Аннотация:ANATOMY OF EXTRAFLORAL NECTARIES
IN LEUCADENDRON MUIRII PHILLIPS
AND MIMETES CUCULLATUS R. BR. (PROTEACEAE)
Maxim I. Antipin, Vladimir V. Choob
Lomonosov Moscow State University, Moscow, Russia
sagefool@rambler.ru
Leaf margin characters are taxa specific and may serve as an important tool
in systematics, including identification of fossil records (Hickey, Wolfe, 1975;
Hickey, 1979). Shapes of lamina and leaf margin in Proteaceae in general are
quite diverse, but Proteaceae of Cape Floristic region are more uniform regarding
leaf margin characters, having either an entire leaf edge or just a few teeth at the
lamina apex. Both entire and dentate leaves often terminate with special structure,
generally referred to as mucro (Rebelo, 2001). Its shape may range from needle-
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or awn-shaped to rounded/hemispherical. It differs from the rest of the lamina by
its indumentum features (it is usually glabrous but in some cases it has trichomes
that are arranged in a specific way), by shape, size and colour of epidermal cells
and underlying mesophyll cells (often brightly colored with anthocyanins) and by
smoother cuticle.
The rounded ‘mucrones’ inherent in several genera of Cape Proteaceae of
so called Cape Clade (Weston, Barker, 2002) may be classified among salicoid
teeth (“a dark, but not opaque, nondeciduous spherical callosity fused to the tooth
apex”) according to the classification of Hickey &Wolfe (1975). Teeth of this type
in Salicaceae (including some genera of former Flacourtiaceae) are often secretory (see Wilkinson, 2007), appearing to be resin glands or foliar nectaries. Rebelo
(2001) prefers to call such mucrones in Mimetes and Leucospermum the ‘glandular
teeth’, but he refers to them simply as ‘glands’ in Leucadendron, the genus whose
leaf margin is entire in all species and only some species have leaf apices terminating with structures of this type.
Brightly coloured rounded leaf glands, or salicoid teeth, of many species of
Mimetes, Leucospermum and Leucadendron are documented to attract insects and
referred to as extrafloral nectaries (Midgley, 1987; Rebelo, 1995; Zachariades,
Midgley, 1999). Although ant-attracting nectaries are usually considered an adaptation that reduces herbivory due to attracted ants preying on herbivorous insects,
experiments had shown that excluding ants from nectary-bearing plants of extant
Cape Proteaceae does not increase herbivory levels (Zachariades, Midgley, 1999).
Lawton & Heads (1984) suppose that nectaries in such cases may be ‘ghosts of
predation past’, meaning that insect phytophages avoid visiting plants with nectaries (and ants), thus the exclusion of ants has no noticeable effect on herbivory.
Midgley (1987) suggests that nectaries in extant Cape Proteaceae represent an
atavistic defence mechanism that eventually became less and less needed with
leaves were becoming smaller, more terete and sclerophyllous in responce to recent climate aridification in the Cape. Existing works on leaf morphology of Cape
Proteaceae did not attribute any functions to these organs, just mentioning them as
‘teeth’ (Rourke, 1984) or ‘apical callus’ (Williams, 1972). No works on anatomy
of these structures or on leaf dentition and distribution of substances they produce
have been published yet.
This work aimed to describe the anatomy features of a typical ‘rounded mucro’
(salicoid tooth, apical foliar nectary) in species of Mimetes and Leucadendron, to
test the structures for mono- and disaccharide occurrence and spatial distribution
using in vivo methods of hystochemical testing, and to compare them with morphologically similar secretory structures of Salicaceae/Flacourtiaceae.
Live shoots of Mimetes cucullatus R.Br. were collected near Fernkloof, South
Africa, in June 2017. Those of Leucadendron muirii Phillips were taken from plants
grown in Botanical garden of Lomonosov Moscow State University from seeds
collected in the wild in the vicinity of Struisbaai, South Africa.
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Live leaves of both species were longitudinally sectioned with hand razor
and mounted on slides. Molisch test was immediately performed on some of the
sections (alpha-naphthol alcohol solution and concentrated sulphuric acid). Both
stained and control leaf sections were observed and photographed using light microscope with digital camera (Olympus CX41).
Fully developed leaves of Mimetes cucullatus vegetative shoots usually terminate with three rounded teeth (some smaller leaves only have a single tooth).
The teeth are somewhat swollen structures thicker than the leaf blade, they are
brightly coloured with anthocyanins, covered with a thick and smooth cuticle,
and glabrous. The rest of the blade is covered with abundant trichomes and its
cuticle is rough.
The leaf blade of Leucadendron muirii acropetally narrows and terminates
with single brightly coloured salicoid tooth with distinctly more smooth cuticle.
Its apical area is somewhat flattened or concave.
