ABSTRACT
Plant succession is a natural phenomenon. It is, however, slow in some
cases and fast in others. All man-made habitats are highly vulnerable to invasion by a
plant/weed, which quickly occupies such areas unless properly cared for. A case in point
is the weed Partheniumhysterophorus L. (Commonly known as parthenium, congress grass,
white top or carrot weed; family – Asteraceae), a herbaceous, annual and poisonous
weed. Widelydistributed in wastelands, degraded soil, rock crevices, along water canals,
bunds, road sides, railway tracks, coalfield areas and recently in cropped areas,
parthenium has become a matter of grave concern. Because of its efficient biological
activity, wider adaptability to varying soil and micro-environment, parthenium has
replaced dominant flora in a wide range of habitats cutting across state boundaries and
agro-climatic region. Very little or sometimes no vegetation could be seen in parthenium
dominated areas. Wherever it invades, it forms a territory of its own replacing the
indigenous natural flora including medicinal herbs utilized by man as a source of
medicine. It is parthenium’s allelopathy, which causes inhibition of germination,
suppression of natural vegetation including medicinal herbs and thereby poses a strong
threat to biodiversity. Besides, its high fecundity [Nearly 7000 cypsella (fruits) &
10,000-15,000 seeds per plant} and small and light weight seeds capable of long distance
travel through wind, water and other means, made parthenium highly proliferative. It is an
ideal example of naturalized weed, which is more virulent and highly adapted with viable
huge standing population all through the year to its new places/countries of introduction
that its original homestead. Its maximum importance value incex (IVI) reported in both
cropped and uncropped aeas itself depicts its higher competitive ability, strong adaptive,
reproductive and regenerative potential.
Since time immemorial, a variety of natural herbs have become the basis
of traditional systems of medicine for treatment of several diseases in various parts of
the globe. These traditional systems have been existing since remote past in countries
such as China and India. The traditional medicinal systems continue to be of vital
importance. It has been estimated by World Health Organization (WHO) that nearly 80% of
the world’s human population depends on the traditional systems for primary health
care.
Amongst the world’s ancient civilization, India has been
recognized as a rich repository of natural herbs, which are being used in different
indigenous systems of medicines e.g. Ayurveda, Siddha and Homoeopathy. Presently there are
9,500 plant herbs used in traditional medicinal systems as well as fold medicines.,
Terrain and longitudinal position encompassing different climatic zones from arid to
temperate region, rainfall zones from highest to lowest ones and phyto-geographical
regions from rich humid tropics to the alpine vegetation made India the home for natural
herbs. Our rich heritage and mosaic ethnic culture too preserve the diversity of natural
herbs widely distributed in different phyto-ecological zones of the country. The medicinal
herbs viz., Swertia chirata, Rheum emodi, Gentiana kurroa, Piccrorhiza sp, Centella
asiatica, Aconetum sp, Inula racemosa, Artimissia sp, Bunium persicum, Onosoma bracteatum,
Orchis latifolia, Saussurea lappa occur abundantly in the western Himalayas. In estern and
north eastern and north eastern region, Chlorphytum sp, Dioscorea sp, Swertia sp, Aconetum
sp, Rheum, sp, Coptis teeta, Aristolochia sp, Mucuna pruriens, Cymbopogon sp, Solanum sp,
Croton tigleum, Curcuma aromatica, Piper cubeba and Rauolfia serpentina are available. The
deccan peninsular and southern region (Western ghats and Malabar region) is rich in
Chlorophytum sp, Curculigo orchioides, Celastrus paniculatus, Plumbago zeylanicum, Gymnema
sylvestris, Strychnos nux vomica, Curcuma sp, piper sp, Dioscorea sp. In gangetic plains,
the prominent medicinal herbs are Psoralea coryifolia, Tinospora cordifoilia, Sida sp,
Phyllanthus fraternis, Cymbopogon sp and Vetiveria sp. Under arid and semi-arid region,
Calotropis sp, Aloe barbadensis, Tribulus terrestris, Withania somnifera are common.
Indian forests are also the reservoir for medicinal herbs. They supported and accelerated
the evolution and speciation of a large number of natural herbs. The medicinal herbs are
adapted to a wide range of agro-climatic conditions and soil texture. Some of them also
prefer to grow under degraded soil and wastelands. However, these areas are being quickly
captured/occupied by parthenium. They, therefore, undergo continuous struggle/interference
with parthenium, in which parthenium definitely becomes the winner. And thus the existence
of economic herbs in these very areas is under threat.
Parthenium lacks natural enemies. Even cattle do not feed on it. As a
result the food chain gets disturbed, the trophic structure changes leading to ecological
imbalance. The chemical constituents mainly sesquiterpence lactones (parthenin &
coronopilin) present in almost all the plant parts impart it a number of ecological and
biological advantages. Parthenin is responsible for toxicity, contact dermatitis and
allelopathic properties. Besides alkaloids, saponins, flavonoids, tannins, unsaturated
sterols, some phenolic acids viz., caffeic acid, p-coumaric acid, p-hydroxybenzoic acid,
anisic and ferulic acids are also present in its aqueous extract as well as in root
rhizosphere soil. All these cause inhibition of germination of other plant/herbs at the
vicinity and allow parthenium to dominate.
Parthenium causes considerable yield reduction to the tune of 40% in
agricultural crops (groundnut, cotton, rice, hybrid sorghum, maize, soybean, safflower,
sunflower, pea, niger, horsegram, sesmum, castor, sugarcane, mulberry, millet, hybrid
Napier grass, vegetables like bean, chilli, tomato and potato) and up to 90% in forage
production in grassland. Even horticultural crops like coconut, sapota, gugava, grapes,
banana, mango and arecanut are not spred.
Parthenium grows well in almost all soil. The growth is more luxuriant in black soil as
compared to laterite soil. It prevails all the year round due to its day-neutral habit,
temperature insensitivity, drought tolerance and absence of seed dormancy. Under arid
conditions, the weed has very high survivability due to low photorespiration and emerges
through out the year, where other plants/herbs hardly withstand. Its seed germinate on a
wide range of soil pH from 2.5 to 10.0 Soil alkalinity may suppress its growth with rains.
High regenerative and reproductive potential of the weed are the other favourable reasons
for its widespread. Root stumps, petiole and even detached midrib have the ability to
regenerate. However, most of the natural herbs are not equipped with these versatile
characters and as a result, they cannot withstand in close competition with it and their
population is under gradual decline. |