Biological control of Congress Grass in NE India using sickle pod
- A report -

Prof N Irabanta Singh & Y Nganthoi Devi *

Parthenium hysterophorus at Salem, Tamil Nadu :: Pix - Wikipedia/Thamizhpparithi Maari

Introduction

One of the major invasive weeds that is threatening natural ecosystem in North East India is Parthenium (Parthenium hysterophorus L.) which is an herbaceous, erect and an annual weed belonging to the family Asteraceae (compositae). It is commonly known as Congress grass or carrot grass, Gajarghas, White top, etc. In India it was first reported from Pune (Rao, 1956). It is native of South and Central America and has been accidentally introduced into India in 1955 along with imported PL480 food grains (Tower et.al., 1977).

Today, Parthenium or congress grass is a household name in the country and has becomes a ‘national weed’. It is capable of establishing four successive generations at the same site in a year under favourable temperature and moisture conditions (Pandey and Dubey, 1988). It is also known to cause a number of environmental and agricultural problems such as loss of crop productivity, fodder scarcity, biodiversity depletion and health problems for livestock (Kohli and Rani, 1994; Evans, 1997).

The sesquiterpene lactones namely parthenin and coronopilin are present in the pollen, trichomes of leaves and stems of Parthenium are responsible for causing various allergies like contact dermatitis, hay fever, asthma and bronchitis in human beings. It also releases chemicals that inhibit the germination and growth of pasture grasses and other plants.

In North East India, it is now found growing gregariously in wastelands, national and state highways, road side, road dividend and is spreading like a wild fire. Due to its prolific cover and hazardous properties to environment, adequate measures are required for the proper management of this weed. Increasing public concern on environmental issues requires alternative weed management systems which are less herbicide dependant or based on naturally occurring compounds.

Allelopathy & Allelochemical: Allelopathy is a mechanism in which chemicals produced by weed plants may increase or decrease the associated plant growth. Rice (1984) defined allelopathy as the effects of one plant (including microorganism) on another plant via the release of chemicals (Allelochemicals) into the environment.

Its allelopathic effect of Parthenium to surrounding is due to allelochemicals like phenolic acids and sesquiterpene lactones (Kapoor, 2012; Veena, 2012). Different studies have proved that certain plants have the capacity to suppress the growth of surrounding plants by their root exudates through allelopathy. Possible control of Parthenium were made in controlled and field condition using Cassia tora.

About Sickle pod (Cassia tora) Plant

Sickel pod (Cassia tora) is a dicot legume known in English as sickle senna, sickle pod, tora, coffee pod; Charota in Hindi, Thaunam macha in Manipuri. It is mostly found in South-East Asia and the South West Pacific as an important weed. It is considered a wild weed, wild peanut or pistache that has many healing benefits. The plant is an herbaceous annual foetid herb.

The plant can grow 30-90 centimeters high and consists of alternative pinnate leaves with leaflets mostly with three opposite pairs that are obovate in shape with a rounded tip. The leaves grow up to 3-4.5 centimeters long. The stems have distinct smelling foliage when young. The flowers are in pairs in axils of leaves with five petals and pale yellow in colour.

Cassia tora yellow flowers occur in pairs with stamens of unequal length producing pods that are somewhat flattened or four angled, 10–15 cm long and sickle shaped, hence the common name sickle pod. There are 30-50 seeds within a pod. The seeds, roots and leaves from this plant has been shown to be very beneficial to the modern system of herbal medicines.

The seeds and leaves are also used to treat skin disease and its seeds can be utilized as a laxative. Since Cassia tora has an external germicide and antiparasitic character, it has been used for treating skin diseases such as leprosy, ringworm, itching and psoriasis and also for snakebites. Other medicinal provisions from plant parts include balm for arthritis using leaves of Cassia tora.

Casia tora is very stress tolerant and is an easy plant to grow. In India, it occurs as a wasteland rainy season weed. Its flowering time is favourable after the monsoon rain. C. tora can grow in dry soil throughout tropical parts and high hills of elevation (Himalayas) of up to 1800 meters as well as the plains. It mainly grows during the period of October to February.

