Ultrastructure and Carotenoid Composition of Chromoplasts of the Sepals of Strelitzia reginae Aiton during Floral Development - Semantic Scholar
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Ann. Bot. 39,175-83, 1975
Ultrastructure and Carotenoid Composition of
Chromoplasts of the Sepals of Strelitzia reginae
Aiton during Floral Development
D. J. SIMPSON, M. R. BAQAR, and T. H. LEE
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Department of Food Technology, The University of New South Wales,
Kensington, N.S.W. 2033, Australia
Received: 2 January 1974
ABSTRACT
Ontogenetic development of chromoplasts from the coloured outer perianth segments of the flower of
Strelitzia reginae was examined with an electron microscope. The plastids evolved through five stages,
namely, colourless leucoplasts, chloroplasts, pale yellow, deep yellow and orange chromoplasts. The
relationship between plastid ultrastructure and carotenoid composition is discussed. The development of
fibrils from osmiophilic globules is shown to occur in chromoplasts which contained only small amounts
of chlorophyll at an early stage of development.
Regular lattices of globular subunits were found which showed a hexagonal or rhomboidal pattern and
which are probably protein in nature. The sudden disappearance of these crystals just before fibrils form,
and the complete absence of starch from all stages of plastid development, suggests that these crystals are
a form of energy storage.
INTRODUCTION
The chromoplasts of the outer perianth segments (sepals) of Strelitzia reginae have been
the subject of light microscopic investigations as early as 1853 (Mohl, 1853; Meyer, 1883;
Schimper, 1883, 1885; Courchet, 1888; Kuster, 1956). The observed spindle-shaped
nature of the mature chromoplasts is characteristic of a fibrillar ultrastructure (Steffen
and Walter, 1958; Frey-Wyssling and Kreutzer, 1958; Bornman, 1968; Simpson, Rahman,
Buckle, and Lee, 1974). Their carotenoids have been mentioned by Meyer (1883), Tammes
(1900), Kohl (1902) and Seybold (1953), although the only qualitative analysis was that
of Tappi and Menziani (1955) who found jS-carotene and cryptoxanthin in major amounts
along with a-carotene, flavoxanthin, xanthophyll (our italics—this pigment is probably
lutein) and xanthophyll epoxide.
To date, most fibrillar chromoplasts examined by electron microscopy have developed
from starch-containing plastids, usually chloroplasts. However, in sepals of Strelitzia
reginae, starch-free plastids, leucoplasts, are transformed through several colour stages
to mature chromoplasts. The present work extends earlier studies on plastid transforma-
tions and fibril genesis (Simpson, Chichester, and Lee, 1974; Simpson etal, 1974). We
have examined the carotenoid composition and ultrastructure of developing Strelitzia
plastids to gain further information on fibril development, and the relationship of caro-
tenoid type to chromoplast ultrastructure.
MATERIALS AND METHODS
Plant source
Five colour stages of the outer perianth segments (sepals) of the flowers of Strelitzia
reginae Aiton (Moore and Hyypio, 1970) were collected from plants growing on the176 Simpson, Baqar, Lee—Ultrastructure and Carotenoid Composition of Chromoplasts
campus from July to September for electron microscopy and carotenoid analysis. The
white, greenish-yellow, and light yellow sepals were dissected out from unopened flowers
inside the spike containing the inflorescence.
Carotenoid analysis
A sample of the sepals (4-6 g) was extracted several times with acetone in a Sorvall
Omnimix and then, if necessary, with methanol-petroleum ether (1:1 v/v) until no further
colour was extracted. The gross extract was separated on thin-layer plates developed by
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petroleum ether :«-propanol (99-9:0-1) or petroleum ether: acetone :«-propanol
(90:10:02) (light yellow, deep yellow and orange stages) and by petroleum ether:ace-
tone :«-propanol (90:10:0-45) (greenish-yellow stage) as previously described (Simpson
et al., 1974).
