Explain the mechanism of biotic stress.

Explanation

Biotic Stresses:
(1) These are adverse effects on plants caused by other living organisms such as viruses, bacteria, fungi, parasties, insects, weeds and competing plants.
(2) Biotic environmental stress is also caused due to the activity of man by cutting herbs and tress, twigs for fodders, fuels and agricultral purposes.
(3) The biotic stresses caused by bacteria, fungi and nematodes that are ever present in the environment are called potential biotic stresses.
(1). Allelopathy:
(i) An organism producing one or more biochemical substances that greatly influence the gemination, growth and reproduction of other organisms is called Allelopathy.
(ii) These biochemicals are known as allelochemicals.
(iii) They are benficial (positive allelopathic) or deteimental (negative allelopathic).
(iv) These allelochemicals are obtained from leaf after leaching on the ground and also from roots.
(v) The term allelpathy is from Greek words allelon - each other and pathos - to suffer and first used in 1937 by Hans Molisch.
(vi) Allelopathich effect may occur with weeds on crops and vice versa.
(vii) One of the most famous allelopathic plants is Black walnut (juglans nigrum).
(viii) The chemical which is present in Black walnut is Juglone and it is a respiratory inhibitor.
(ix) Solanaceous plants such as tomato, capsicum and eggplant are susceptible to juglone.
(x) These plants when exposed to these allelochemicals exhibit symptoms such as wilting, chlorosis and death.
(xi) Tree of heaven (Ailanthus altissima) is a recent addition to the list of allelpathic trees.
(xii) Ailanthone an allelochemical extrached from the root of Ailanthus acts as potent herbicide.
(xiii) In Sorghum plant hte allelochemical sorgolone possess allelopathic activity. It is found in root exudates of most Sorghum species.
(xiv) Root exudation of maize inhibits the growth of some weeds such as Chenopodium album and Amaranthus retrofllexus.
(xv) The seed exudates of oat (Avena fatua) affect the germination of wheat seedling.
(2). Pathogenecity:
(i) The effect of microbes that cause diseases in plants.
(ii) Example: Xanthomonas citri.