Taxonomy, Reproductive Biology and Medicinal uses - URP Journals

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International Journal of Research in Biological Sciences
Universal Research Publications. All rights reserved
ISSN 2249–9687
Review Article
Betula L. A wonder plant: Taxonomy, Reproductive Biology and Medicinal uses
Mohammad Saleem Wani 1 , A. H. Munshi 1 , R.C Gupta 2 and Ashiq Hussain Lone 1
1
Department of Botany, University of Kashmir Hazratbal Srinagar 190006 India.
2
Department of Botany Punjabi University Patiala 147002 India.
Growth habit and occurrence: The genus Betula,
commonly known as birch, belongs to the family
Betulaceae .It is represented by 40 to 50 species of
deciduous trees and shrubs occurring in the cooler parts of
the Northern Hemisphere (Weaver 1978). It attains a height
of 15-20 meters from the ground. Their bark is smooth,
shining, reddish white or white, with white horizontal
lenticels. The outer bark consists of numerous thin papery
layers, exfoliating in broad horizontal rolls. The inner
cortex is red and moist. They are fast growing and many
are short lived. White barked species are weak-woody and
their twiggy crowns collect and hold ice and snow.
Therefore they are subject to damage from winter
stroms.Because of these characteristics few birches are
recommended as street trees. For home gardening they are
most useful when grown in association with other trees,
although a few are most effective when planted as
specimens. Care should be taken to prevent the outer bark
of the white-barked species from being stripped or marred
since the injured area will always remain green (Hylander,
N 1957).Their leaves are ovate-acuminate, elliptic and
irregularly serrate.
Taxonomy: The taxonomy of Betula is controversial, and
various classifications have been proposed. The first
monographer who provided an extensive review of the
genus was Regel (1865), who divided the genus into
subgenera Betulaster and Eubetula. Subgenus Betulaster
contains only one section, the Acuminatae. Subgenus
Eubetula comprises six sections, namely Costatae, Lentae,
Nanae, Albae, Fruticosae, and Dahuricae. Winkler (1904)
proposed a slightly different division, lowering the status of
the sections to that of subsections, merging the Fruticosae
and the Dahuricae with the subsection Albae and merging
the Lentae with the Costatae. More recently, De Jong
(1993) proposed a division into five subgenera, namely
Betulenta, Betulaster, Neurobetula, Betula and
Chamaebetula. Subgenus Betulenta is considered the most
primitive subgenus, followed by Betulaster and
Neurobetula. Neurobetula is considered a very
heterogeneous and partly artificial group (De Jong 1993).
Received 23 March 2013; accepted 24 April 2013
The subgenera Chamaebetula and Betula are considered to
be more derived. Hybridization and introgression are
common in Birches where the natural distributions of birch
species overlap, for example among the European birch
species B. pendula, B. pubescens, and B. nana (Palme et al.
2004). Betula utilis D.Don the only critically endangered
species has been reported from North-West Himalaya and
Kashmir but Steward (1982) has collected number of
accession mostly described them either as sub-species
/varieties due to which the taxonomy of the species is
doubtful and need further scientific investigation for their
correct taxonomic structure. Moreover, several of the
recognized Betula species have a hybrid origin (Nagamitsu
et al. 2006). Hybrids generally show morphology
intermediate between the parental species but are not
always morphologically distinct as a group (Thórsson et al.
2001). This overlap in morphological features complicates
species and hybrid identification. Introgression appears to
be bidirectional (Williams and Arnold 2001) but
asymmetrical (Palme et al. 2004). Hybridization and
introgression are further facilitated by the introduction and
distribution of artificially propagated cultivars outside the
natural distribution range. The simultaneous occurrence of
polyploidization, extensive hybridization, and introgression
complicates taxonomical studies in the genus Betula.