The carotene fraction eluted from the plates was rechromatographed on an MgO: Hyflo-
Supercel (1:2) column developed with increasing amounts of acetone in petroleum ether.
Individual pigments were identified by comparing chromatographic and spectrophoto-
metric characteristics with published values (Davies, 1965; Foppen, 1971), and where
possible, by co-chromatography with authentic carotenoids.
The cis-trans configuration was determined where necessary, by the iodine isomeriza-
tion test (Jungawala and Cama, 1962). Carotenoids having epoxide groups were charac-
terized by the addition of a drop of ethanolic 005 N-HC1 to an ethanolic solution of
pigment (Jungawala and Cama, 1962). The concentrations of individual carotenoids
were calculated using published E\°i, values (Davies, 1965). An E\°^ value of 2500 was
assumed for pigments with no published value.
Electron microscopy
Transverse sections of the sepals were pre-fixed for 1 h in a 3 per cent formaldehyde-
3 per cent glutaraldehyde solution buffered with 0-1 M-sodium cacodylate to pH 7-4.
After pre-fixing, the tissue was washed in cacodylate buffer to remove the aldehydes
and fixed in 2 per cent OsO4 buffered to pH 7-4 for 2 h. The material was then post-fixed
for 1 h in a 2 per cent uranyl acetate solution. The tissue was dehydrated in increasing
concentrations of ethanol and then acetone, and embedded in a low viscosity epoxy resin
(Spurr, 1969).
Ultra-thin sections with silver interference colours were cut with glass knives on an
LKB-Huxley Ultramicrotome and mounted on 200 mesh copper grids (Athene) which
had been coated with collodion followed by carbon. The sections were stained with
Reynolds (1963) lead citrate solution for 2-5 min. The specimens were examined in a
Philips EM 300 electron microscope, and electron micrographs were recorded on 35 mm
Fine Grain Positive Film (Kodak).
RESULTS AND DISCUSSION
Carotenoid composition
The carotenoid content of Strelitzia reginae sepals increased with age (Table 1); the
orange sepals (656 fig g"1) having some 800 times the content of the white sepals (0-75
Hg g" 1 ). In earlier work, Seybold (1953) had reported similar carotenoid contents in
gelben Blumenbldttern (445 /ig g"1) and gelbe Perianthblatter (853 fig g"1)-
The sepals contained at least 19 known carotenoids during their development (Table
2). One carotenoid, which was the third most abundant pigment in the deep yellowSimpson, Baqar, Lee—Ultrastructure and Carotenoid Composition of Chromoplasts 177
T A B L E 1. Total carotenoid content and dimensions of plastids of sepals of the flowers of
Strelitzia reginae at different colour stages
Colour stage Carotenoid content* Diameter Length
Og per g f.wt) 0/m) (tun)
White 0-75 0-94 2-20
Greenish-yellow 29-3 0-55 3-40
Light yellow 190 0-71 2-56
Deep yellow 619 0-83 19t
Orange 656 0-89 21t
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OrangeJ 101 1-61
* Expressed as /?-carotene.
t Light micrographic measurements.
j Chromoplasts from cells adjacent to xylem elements.
T A B L E 2. Pigments in the sepals o/Strelitzia reginae at different colour stages
Pigment Proportion of total pigment (%)
White Greenish Light Deep Orange
yellow yellow yellow
Carotenoids
Phytofluene * — trace trace 2-2
a-Carotene 2-6 1-4 trace 0-6
/?-Carotene 23-2 23-9 53-2 49-9 45-8
/?-Zeacarotene — — — 3-9 1-2
C- Carotene — — — trace 0-6
5,6-Monoepoxy-/?-carotene — 41-7 13-8 — 2-3
cis-5,6:5',6'-Diepoxy-/?-carotene 31-5 — — — —
trans-5,6:5',6'-Diepoxy-/?-carotene 42-2 11-3 2-4 — —
Mutatochrome — — — — 2-3
Hydroxy-a-carotene — 9-5 — — —
Hydroxy-a-carotene-5,6-epoxide — — 2-0 — —
Cryptoxanthin 200 22-9 24-6
Unknown 10-5 7-7
Flavochrome — — — 3-5 3-9
Chrysanthemaxanthin + flavoxanthin — — — — 1-3
Zeaxanthin — — 7-0 91 5-7
Antheraxanthin — — trace 10
Neoxanthin trace 0-9
Lutein 80
Violaxanthin — 2-8 — — —
Chlorophylls
Chlorophyll a trace 2-67t — — —
Chlorophyll b trace 0-94t — — —
* Not detected.
