Biology_Today_-_October_2016

(a) Bioherbicides

s Bioherbicide is an organism

which controls or destroys

unwanted plant growth

without harming the

useful plants. These are the

compounds which are derived

from microbes such as fungi,

bacteria, phytotoxic plant residues

etc. Bioherbicide based on a fungus is called

mycoherbicide.

(b) Bioinsecticides

s Bioinsecticides are those biological agents that are used to

control harmful insects. They include the following:

(i) Predators: Destructive insects or plant pests can be

brought under control through introduction of their

natural predators. The predators should be species

specific and unable to harm the useful insects.

Introduction of ladybugs (Lady bird beetles) and Praying

Mantis has been successful in combating scale insects

or aphids which feed on plant sap.

(ii) Parasites and Pathogens: This is an alternate biological

control of plant pests through the search of their natural

parasites and pathogens. They include viruses, bacteria,

fungi and insect parasitoids. Parasitoids are organisms

that live as parasites for some time (as early or larval

stage) and free living at other times, e.g., Trichogramma.

Nucleopolyhedrovirus (NPV) are species

specific, for example, Baculovirus

heliothis (a virus) can control

Cotton bollworm (Heliothis zea).

Similarly, Bacillus thuringiensis

(a bacterium) is effective against

the cabbage looper.

(iii) Natural Insecticides: These

are insecticides and related pesticides

which are obtained from microbes and

plants. A number of natural insecticides are

available which are used to kill insects. The common

ones include azadirachtin, rotenone, nicotine, pyrethrum

and thurioside

BIOFERTILISERS

s Biofertilisers are microorganisms which bring about nutrient

enrichment of soil by enhancing the availability of nutrients

to crops. The microorganisms which act as biofertilisers are

bacteria, cyanobacteria (blue green algae) and mycorrhizal

fungi. Bacteria and cyanobacteria have the property of

nitrogen fixation while mycorrhizal fungi preferentially

withdraw minerals from organic matter for the plant with

which they are associated.

s Nitrogen fixation is the process of conversion of molecular

or dinitrogen into nitrogen compounds. Insoluble forms of

soil phosphorus are converted into soluble forms by certain

microorganisms. This makes the phosphorus available to the

plants. The various biofertilisers are as follows:

Table: Various biofertilisers and their examples

Biofertiliser Nutrition/Association Examples

(i) Free living nitrogen fixing bacteria (i) Saprotrophic

(ii) Photoautotrophic

Azotobacter, Clostridium,

Rhodospirillum, Chromatium

(ii) Free living nitrogen fixing cyanobacteria Photosynthetic Anabaena, Nostoc, Aulosira

(iii) Symbiotic nitrogen fixing bacteria Mutual association between bacteria and plants Rhizobium

(iv) Symbiotic nitrogen fixing cyanobacteria Mutual association between cyanobacteria and

plants

Anabaena

(v) Mycorrhiza Symbiotic association of fungus with roots of

higher plants.

Glomus

Symbiotic nitrogen fixing bacteria

s They form mutually beneficial association with plants. The bacteria obtain food and shelter from plants and in return provide
nitrogen to plants.

s Bacteria form nodules on roots of plants, as they develop ability of nitrogen fixation only when present inside root nodules.

s Bacteroids in nodule cells are surrounded by membrane of host lined by red-pink pigment, leghaemoglobin, which acts as
oxygen scavanger.