PhD

Mimosa invisa Mart. ex. Colla, called giant sensitive plant (Anathottavodi or Padayincha in Malayalam), a native of Tropical America, introduced in Kerala in 1964, is now a problem to both cultivated and wastelands. Being a comparatively new introduction, no systematic and comprehensive work has been undertaken to study the problems associated with the weed and strategies to manage it. Hence, the project entitled “Biology and management of Mimosa invisa Mart in Kerala” was taken up at the College of Horticulture, Vellanikkara during 2001-2004 with the intention to study the distribution and severity of infestation and biology of the plant, the efficiency of different control methods, utility of the plant and its harmful effects.

A survey revealed medium to severe infestation in Kollam, Kottayam, Ernakulam, Alappuzha, Thrissur and Kozhikode districts. It had also spread to high altitude regions in Pathanamthitta, Idukki, Palghat and Malappuram where sporadic to medium infestations were noted.

During the survey, based on morphological characters, two types of M. invisa, spined and spineless, were noticed. The spined type was found to be predominant and more invasive compared to the spineless type in all the areas observed. M. invisa is strongly heliophytic and could climb on any support available - dead (stay wire, electric posts, sign boards, walls etc.) or alive (tapioca, banana, polyalthia, jack tree etc.). The infestation was severe in open sites like road sides, banks of irrigation cum drainage channels, and in neglected areas.

The important growth stages of the plant included seedling, active vegetative, flowering, seed set and dehiscence stages. The duration from flower bud appearance to pod dehiscence was 50-60 days. The seed production potential of a single plant was 72,650 seeds. The germination percentage decreased with time. Still the seeds could retain more than 45 per cent germination even after three years. High dormancy was exhibited by the seed. Hot water treatment at 60°C for five minutes, 80°C for two minutes or scarification with concentrated sulphuric acid for upto 15 minutes gave good germination above 85 per cent. Flaming for 30 seconds was also equally efficient.

Seeds are the main propagule for M. invisa. It is not propagated by stem cuttings. Root clumps could survive the summer months if soil moisture was available.

Sickle weeding at monthly interval gave the best control, followed by digging and removing the plant twice, and sickle weeding bimonthly, which were on par. Although production of new shoots and dry matter per square metre were effectively controlled by these treatments, they were not cost effective.

In nursery beds and small compact areas where high value crops are raised, or for soils collected from M. invisa infested fields, the solarization for 40 days could effectively prevent the establishment of M. invisa seedlings.

Among the pre emergence herbicides, Atrazine, Diuron, Flucloralin, Metolaclor and Pendimethalin gave almost 100 per cent control. These herbicides did not prevent the emergence of seedlings, but killed them within few days after emergence.

Glyphosate was the most effective post emergence herbicide against M. invisa and the optimum dose was found to be 0.6 kg ha-1. 2,4-D was not at all effective even at high dose of 5 kg ha-1. The growing tips of M. invisa were found to recover from epinasty caused by 2,4-D within a few days. Paraquat could completely defoliate the plants, but the lower canopies of the thick growth of the weed escaped the herbicide and regrowth was observed within weeks. Application of post emergence herbicides at active vegetative stage (100 days after germination) was more effective compared to the application at seedling stage (45 days after germination).

The survey on insect pests and diseases of M. invisa lead to the identification of two flower bud feeders, four defoliaters and a pathogen. Detailed studies are to be conducted on Porthesia sp. and Ericeia optature (defoliaters), to ascertain the host specificity and intensity of feeding of these insects, before using them as bioagents.

The utility of M. invisa as a green manure crop was assessed based on dry matter production potential, nutrient content and litter decomposition pattern of the plant in the soil. The very high dry matter accumulation of upto 3.63 kg plant-1 and the ability to decompose 60 per cent of the litter within the first fortnight of incorporation in soil upholds its use as green manure crop. The high content of nitrogen (3.85%) and exceptionally high content of Zinc (90-100 ppm) combined with the nitrogen fixing capacity of the legume add to its utility as green manure.

The earthworm species, Eisenia foetida could effectively compost M. invisa and banana pseudostem (1:1) within 60 days, thus enriching the content of primary and secondary nutrients in the substrate. The positive response of the growth parameters of amaranthus confirmed the quality of the compost.

The harmful effects of the M. invisa included its negative effects on biodiversity of native flora, allelopathic effects and the toxicity problems to animals due to the mimosine. The smothering efficiency of M. invisa gradually increased to above 38 per cent within three years of infestation of a new area, destroying the biodiversity of the native flora.

Application of the plant as mulch or water extract had noticeable allelopathic effects on the germination of rice (cereal) and cowpea (legume). Application at 10 tons ha-1 as mulch or as ten per cent water extract almost completely inhibited germination of cowpea and negatively effected the growth parameters. But in rice, the application at 10 tons ha-1 as incorporation, eventhough severely reduced germination initially, the positive effect on growth parameters was more significant towards 30 days growth. This can be attributed to the green manurial effect of the plant when incorporated.

Presence of mimosine, a toxic non protein plant amino acid, is the reason for the animal toxicity of M. invisa. The content of mimosine was highest (9.22%) at 90 days growth, which is very high, compared to the 3.0 per cent in subabul, at active vegetative stage. The mimosine content dropped at flowering stage. Among the plant parts highest content was in the growing tips at active vegetative stage (10.42%), which is generally grazed by live stock.

Intake of mimosine through M. invisa caused nephrotoxic and hepatotoxic symptoms in animals. Ensiling of fodder containing M. invisa under anerobic conditions is the best method for reducing mimosine toxicity. Ensiling M. invisa for 60 days could reduce the mimosine content by 40 percent to 46 percent. Drying and powdering of the plant could also reduce the mimosine content. Feeding trials in rabbits using ensiled M. invisa and fodder in different proportions, demonstrated the reduction in toxicity due to ensiling. It is possible that pastures infested upto 50 percent with M. invisa could be cut, ensiled and used for feeding animals after reducing the mimosine content significantly.