t Values in n% g~l f.wt
and orange sepals, was not identified. It had similar spectral properties to cryptoxanthin,
but was more polar, did not react with ethanolic HCl and had a trans structure.
In the light yellow, deep yellow and orange sepals, ^-carotene and its hydroxy deriva-
tives, cryptoxanthin and zeaxanthin, comprised over 75 per cent of the total carotenoids
(Table 2). Tappi and Menziani (1955) had found ^-carotene and cryptoxanthin in major
amounts, and in minor amounts, a-carotene, flavoxanthin, xanthophyll and xanthophyll
epoxide.178 Simpson, Baqar, Lee—Ultrastructure and Carotenoid Composition of Chromoplasts
The greenish-yellow as well as the white sepals contained a high percentage of 5,6-
epoxides; these had largely disappeared in the deep yellow sepals although 5,8-epoxides
were identified in these and orange sepals. Valadon and Mummery (1972) found a similar
reduction in 5,6-epoxycarotenoids in ageing berries of Sorbus aucuparia followed by
accumulation of 5,8-epoxycarotenoids. Many flowers and fruit accumulate epoxycaro-
tenoids as they mature although it has been claimed that the 5,8-epoxides are isolation
artefacts (Egger, 1968). The present work and that of Valadon and Mummery (1971)
rebuts this suggestion as no 5,8-epoxides were found in the extracts of white and greenish-
yellow sepals, which contained high percentages of 5,6-epoxides. There has been much
speculation as to the function of the epoxycarotenoids in these tissues and also in photo-
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synthetic tissues (Cholnoky, Gyorgyfy, Nagy, and PanczeU, 1956; Goodwin, 1966) but as
yet no definite biochemical or physiological role has been confirmed. The formation of
oxygenated carotenoids in non-photosynthetic tissue is often thought to be an uncontrolled
oxidative or degenerative process (Goodwin, 1966; Valadon and Mummery, 1969).
However, our ultrastructural observations of the maturing sepals show an increasingly
organized and complex plastid structure which suggests that the metabolism of the plastid
is still under tight control.
Plastid development
Plastids of Strelitzia reginae sepals were concentrated mainly in the outer two or three
layers of cells. Plastid size increased as the colour of the tissue progressed from white to
orange (Table 1) due to a tenfold increase in the length of the plastids.
White sepals contained small, undifferentiated leucoplasts which were only slightly
larger than mitochondria (Plate 1A). A variety of types was observed; the majority con-
tained a small number of osmiophilic globules and pale vesicles, although occasionally
regular crystalline structures were found (Plate 1A). In other plastids primitive grana were
found, and in all types, the stroma was noticeably granular.
Plastids from the surface layers of greenish-yellow sepals contained large grana with
up to 20 thylakoid discs (Plate 1B). The stroma was again granular and contained osmio-
philic globules and swollen intergranal thylakoids. The plastids from inner cells were
characterized by large crystals and lacked grana.
The presence of crystals in almost every plastid, and osmiophilic globules about 100 nm
in diameter (Plate lc) was the most obvious feature of chromoplasts of light yellow sepals
from unopened flowers. A few chromoplasts contained transition forms between globules
and the fibrils characteristic of the chromoplasts of the orange sepals.
