Hyptis is a genus of Lamiaceae
with about 400 species (Willis 1973). Most of the species are native
to the tropical America (Hickey & King 1988, Hutchinson &
Dalziel 1963) and extends from Southern United States through the
Caribbean region and Central America, south to Argentina (Srivastava
1976, Aluri & Reddi, 1989). But few of these species has
naturalized in the warmer parts of the old world (Harley 1988; Aluri
& Reddi, 1989, Holm et al. 1979).
One of the species from genus, Hyptis
suaveolens Poit (referred as Hyptis hereafter) is
naturalized in India. It has now established in Deccan Peninsula,
North East India, Vindhyan Highland and Andaman and Nicobar Islands
(Wealth of India 1959; Yoganarasimhan 2000). Hyptis is of
common occurrence along the rail tracks, roadsides (Verma &
Mishra 1992), foothills of open forests, forest clearings (Mudgal et
al. 1997) and can heavily infest wastelands particularly arid and
rocky substrates. It is a ruderal weed (Walter 1963 Keller &
Armbuster 1989, Aluri 1990) and is capable of heavy infestations
displacing native flora and is said to be a potent invader of
Vindhyan Highlands (Sharma et al. 2007). Present paper
investigates ecological and vegetative features of Hyptis which
favour its growth and invasiveness.
Ecology and Phenology:
Hyptis
is a rigid annual herb of aggressive nature (Mudgal et al
1997). It starts its vegetative phase either from perrenating
rootstock or viable seeds either from persistent seed bank or from
fresh stock with the onset of monsoon rains (Figure 1). It can attain
height of approximately 2.5 meters within a growing season. Its stem
is quadrate and bears hair. Leaves are either ovate or obovate.
Leaves are generally 3-5 cm long and 2-4 cm wide with serrulate
margins and a long petiole. Lower surface of the leaves bears hairs;
petioles up to 3 cm long. Flowering starts in it at an early age of
two to three months. It produces copious blue flowers in small cymes
along branch that ends with reduced leaves. Calyx is hairy in nature
and is nearly 5 mm long in flower while it enlarges to 10 mm long in
fruit and become ribbed. Corolla is blue, strongly zygomorphic
and bilabiate, declinate, and about 8 mm long, with a limb 5 mm in
diameter. The flower has 4 stamens. Hyptis flowers are pollinated by
a large number of pollinators leading to enormous seed production
(Plate 1).
Cyclic representation of
Hyptis life cycle and factors conferring invasive potential
Morphological description of Hyptis. a.-Hyptis
in full bloom, b.-Mature twigs, c.- Twig bearing fruits, d.-Flowers,
e.-Seeds in spined burr, f.-Dimorphic seeds, g.-Vegetative
propagation
Hyptis
fruits (nutlets) are about 1.2-1.5 mm long and seeds are protected in
spined burr which help in its dispersal and are slightly notched at
the end (Stone 1970). Seeds of Hyptis are characteristically
dimorphic. This dimorphism is reflected in the seed size (Wulff 1973)
and a inverse relationship of seed size with light requirement has
been observed which confers a good spectrum for the seed germination
across a range of germination temperature, so seeds are capable of
germinating across a range of temperature within 10ºC to 40ºC
but 25-30ºC seems to be optimum for its growth (Felippe et
al. 1983). Seeds produce mucilage after getting in contact of
water. Population expansion takes place by heavy sprouting after
rains and through autogamic and allogamic reproduction (Aluri &
Reddi 1989).
No evidence of substantial
reproductive failure in native habitat indicates production of a good
quantity of viable seed. Due to successful reproduction, Hyptis
covers a large area after the rains and not allows the adjoining
native species not to flourish. So it has potential for a successful
invader. Probable factors which can make Hyptis a successful
invader of grasslands are discussed in detail as follows,
Factors conferring invasive
potential to Hyptis
As each plant species displays its
own reproductive strategy, i.e. a set of characteristics, which
maximizes the chances of its offspring to overcome the hazards posed
by environmental stress, competition, predation and disease (Fenner
1985) Hyptis also shows strategy for better survival and
establishment. It exhibit vigorous growth on the agricultural fields
and wastelands. A number of vegetative characters are helpful in the
heavy proliferation of Hyptis such as small seed size,
prolific seed production (>2000 m-2) forming persistent
propagule bank within short period, seed dimorphism, autogamic and
allogamic mode of reproduction, good proliferation from the
perrenating rootstock, probable allelochemicals and presence of
essential oil conferring resistance to it against variety of
pathogens. Description of life cycle of Hyptis along with
factors favoring its growth is presented in figure 1.