The seed has vast soil reserves that can remain viable for up to twenty years and can produce up to 1000 emerged plants per square meter following a precise germinating rain. C. tora has many uses. It is used as a natural pesticide in organic farms and its powder is most commonly used in the pet food industry.

Management of Parthenium in NE India using Cassia tora

The biological control of Parthenium through competitive plant has been carried out in India and now gaining momentum in other countries too. From our study in controlled condition, it was found that leaf and stem aqueous extracts or leachates of Cassia tora exhibit phytotoxic activity against germination and seedling growth of P. hysterophorus. Cent percent germination inhibition was observed in the leaf (20%) and stem extract (30%) of Cassia tora. The inhibition of germination and seedling growth indicates that allelochemicals released by these plants had a negative allelopathic effect on the growth of the Parthenium seedlings.

With the increase of the soaking periods and concentration the inhibitory effect on germination and seedling growth of Parthenium were also increased which mean that the inhibitory effect was directly proportional to the soaking period and concentration. Leachates obtained after 12 days immersion period were more inhibitory as compared to 8 day and 4 day old leachates, which revealed leaching out of allelochemicals from the part of the Cassia tora increased with the increase of soaking period.

The inhibitory effect of prolonged soaking period in different concentration of leachates on the germination might be due to the imbalance in metabolism regulated by various enzyme activities. The allelopathic impact of leachates or extract is more harmful to radical. Roots absorb water for the growth of plants, which is done by the root cells that are directly exposed to the solution of leachates containing the allelochemicals, which affect the growth of the root and shoot.

An attempt was also made by growing together the Cassia tora and Parthenium in microplot field condition at different population ratio. From our study, it was found that Cassia tora population suppressed Parthenium in all the tested ratios. It was observed that after the initial monsoon, Parthenium, and Cassia tora seeds germinate .Young seedlings of these plants grow during the first 35 days of the initial monsoon .

However, seedling of Cassia tora were found to be larger than Parthenium due to which C. tora grew faster and had taken over the Parthenium. Due to faster growth of Cassia tora in its early stage, Parthenium population got suppressed. Later on, the whole area was covered by Cassia tora in each plot and Parthenium was not able to compete with them. These resulted in the reduction of Parthenium plant height, its number of branches and flowers per plant.

Even though Parthenium is known for its allelopathic nature on the other weeds or plants, Cassia tora can suppress the growth of Parthenium to a greater extent. The reason may be due to their larger leaf size in early seedling stage than Parthenium, which overtakes Parthenium and keeps them under control. The plants in early phenological stages exhibit higher metabolic activities resulting in biosynthesis and accumulation of potential allelochemicals and releases them into the soil as root exudates.

In the early stages, plants have to strengthen the defence mechanism to establish the seedling (Narsingh Bahadur et al., 2013). The Parthenium plant could not get sufficient sunlight, which is needed for plant growth. The allelochemicals releasing stages of Parthenium and Cassia tora plants may have different timing. As the early seedling growth of Cassia tora was larger in size, the physiological development of Cassia tora may be faster which causes the release of allelochemicals in the rhizosphere first and suppresses the growth of Parthenium.

Discussion: Researchers from different places of India have also demonstrated the use of competitive plant species to suppress the growth of Parthenium. It has been reported that Parthenium intensity was reduced by 95% when Cassia tora was broadcasted before monsoon in the Parthenium infested areas (Tiwari et al., 1997).

In Madhya Pradesh, heavily infested Parthenium sites were replaced at many places by deliberate broadcasting of the seeds of C. tora during March- April (Sulshilkumar and Varshney, 2007). Cassia tora have low seed productivity and spreading ability than Parthenium and can adapt to wide range of climatic conditions of India. Besides these, the seeds of C. tora are heavy in weight so there are least chances of its spread from the infested sites to crop fields.

Field observation and Future prospect

During our recent visit to Assam and Arunachal Pradesh the author came across massive growth of C. tora on the road median, road side in the National Highways resulting complete suppression of Parthenium on either sides of the road.

From these findings we can recommend that Cassia tora seed be collected in large quantity and broadcasted in previously Parthenium infested regions of North East India before the onset of monsoon will suppress the growth of Parthenium.