Few chromoplasts contained crystals in the deep yellow sepals from the opened flower.
Instead, they were filled with extremely osmiophilic globules, the majority of which
were elongated at opposite ends to form fibrils (Plate 2A). A lighter central band continuous
with the fibrils extended from some globules (Plate 2E). The chromoplast stroma was
granular with lighter vesicular regions.
The orange sepals contained three distinct chromoplast types (Plates 2B, 3A and 3B).
Chromoplasts from the inner cell layers (Plate 2B) had much longer fibrils and fewer
osmiophilic globules than chromoplasts from deep yellow sepals, and the globules were
mainly associated with fibrils. The particular chromoplast shown in Plate 2B is annular
in shape and probably corresponds to the ring-shaped forms visible under the light
microscope. Most chromoplasts at this stage of development were more elongated and
narrower than the chromoplast shown in Plate 2B. When sectioned longitudinally, their
tapered extremities consisted of parallel, often wavy fibrils, with no globules and almost
no stroma.
Chromoplasts from the outer two or three cell layers of the orange sepals were the most
developed, with the greatest proportion of their interior being filled up with parallelSimpson, Baqar, Lee—Uhrastructure and Carotenoid Composition of Chromoplasts 179
fibrils (Plate 3A). Few osmiophilic globules were found in these chromoplasts which
were the only ones observed to be clearly limited by a double membrane. An interesting
feature of these chromoplasts was the presence of wavy fibril bundles and what appeared
to be fused fibrils (Plate 3A).
We have been able to confirm with the electron microscope, and present greater ultra-
structural details about, the long-observed (Meyer, 1883; Schimper, 1883,1885; Courchet,
1888) ontogenetic development of Strelitzia sepal plastids from small, colourless leuco-
plasts, to increasingly large spindle-shaped chromoplasts. In at least two of the colour
stages, greenish-yellow and orange sepals, plastids located in the two or three epidermal
cell layers show more advanced ultrastructural development than plastids from inner cells.
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Also, plastids in the layer of long, narrow cells on either side of xylem elements in orange
sepals, and only in these cells, closely resemble those from white sepals (Plate 3B) in size
(Table 1), shape and ultrastructure.
Protein crystals
The occurrence of protein crystals in the plastids of Strelitzia sepals (Plates 1A, l c and
3B) confirms the light microscopic observations of Schimper (1885) who reported the
presence of cubic protein crystals in the stroma; they were not seen by Courchet (1888).
Two regular arrays were observed (i) a distorted hexagon with centre-to-centre spacings
of 16-4 nm in two directions, and 13-7 nm in the third (Plate 3c), and (ii) a rhomboidal
pattern (Plate 3D). The centre-to-centre spacings of the two directions of periodicity in
the latter pattern were different. In Plate 3D they are 14-9 nm and 13-9 nm, and the two
directions are inclined at an angle of 75° to one another. Other rhomboidal arrays were
measured as having angles of 86-5° and 82-5°.
Since the section thickness is about 80 nm, it follows that five or six crystal subunits
can be accommodated vertically in a section. Furthermore, they must be aligned exactly
one on top of one another to appear as clearly as shown in Plate 3c. Since no square
arrays were seen the angle of section must be inclined to the crystal plane in such a way
that subunits are aligned vertically. A model of hexagonally close-packed spheres was
constructed, with every third layer corresponding. By viewing the model at various angles
so that the spheres were aligned, only two patterns were observed, similar to those shown
in Plate 3c and D. The hexagonal pattern was observed when the model was tilted 30°
which is almost exactly the angle calculated to produce the 1-18:1-0 ratio of the distorted
hexagon (Plate 3c). The theoretical angle for the rhomboidal array is 70°32' which differs
from all those measured in crystals of Strelitzia. The three-dimensional structure is
possibly a non-close-packed hexagonal array, although it may require two crystalline
arrays to explain all of the various patterns seen.