Flowering and fruiting: Mature birch trees carry large
amounts of female and male catkins and the flowers are
monoecious bloom in May June (Koski and Tallqvist,
1978). Staminate catkins are formed in late summer or
autumn, remain naked during winter, and open after
considerable elongation in the spring. The pistillate catkins,
which are cone-like with closely overlapping scales are
born terminally on short, spur-like lateral branches and
appear with the leaves .When the female catkins (strobilus)
ripen in late summer or autumn, they become brown and
woody and are either erect or pendulous. Each scale may
bear a single small, winged nut (seed) that is oval, with two
persistent stigmas at the apex. The seeds turn from greenish
tan to light brown or tan when mature (Brinkman 1974).
Seeds disperse from late fall until the following spring
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(Houle and Payette 1990; Matlack 1989) and are thin and
winged. Although seeds can begin to disperse in late
summer, these early-shed seeds may be of poor quality.
Wind play important role and blow seeds along the surface
of the snow up to 80 m from the mother tree. Ford et al
(1983) trapped about 5% of the total seed-fall from roundleaf
birch at nearly 100 m from the parent tree. An
abundance of seeds can be found in the forest soil, but these
seeds are found to be short-lived. Most seeds are nonviable
after the second or third year (Granstrom 1987; Granstrom
and Fries 1985; Johnson 1975; Moore and Wein 1977;
Perala and Alm 1989; Steijlen and Zackrisson 1986). The
abundance of seeds in the forest soil is, therefore, likely
supported by regular replenishment from new crops
(Komarova 1986). A rare case of excessive seed production
has been observed to lead to crown deterioration and
reduced growth of the parent trees (Gross 1972). Seeds of
yellow birch shed in August were found non-viable. Viable
seeds were not released in meaningful amounts until
September, with the maximum of good seeds being
released in October (Houle and Payette 1990). After seed
fall, the strobilus slowly disintegrates on the trees, with the
axes persisting on the branch lets.
Seed production, collection and extraction: Lepisto
(1973) reported Birches tends to flower at the relatively
young age of 10 to 15 years .Some individuals are
precocious in flowering and this appears to be under
genetic control (Huhtinen and Yahyaoglu 1974). In
greenhouse conditions with irrigation, fertilizers and CO 2 -
enriched air, European white birch seedlings have produced
male catkins as early as 9 months and commercial
quantities of seed from 5-year-old trees (Lepisto 1973).
Bjorkbom and others (1965) reported that paper birch
produced a higher proportion of viable seeds in good seed
years than it did during poor seed years. The percentage of
viable birch seeds can be estimated by examining the seeds
on a light table (Patterson and Bruce 1931).The number of
seeds per square meter in a birch stand can be hundreds of
thousands (Koski and Tallqvist, 1978). When birch
seedlings are raised in continuous light together with high
nutrient level and elevated CO 2 concentration in a
greenhouse, the generation time from seed to seed can be
reduced to less than 1 year (Kärki, 1976). Commercial seed
production starts 2 years after establishment in a less
intensive polythene seed orchard. Birch flowering starts
early in the spring after a certain threshold temperature is
reached. Due to the elevated temperature in the poly house,
the flowering time is some weeks earlier than outside,
which totally eliminates the risk of pollen contamination.
The ability of birch to „accelerate breeding‟, making it is
possible to get any seedling or graft to flower and set seed
within a few years, is a crucial advantage relative to
conifers.
Birch seeds are collected by picking or stripping the
strobilus from standing trees or shrubs or from trees. This is
done best when the strobiles are still green enough to hold
together. Because ripe strobilus shatters readily, they are
put directly into bags rather than allowed to fall onto the
ground or tarps, which can result in loss of seed.
Freshly collected strobilus can be subject to heating as they
are usually green. They should be spread out to dry for
several weeks until they begin to disintegrate. Low relative
humidity is the most important way in drying the strobiles.
Once the strobilus begin to fall apart, they can be shattered
by rubbing or shaking or can be put in the blender, and the
seeds can be separated from most of the scales and debris
by screening and fanning. Round-hole screens of the
following sizes have proved satisfactory for the following
species: glandulose birch, 2.38 mm; yellow birch, 3.2 mm;
river birch, 4 mm; paper birch, 3.2 mm; European white
and downy birches, 2.6 mm. The remaining scales can be
removed by fanning (Brinkman 1974). Birch seeds are very
small and light and show considerable variation with the
number per weight and yield per volume among species.
