Tetrad size
35.0
0.89
dimension D
dimension d
D/d
The pollen of Rh. luteum is white in
colour. The average weight of pollen
produced by 10 flowers was 9.5 mg. The
weight of pollen from 100 stamens was
twice as high (Table 1).
The pollen grains of Rh. luteum occur in
tetrad that form a tetrahedral system. The
average tetrad size as defined by D and d
values was 54.8 and 36.9 µm, respectively.
The index that described tetrad shape (D/d)
reached a value of 1.5 which indicated
little relative globularity. The average size
of pollen grains within the tetrads was 31.1
µm for the polar axis (P) and 35 µm for the
equatorial axis (E). The shape index (P/E)
of 0.89 allows those grains to be classified
as oblato-spheroidal (Table 2).
Viable pollen grains accounted for 74%
of the anther content. From the
examinations of pollen viability within the
(µm)
54.8
36.9
1.49
tetrads it appears that the tetrads with four
viable pollen grains were the most frequent
(40%). Many tetrads contained two or
three sterile grains, accounting for 38 and
15 % of the total number, respectively. The
tetrads with all sterile grains accounted for
4% (Fig. 1). After the treatment of the
anther content with Lugol’s solution three
categories of pollen grains were observed
in pollen tetrads of Rh. luteum: grains with
a protoplast containing protein substances
(yellow), grains filled abundantly with
starch (dark blue) and empty sterile grains
without protoplasts (light) (Fig. 11).
Nectary structure. Sizeable flowers of
Rhododendron luteum have a pentamerous
structure (Fig. 2). Nectaries of that species
are hidden deep in the flowers as they are
situated at the base of the ovary which is
embedded in the corolla tube which attains
Vol. 49 No. 2 2005
Journal of Apicultural Science
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Fig. 6. Closed stoma in the nectary epidermis
(SEM), x 1400.
Fig. 5. Epidermal cells of the nectary with
stomata of different size (SEM), x 630.
Fig. 8. Basal part of the ovary with the visible
bulge of the nectary tissue (longitudinal
section), x 50.
Fig. 7. Longitudinal section of the ovary with
visible nectary (arrows), x 15.
Fig. 9. Fragment of the cross-section of the
ovary with peripherally situated nectary tissue
(arrowheads), x 60.
Fig. 10. Fragment of the cross-section of
the ovary with numerous conductive bundles
which supply the nectary tissue (arrows) x 80.
10
Fig. 11. Pollen tetrads of Rh. luteum containing pollen grains with different cellular content. Atetrads with a cell wall stained with safranin, x 220; B, C, D - pollen grains following acetocarmine
treatment, x 440; E, F, G, H, I - pollen grains after Lugol’s treatment, x 440; B, E - tetrads with all
viable grains; C, F - tetrads containing three categories of pollen grains: 1) viable grains with high
protein compounds content; 2) viable grains with high starch content; 3) sterile grains (devoid of
cytoplasm); D, G - tetrads with one viable grain; H, I - tetrads with one grain of high starch content.
(Fig. 4). Next to the ovary are awl-shaped
non-glandular trichomes whereas in the
upper parts of the nectary there can be
seen, along with non-glandular trichomes,
more hefty glandular trichomes with a
clearly marked head.
The epidermis of the nectary is formed
by small, multigonal cells with convex
outer walls (Fig. 5). In their midst there are
stomata slightly protruding above the
surface of the remaining epidermal cells.
There are characterized by varied size.
During the anthesis closed stomata were
observed (Fig. 6) which may indicate their
ability to adjust the size of the pore
between the guard cells.
At the site of the nectary there is a
distinct dilatation of the ovary (Fig. 7). The
nectary tissue forms a bulge in the lower
part of the organ (Fig. 8). The cells of that
tissue are smaller in size than the adjacent
parenchyma cells. In the ovary walls
occurred numerous conductive bundles the
ramifications of which penetrate into the
glandular layers of the nectary (Figs 9, 10).
Vol. 49 No. 2 2005
Journal of Apicultural Science
DISCUSSION
The localization of the nectary in
Rhododendron luteum corresponds to that
in other Rhododendron species described
by Philipson (1985). However, their
shape and size differ substantially among
species.
