Population Interactions – Parasitism


It is a relationship between two organisms in which one organism lives in or on the body of another organism.

The former one is referred as the parasite and the later as host.

Ø   Parasites are usually smaller in size and majority of them are host specific.

Ø   Both host and the parasite tend to co-evolve, i.e., if the host evolves a mechanism for resisting the parasite, the parasite must evolve mechanisms to counteract and neutralize them, to be successful with the specific host.

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Population Interactions – Mutualism


An association between two organisms in which both are mutually benefitted. It is a close physical & an obligatory relationship. Organisms provide resources to each other.

  • Lichens – an association between fungus and algae.
  • Mycorrhizae – an association between fungi and roots of higher plants.
  • Plant-animal relationships – animals help in pollination and seed dispersal.
  • Rhizobium grows in the root nodules of leguminous plants.
  • Coralloid roots in Cycas.
  • Ruminant mammals have microorganisms in their intestine that help in cellulose digestion.

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Population Interactions – Competition


Competition is the struggle between two or more organisms of same or different species for available resources.

Intraspecific competition is competition between individuals of same species. It is more acute as the requirements are same and can reduce the size of the population.

Interspecific competition is competition between individuals of different species. It occurs when organisms of different species have similar feeding habits. They belong to the same trophic level.

  • E.g., In grassland, there is competition between herbivores like rabbit, deer, bison.
  • Competition between resident fishes and visiting flamingoes for the zooplankton in the lake.
  • Carnivores like tiger and leopard compete for the same prey.
  • In forests trees, shrubs and herbs compete for water, minerals, and sunlight.

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Population Interactions


  • Population: the group of individuals of a species inhabiting a given area.
  • Habitat: it is the place where the organism lives. It is never occupied by a single species. It is occupied by a biological community.
  • Biological community: Populations of animals, plants, and microbes living in an area and interacting with each other.
  • Ecological niche: each species has a functional role to play in the ecological system.

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Vernalization is the effect of cold temperature on flowering.

  • It prevents precocious reproductive development late in the growing season and enables the plant to have sufficient time to reach maturity.
  • These plants remain vegetative during the warm season, receive low temperature during winter, grow further & then bear flower & fruits.

  • Lysenko 1st reported the effect of cold temperature on flowering in the wheat seedling.

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Photoperiodism is the effect of light on flowering.

  • Garner & Allard 1st reported the effect of light on flowering in Maryland Mammoth (variety of tobacco) & Glycine max.
  • On the basis of photoperiodic requirements, plants are divided into 3 types:

  • Short day plants: in these plants, the dark period is very important for flowering. The dark period should exceed a critical length. Light has no effect on flowering. E.g., rice, sugarcane, potato, tobacco, soybean, Chrysanthemum.
  • Long day plants: in these plants, the light period is very important for flowering. The light period should exceed a critical length. Darkness has an inhibitory effect on flowering. E.g., wheat, oat, spinach, radish, barley, lettuce.
  • Day-neutral plants: these plants blossom throughout the year. Length of light or dark period has no effect. E.g., tomato, cucumber, sunflower, cotton, maize, pepper.

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Abscisic acid

Abscisic acid:

Abscisic acid (ABA) is a growth inhibitor.

  • Discovered by 3 independent researchers as inhibitor-B, abscission II & dormin.They were chemically identical.
  • Addicott isolated it from cotton balls. Later renamed as abscisic acid.
  • Site of synthesis:  chloroplast in leaves.
  • Translocated through phloem.
  • Formed by oxidation of carotenoids.

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Ethylene is a volatile gaseous compound that stimulates transverse growth but retards longitudinal growth.

  • Cousins confirmed its presence in ripened oranges, that hastened the ripening of stored unripe bananas.
  • Formed in all parts of the plant i.e., roots, leaves, flowers, seeds.
  • Maximum synthesis occurs in ripening fruits & tissues undergoing senescence.
  • Synthesized from Methionine.
  • Excess of auxin induces synthesis of ethylene.
  • Biologically active at low concentration.

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  • Skoog & co-workers showed that callus formation occurred when internodal segments of tobacco are cultured. In the nutrient medium in addition to auxin, they provided extracts of yeast, vascular tissues, coconut milk or DNA.
  • Miller first discovered it from autoclaved Herring sperm DNA. He named it as kinetin.
  • Letham first obtained natural cytokinin from maize grain & coconut milk, it is referred as Zeatin.
  • Site of synthesis: Major source is roots & regions where rapid cell division occurs like meristems, root tip, buds, young leaves & developing fruits.
  • Transported through the xylem.
  • Derived from Adenine, it is 6-furfuryl aminopurine.

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  • A Japanese farmer in his rice field found a seedling that is extremely tall, he named it as Bakanae means ‘foolish seedling’.
  • Kurosawa showed that it was due to an infection by fungus Gibberella fujikorai (Fusarium monoliforme).
  • Yabuta & Sumuki isolated it & named it as Gibberellins.
  • Chemically it is Gibberellic acid (C19H22O6).
  • More than 100 different types of Gibberellins are known, of which GA3 is the most common.
  • Precursor of gibberellins: Mevalonic acid (derived from Acetyl CoA).
  • Site of synthesis: shoot buds, young leaves & developing seeds.
  • Transported by simple diffusion.

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