The centre-to-centre spacing is probably a more accurate estimate of the diameter of
the subunits of the crystal, since slight misalignment of the electron-transparent spheres
would result in a reduction in apparent diameter. A rough estimation of the molecular
weight was obtained by comparison with fraction I protein which has a diameter of 9 0 nm
and a molecular weight of 570 000 (Gunning, Steer, and Cochrane, 1968). Assuming that
both molecules have the same density and shape, the molecular weight of Strelitzia
protein was calculated to be approximately 3 500 000.
The function of the protein crystals in these plastids is unknown. It may be a reserve
energy store, as in the plastids of bean root tips (Newcomb, 1967) which show a similar
pattern, but the subunits are smaller. Although there is a dramatic decrease in the number
of crystals with the appearance of fibrils, it is doubtful that the crystal subunits are directly
reassembled to form the fibrils, since the fibrils are about 50 per cent thicker and such
crystals have not been reported in the development of fibrillar chromoplasts in any other
plant. However, metabolism of the protein crystal may supply the energy required for180 Simpson, Baqar, Lee—Ultrastructure and Carotenoid Composition of Chromoplasts
the synthesis of carotenoids and other fibril components during chromoplast development,
since starch is completely lacking from all stages of plastid ontogeny.
Fibril formation
It has been demonstrated that the osmiophilic globules of chromoplasts and degenerat-
ing chloroplasts contain carotenoids (Sprey, 1970). Although pure carotenoid pigments
are extremely osmiophilic (Harris and Spurr, 1969), the electron density of globules is
more dependent on reducing substances such as plastoquinones and a-tocopherol
(Lichtenthaler, 1970). In the plastids of Strelitzia reginae sepals, the increase in carotenoid
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content is at first manifested by a progressive increase in the number of osmiophilic
globules in the greenish-yellow and light yellow sepals, as well as an increase in the number
and size of plastids (Table 1). The threefold increase in carotenoid content from light
yellow to deep yellow sepals corresponds to a change in the ultrastructure of the globules.
This change can be seen in progressive stages (Plates 2c, 2D and 2E). Initially an electron
transparent band develops (Plate 2c) and the globule then becomes ellipsoid due to the
formation of a fibril which extends right through the globule (Plate ID). AS the fibril
grows, the globule stretches out along it (Plate 2E) and finally disappears, or is present
merely as a swelling on the fibril (Plate 3A).
We regard these electron micrographs as further evidence (Simpson et al., 1974) for the
theory of Steffen and Walter (1958) that fibrils develop from globules. Furthermore, the
extremely small amount of chlorophyll present, makes it most unlikely that the fibrils
are formed by reassembly of photosynthetic lamellar structural protein as suggested by
Steffen and Walter (1958) and Frey-Wyssling and Kreutzer (1958) for the chromoplasts
of Solanum capsicastrum and Capsicum annuum, respectively. These results support the
suggestion of Kirk and Juniper (1967) that fibril formation from globules may require
the synthesis of a protein since there is no pre-existing lamellar protein suitable for fibril
development.
It is interesting to note that chromoplast elongation occurred some time before the
formation of well-developed fibrils (Plate 2A). In addition there were vesicle-like forma-
tions which were aligned parallel to the major axis of the chromoplast and the direction
of globule elongation. It is possible then, that the same force which causes the chromoplast
to lengthen is also responsible for determining the direction of fibril development.