The study showed that the nectar
secreted by Rh. luteum contained on
average 39% of sugars. Maurizio and
Grafl (1969) report that the sugar content
in the nectar of various Rhododendron
species is ca. 24%. Szklanowska and
Denisow (1994) demonstrated that, under
Poland’s conditions, the concentration of
sugars in different species of the genus was
from 30% to 70%. Instead, Martini et al.
(1990) report that the nectar of Rh.
arboretum and Rh. barbatum contained
from 17% to 60% of sugars among which
glucose and fructose were present in
similar quantities and only in one sample
sucrose was found. Other authors maintain
that sucrose is the prevalent sugar in the
nectar of the representatives of the
Rhododendron genus (Maurizio and
Grafl 1969, Sai-Chit et al. 2000).
The weight of pollen from 10 flowers as
determined in the study of Rh. luteum
(9.5 mg) provides a sufficient amount for a
bee to form a pollen load which, according
to Maurizio and Grafl (1969), averages
6.8 mg for azaleas.
One of the parameters that define the
larger dimension of pollen grain tetrads in
Rh. luteum averaged 54.8 µm in this study.
The value is lower than that reported for
the species by Szklanowska and
Denisow (1994) (59,6 µm) but fits the
range 51.0-74.0 µm reported by Beug
(2004) for Rh. flavum.
The formation by Rh. luteum of two
categories of viable pollen grains:
1. abundant in protein and 2. abundant in
starch may provide insects with a valuable
supply of diversified food.
11
The determination of the percentage of
sterile pollen grains in a tetrad may be of
importance when choosing Rh. luteum as a
parent species to obtain new hybrids.
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CECHY BUDOWY NEKTARNIKA I ZIAREN PY£KU ORAZ
WARTOŒÆ PO¯YTKOWA KWIATÓW RÓ¯ANECZNIKA
¯Ó£TEGO (Rhododendron luteum Sweet)
Weryszko-Chmielewska
E.,
Chwil
M.
S t r e s z c z e n i e
Przeprowadzone badania dotyczy³y iloœci po¿ytku dostarczanego przez kwiaty, struktury
nektarników oraz w³aœciwoœci ziaren py³ku Rhododendron luteum Sweet. Okreœlono masê
nektaru wytworzonego w ci¹gu ca³ego ¿ycia kwiatu oraz py³ku uwalnianego przez kwiaty.
Wykonano pomiary wielkoœci tetrad py³kowych i pojedynczych ziaren py³ku, a tak¿e okreœlono
jego ¿ywotnoœæ. Mikromorfologiê i anatomiê nektarników badano w skaningowym mikroskopie
elektronowym oraz w mikroskopie œwietlnym. Roœliny Rh. luteum produkowa³y przeciêtnie
32,7 mg cukrów i 9,5 mg py³ku z 10 kwiatów. Ziarna py³ku Rh. luteum zaliczono do œrednich
o kszta³cie p³asko-kulistym. Ziarna wystêpuj¹ w tetradach o niewielkim stopniu kulistoœci,
tworz¹c uk³ad tetraedralny. ¯ywotne ziarna py³ku stanowi³y 74% zawartoœci pylników.
Najwiêkszy udzia³ procentowy mia³y tetrady zawierajêce 4 ¿ywe ziarna (40%), zaœ wykazuj¹ce
wszystkie ziarna sterylne stanowi³y 4%. W tetradach py³kowych obserwowano trzy kategorie
ziaren py³ku: ¿ywe zawieraj¹ce g³ównie substancje bia³kowe, ¿ywe wype³nione obficie skrobi¹
i sterylne bez protoplastów. Gruczo³ nektarnikowy po³o¿ony jest u nasady zal¹¿ni s³upka
i tworzy jej zewnêtrzn¹ warstwê. W epidermie nektarnika wystêpuj¹ komórki szparkowe
posiadaj¹ce zdolnoœæ regulacji wielkoœci poru. Komórki sekrecyjne maj¹ mniejsze rozmiary ni¿
s¹siaduj¹ce z nimi komórki miêkiszu. W œcianach zal¹¿ni wystêpuj¹ liczne wi¹zki przewodz¹ce,
których odga³êzienia przenikaj¹ do warstw gruczo³owych nektarnika.
S³owa kluczowe: Rhododendron luteum, nektarowanie, py³ek, struktura nektarnika.