The small seed size facilitates burial of seeds because
of the ease with which such seeds would penetrate into cracks or
small openings in the soil. Reduction in size along with large number
of seeds has also been associated with predator avoidance as reported
by Fenner (1985).
Seed dimorphism in Hyptis is
helpful in its germination across a range of temperature conferring
year round seed germination. In addition to it small seeds have
greater surface to volume ratio showing greater germination rates
than in the large seeds (Cideciyan et al. 1982). Seed
polymorphism is also helpful in exploration of different microsites
(Sagar 1960) or safe sites as described by Harper et al.
(1961. Polymorphic seeds as produced by some other species often
differ in their germination response to temperature and light as
demonstrated by Caves et al. (1966). Different soil qualities
viz. soil temperature and irradiance in addition to spectral
quality of light reaching to the floor may vary widely over short
distances within range of dispersion of seeds of a single individual
(Thompson & Grime 1979) so Hyptis could potentially be
favoured in different microsites owing to the differential seed size.
Further
dispersal of seeds across a range of area and different places is
attributed to difference in seed size because as compared to larger,
smaller seeds may be more easily dispersed; less predated and
maintains more favourable water balance on soil surfaces (Harper &
Benton 1966). Seeds of Hyptis have spined burr which catches
in wool, fur, clothing and other fibrous materials and also floats on
water and some movement is also with animal hooves in mud,
machineries and other vehicles (Parsons & Cuthburston 2000).
. Although allelopathic effect of
Hyptis is not well documented but essential oils present in it
may confer competitive advantage to it as some members of Lamiaceae
family viz.Trichostema lanceolatum (Heisey &
Delwiche 1985), Oscimum sanctum, Nepata cataria and
Salvia species possesses allelopathic properties owing to
presence of essential oils, volatile oils, exudates, leachates and
inhibitory zones (Qasem & Foy 2001). Essential oils of Hyptis
has nearly 2.3 % terpinene 4-ol (Peerzada 1997) which is reported as
main allelopathic inhibitory compound of Trichostema lanceolatum
(Heisey & Delwiche 1985). So we can predict that pathogen
resistance and decreased recruitment of natives by Hyptis is
due to presence of allelopathicals. In addition to all these factors
another factor which plays an important role in the species loss in
the area occupied by Hyptis is its unpalatability to
livestocks due to presence of essential oils so because of avoidance
as a fodder other species are heavily used as fodder by livestocks
resulting in the loss of other species.
Biological attributes viz.
superior reproduction, dispersal benefit and probable allelopathic
chemicals than the native flora are the factors which may be
attributed for its spread at a fast pace on the open areas interferes
with the recruitment of nearby herb and shrub causing threat to the
pre-existing flora and ultimate decrease in the diversity of the area
as in the case of Vindhyan highlands where it has covered a large by
creating monospecific thickets and displacing native species. Its
reproductive capacity is so good that it has been referred as high
fertility weed(Sturtz et al. 1975, Tothilll et al.
1982.)
Hyptis
also has good medicinal value owing to the presence of essential oil,
a characteristic feature to the family Lamiaceae. After hydro
distillation of its leaves, an average yield of 0.1 % is obtained.
Main constituents of the distillate are 1, 8-cineole (32%) and
o-caryophyllene (29%) (Peerzada 1997). Hyptis is known to be used for
traditional medicine for the treatment of various illnesses and has
been found to possess significant pharmacological (Kuhnt et al.
1995), anti-cancerous properties (Mudgal et al. 1997) and
tumorigenic (Peerzada 1997) properties. In addition to above, it also
has mycotoxic activity against fungus Candida albicans,
antimicrobial activity against both gram positive Staphyloccus
aureus and Basillus cereus and negative strains of E.
coli, Pseudomonas (Olayinka et al. 1999). Besides all
these properties it also has insecticidal properties and said to be
mosquito repellant (Mudgal et al. 1997).
Although
Hyptis possess medicinal properties but it is not efficiently
utilized in this context. Further damage to the biodiversity of
adjoining areas is much greater than its utilization as medicinal
plants. To avoid its spread, small plants or small infestation should
be pulled out by grubbing. Removal of larger colonies depends on
planned use of the area. Use of tall growing crops viz. pearl
or bulrush millet (Pennisetum glaucum ) is effectively used to
shade out Hyptis. It can also be controlled chemically with
atrazine. Exploration of suitable host against the plant may prove to
be a beneficial in its removal. Rust disease from Pucinia may
seem to be promising but it further needs to be investigated (Cullen
& Delfosse 1990). So efforts should be done to check it's
spread so that it may not become a successful invader in near future
like other invaders in the Indian forests.
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