Carotenoid composition and plastid ultrastructure
The generally variable relationship between carotenoid composition and the presence
of fibrils in chromoplasts of Capsicum annuum and the inhibition of fibril formation by
the chemical 2-(4-chlorophenyl thio)ethyl diethyl ammonium chloride (CPTA) (Simpson
et al., 1974) led us to suggest that fibril development is not necessarily connected with
carotenoid synthesis and may be under separate genetic control. Kirk and Juniper (1967)
proposed that fibril development might require the formation of a certain minimum
level of carotenoids. However, the carotenoid composition and content of deep yellow
and orange sepals are comparable (Tables 1 and 2), yet the chromoplast ultrastructure is
considerably different. The carotenoids, which are located in the osmiophilic globules in
deep yellow sepals, become associated with fibrils in orange sepals. It appears more likely
that fibrils develop independently of carotenoids, and not as a result of an accumulation
of a large quantity of them, especially as the transition from deep yellow to orange sepals
takes several weeks.
Further evidence that fibril development is controlled by genes other than those
influencing carotenoid synthesis is provided by Laborde and Spurr (1973). Fibrils were
observed in the plastids of the ripe fruit of four isogenic lines of Capsicum annuum whichSimpson, Baqar, Lee—infrastructure and Carotenoid Composition of Chromoplasts 181
differed in chlorophyll retention and carotenoid composition. However, globule develop-
ment is limited in green and yellow genotypes with a corresponding reduction in the
number of globule-associated fibrils. As the fibril develops during the transition from
deep yellow to orange sepals in Strelitzia, it is possible that the carotenoids of the globules
may coat the surface of the fibril. This is supported by the fact that the globules elongate
as the fibril grows (Plate 2D and E) and that the surface of the fibril is the site of the caro-
tenoids (Frey-Wyssling and Kreutzer, 1958). In addition, fixation with potassium
permanganate, which stains carotenoids intensely shows that the fibrils have an electron
dense outer cortex when seen in transverse section. There may, furthermore, be an inter-
action between the deposition of carotenoids and fibril development which would explain
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the presence of only short fibrils in the chromoplasts of the fruit of green and yellow
genotypes of Laborde and Spurr (1973).
The concentration of ^-carotene in orange and deep yellow sepals of Strelitzia (over
300 ng per g f.wt) is much higher than that reported by Harris and Spurr (1969) in the
high-beta tomato fruit mutant (673 ±315 ^xg per g d.wt). However, in the chromoplasts of
high-beta tomato fruit, the yS-carotene crystallizes out of the globules. No such crystals
were observed in Strelitzia chromoplasts, even when fixed in formaldehyde-glutaralde-
hyde/potassium permanganate which is known to preserve the structure of the crystalline
carotenoids /?-carotene and lycopene (Simpson et al., 1974). A major difference between
the carotenoid compositions of the two types of chromoplasts is that /J-carotene makes
up about 50 per cent of the total in Strelitzia and 82 per cent in high-beta tomatoes. It is
possible that /?-carotene is prevented from crystallizing out in Strelitzia initially by the
other carotenoids, particularly xanthophylls, and then by the development of fibrils.
ACKNOWLEDGEMENTS
The authors wish to express their appreciation of the assistance and advice given by
Dr M. R. Dickson of the Biomedical Electron Microscope Unit, University of New South
Wales, and by his assistant, Mr A. B. Martin. We also thank Ms Eva Kowal of the School
of Botany for identifying the species of Strelitzia used. We are grateful for financial
assistance in the form of a CSIRO studentship to D.J.S. and a Colombo Plan Fellowship
awarded by the Government of Australia to M.R.B. This work was partly supported by
a grant to T.H.L. from the Rural Credits Development Fund administered by the Reserve
Bank of Australia.
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E X P L A N A T I O N OF PLATES
ABBREVIATIONS
C = protein crystal OG = osmiophilic globule
CW = cell wall PE = plastid envelope
Gr = granum SW = swelling on fibril
M = mitochondrion X = xylem element
PLATE 1
A. Leucoplasts from white sepals showing a few osmiophilic globules and vesicles. Occasionally a protein
crystal or thylakoids are seen. The stroma is markedly granular and no limiting plastid envelope is
visible with this fixation method. x34 200.