Molisch test revealed sugars in vascular bundles reaching the tooth in
Mimetes cucullatus. The tooth part of the section is wider than the blade per se;
its epidermal and subepidermal cells are red of anthocyanin. Sugar is also noted
in large polygonal mesophyll cells situating abaxially in relation to the vascular
bundle and almost lacking the chlorophyll. It is detected in loose round parenchyma cells between these polygonal cells and the abaxial epidermis. Cells of
tooth epidermis are square or elongated (palisade-like sensu Wilkinson, 2007),
noticeably longer radially than flatter epidermal cells of the blade; they are longest on the abaxial side of the tooth. Mesophyll cells and intercellular spaces
underlying the epidermis are most stained in the proximal part of the tooth at its
abaxial side. This seems to be an area where exudates reach the gland surface
through epidermis.
In Leucadendron muirii leaf sections, the borderline between the gland and
the rest of the lamina is clearly seen in the most apical narrowing part of the leaf.
Epidermal cells of the gland are only slightly elongated compared to the blade
epidermal cells, but mesophyll cells underlying them are quite distinct as they are
bright purple-red and contain no chloroplasts. The most apical, flattened part of the
gland is characterized by the longest, square epidermal cells. Parenchyma cells in
the flattened apical part of the gland are large and polygonal and lack both chlorophyll and anthocyanins. There is a small space between these large parenchyma
cells and the epidermal layer of the flattened apex of the gland, which stains orange
with Molisch reagent.
In genera of Salicaceae s. str. and former Flacourtiaceae, the secretory epidermis of the glandular salicoid teeth consists of elongated, palisade-like cells perpendicular to the leaf surface (Wilkinson, 2007). Subepidermal mesophyll consists
of rounded or polygonal cells lacking chloroplasts. It is supplied by one to several
veins and more or less modified for nectar or resin production; some authors refer
to it as ‘nectariferous parenchyma’ (Thadeo et al., 2008). Nectar or other excretes
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may concentrate either in a cavity between the outer epidermal walls and the cuticle
(Prockia, Thadeo et al., 2008), or between the loose spongy cells of partly disintegrated mesophyll under palisade-like epidermis (Polyothyrsis, Idesia, Wilkinson,
2007). Secretory teeth of Idesia and Populus are glabrous, while the rest of the leaf
is covered with trichomes (Wilkinson, 2007). The same pattern of indumentum
distribution is observed in apical leaf glands of Proteaceae.
Teeth of Leucadendron muirii and Mimetes cucullatus strickingly liken Idesia
foliar nectaries in their morphology and anatomy. In spite of that, the epidermal
cells on Proteaceae salicoid teeth are less elongated radially than the typical
palisade-like epidermal cells of secretory glands of Salicaceae/Flacourtiaceae,
these cells seem to have changed their shape due to their secretory function. As
the secretory teeth usually actually produce nectar only at certain stages of leaf
developmetn, it is worth to perform a detailed study of the changes these structures experience in the course of leaf development. The genus Leucadendron is
characterized by a wide adaptive radiation. Therefore, studies to cover the whole
range of mucro forms within the genus could shed some light on evolutionary
history of foliar nectaries as a highly specialized structure of great ecological
importance.
References
Hickey L.J. 1979. A revised classification of the architecture of dicotyledonous leaves. In: Metcalfe C.R., Chalk L. (eds.): Anatomy of the dicotyledons. 2nd ed. Oxford: Clarendon Press.
V. 1. P. 25–39.
Hickey L.J., Wolfe J.A. 1975. The bases of angiosperm phylogeny: vegetative morphology. Ann.
Mo. Bot. Gard. 62: 538–589.
Lawton J., Heads P. 1984. Bracken, ants and extrafloral nectaries. I. The components of the system.
J. Animal Ecol. 53: 995–1014.
Midgley J.J. 1987. Aspects of the evolutionary biology of the Proteaceae, with emphasis on the genus
Leucadendron and its phylogeny. Ph.D. Thesis. University of Cape Town.
Rebelo A.G. 1995. A field guide to the proteas of Southern Africa. Vlaeberg: Fernwood Press.
Rebelo A.G. 2001. A field guide to the proteas of Southern Africa. 2nd ed. Vlaeberg: Fernwood
Press.
Rourke J.P. 1984. A revision of the genus Mimetes Salisb. (Proteaceae). J. S. Afr. Bot. 50:
171–236.
Thadeo M., Cassino M.F., Vitarelli N.C., Azevedo A.A., Araújo J.M., Valente V.M., Meira R.M.
2008. Anatomical and histochemical characterization of extrafloral nectaries of Prockia crucis
(Salicaceae). Am. J. Bot. 95: 1515–1522.
Weston P.H., Barker N.P. 2006. A new suprageneric classification of the Proteaceae, with an annotated checklist of genera. Telopea 11: 314–44.
Wilkinson H.P. 2007. Leaf teeth in certain Salicaceae and ‘Flacourtiaceae’. Bot. J. Linn. Soc. 155:
241–256.
Williams I.J.M. 1972. A revision of the genus Leucadendron (Proteaceae). Contr. Bol. Herb. 3:
1–425.
Zachariades C., Midgley J.J. 1999. Extrafloral nectaries of South African Proteaceae attract insects
but do not reduce herbivory. Afr. Entomol. 7: 67–76.