Seed Germination: European white birch (Black and
Wareing 1955, Vaartaja 1956) and paper birch (Bevinton
and Hoyle 1981) can germinate in the dark, whereas
monarch and Japanese white birches and Erman birch
require light (Nagata and Black 1977; Nagata and Tsuda
1975; Odani and Anma 1986). Giberellic acid (GA3) in
concentrations of 50 to 100 ppm could substitute for the
light with Erman birch (Odani and Anma 1986).
The basic chromosome number of Betula is n=14, and the
species form a series of polyploids with chromosome
numbers of 2n=28, 56, 70, 84, 112, and 140 (Furlow 1990).
Some robust and fast-growing silver birch individuals have
been found to be triploid (2 n = 42) (Sarvas, 1958).
Polyploidy is a common feature among Betula species, and
its presence within at least four of the five recognized
subgenera suggests several independent polyploidization
events.
Medicinal Use: Betulin, lupeol, oleanolic acid, acetylo-
-heanolic acid, betulitc acid, lupenone, sitosterol, methyl
betulonate, and methyl betulate have been extracted from
its bark. A new triterpenoid, called karachic rustics which
is aromatic and has antiseptic properties (Barnes et al 1974;
Bevington et al, 1981) have also been extracted. The
compound Betulin shows anticancer activity by suppressing
growth of malignant melanoma and cancer of liver and
lungs (Kikuzaki, H and Nakatani, 1993). According to
literature Betulin is a pentacyclic triterpene with the lupane
skeleton whose existences in nature have been reported
since 1788, isolated from birch bark as pure chemical
substance by sublimation and elemental analysis of this
natural constitutes was done in 1876 (Hayak et. al., 1989).
Traditionally birch bark has been used as rheumatism, gout,
malaria (Hager 1967) by human being. Betulin can be
isolated (Ender et. al. 2003) from the outer layer of the bark
of Betula alba with maximum yield. It can also be isolated
from the Himalayan birch tree, Betula utilis that contains
betulin up to 12% of its weight. Betulin is an ideal starting
material for the synthesis of biologically active betulinic
acid via jone‟s oxidation method. Betulin and its
derivatives possess biological effects, e.g. Antiinflammatory,
antiviral, anti-HIV, hepato protective.
(Dzubak et. al. 2006). Some of derivatives of betulinic acid
also show high anti-HIV and antiviral activities (Dzubak et.
al. 2006). Betulinic acid has been studied more extensively
then betulin, having selective antitumor activity against
human melanoma cell culture and anti- HIV activity.
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Several species produces valuable lumber. In Northern
Europe, pollen of birch is a major cause of hay fever
complaints, as is pollen of the Fagales species hazel and
alder (Breiteneder et al. 1989, 1992; Lüttkopf et al. 2002).
The major allergen involved is Betv 1 of which several
variants exist that may differ in their allergenicity (Ferreira
et al. 1996, 1997; Schenk et al. 2006).
The bronze birch borer, Agrilus anxius Gory, is
the major lethal pest of white-barked birches. The larvae of
this native buprestid beetle form extensive mines in the
cambial-phloem region beneath the bark, leading to the
death of large branches and entire trees. Fisher (1928) listed
B. papyrifera, B. alba L. (which includes both B. pendula
and B. pubescens), B. populifolia, B. lenta, and B. lutea (=
B. alleghaniensis) as hosts of bronze birch borer. Weaver
(1978) based his observations on trees growing at the
Arnold Arboretum in Massachusetts and rated B. pendula,
B. populifolia, and B. albosinensis as susceptible, with B.
papyrifera being slightly less susceptible. Weaver listed B.
davurica, B. ermanii, and B. maximowicziana (monarch
birch) as probably resistant species.
AKNOWLEDGEMENT:
The concept of the article has been conceived while
working on “ALL INDIA COORDINATED PROJECT ON
REPRODUCTIVE BIOLOGY OF RARE, ENDANGERED
AND THREATENED (RET) TREE” (Betula utilis D. Don)
sponsored by Ministry of Environment and Forest
Government of India New Delhi.
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Source of support: Nil; Conflict of interest: None declared
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