B. Chloroplast from the outer cell layer of a greenish-yellow sepal. The grana are well-developed and
appear functional, although intergranal thylakoids are swollen. Osmiophilic globules are found in
the granular stroma. x41 040.SIMPSON, BAQAR, LE E—Ultrastructure and Carotenoid Composition of Chromoplasts
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PLATE 1
Ann. Bot. 39, 175-83, 1975 (facing p. 182)SIMPSON, BAQAR, L E E — Ultrastructure and Carotenoid Composition of Chromoplasts
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PLATE 2
Ann. Bot. 39, 175-83, 1975SIMPSON, B A Q A R , LE E—UJtrastructure and Carotenoid Composition of Chromoplasts
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PLATE 3
Ann. Bot. 39, 175-83, 1975Downloaded from https://academic.oup.com/aob/article-abstract/39/2/175/173106 by guest on 19 March 2019
Simpson, Baqar, Lee—Ultrastructure and Carotenoid Composition of Chromoplasts 183
c. Young chromoplasts from the subepidermal cell layer of light yellow sepals. Chartereristically, these
contain hexagonal crystals with a few osmiophilic globules in the stroma. Although mitochondria are
clearly bound by a double membrane, no similar structure is seen around the chromoplasts. A few of
these chromoplasts appear amoeboid, encircling an area of cytoplasm. x22 050.
PLATE 2
A. Elongated chromoplasts from deep yellow sepals, containing electron transparent vesicular regions,
oriented in the same direction as the long axis of the chromoplast. The stroma is muchfinerin appearance
and there are many large (130 run wide) osmiophilic globules, a number of which can be seen in an
early stage of transformation into fibrils (arrows). x23 220.
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B. Annular chromoplast from an inner cell layer of an orange sepal. Fibrils are further developed and
most osmiophilic globules at this stage are associated with a fibril. A double lamella bounding the
chromoplast is visible for the first time in the ontogenesis of the plastids. x36 990.
c. Osmiophilic globules at an early stage of fibril development. The lighter central bands seem to be
oriented in roughly the same direction. Their staining behaviour is similar to that offibrilsand probably
indicates a different composition compared with the globules. x72 810.
D. A later stage of fibril development showing the projection of fibrils beyond the limits of the osmiophilic
globules, and quite clearly continuous with them. x75 870.
E. A still later stage of development clearly demonstrating the continuity offibrilsthrough the osmiophilic
globules. The globules are elongated in the direction of fibril development. In some places (arrows)
the fibrils appear to be stained more densely at their centre. x46 620.
PLATE 3
A. Mature chromoplast from the epidermal cell layer of orange sepals. The stroma is reduced in volume
and most of the chromoplast is occupied by parallel fibrils. Occasionally these cross over (arrow)
and this is probably the explanation for the apparent fusing of three fibrils into one (double arrow)
since the section thickness is three to four times the fibril diameter. The wavy configuration is typical
of the fibrils at the pointed ends of the chromoplasts and may be due to mechanical deformation,
although such formations are regular and have not been reported in any other fibrillar chromoplast.
A few small osmiophilic globules (60 run) are seen mainly at the periphery of the chromoplast. Some
remain as swellings of the fibrils. x30 600.
B. Chromoplasts from an elongated cell adjacent to a xylem element. They are similar to plastids of white
sepals (Plate 1A) although the stroma has a finer grain and there are more osmiophilic globules. Protein
crystals are a common feature. x25 200.
c. High magnification micrograph of the regular hexagonal lattice of protein crystals found in most
chromoplasts from light yellow sepals. The hexagons show a distortion corresponding to that produced
by a rotation of 30° about the longest side of the hexagon. x417 600.
D. A protein crystal sectioned at a different angle showing a rhomboidal pattern, with directions of
periodicity inclined at 75° to each other. The dark spaces are holes between protein sub-units. xl31 400.Downloaded from https://academic.oup.com/aob/article-abstract/39/2/175/173106 by guest on 19 March 2019
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