Allelopathic is any direct- or – indirect harmful effect that one plant has on another through the relates of chemical substances- or – toxins into the root environment.
Some crops may be unsuitable to be grown as inter- crops because they may produce and excrete toxins into the soil, which are harmful to other components. Allelochemical produced from the leaves of eucalyptus globules drastically reduce the germination of mustard sown underneath.
On the contrary, negative allelopathic i.e stimulates the growth of this associated crop by release hormone like substances, is also possible (Tukey, 1970). But release of N from root nodules of legume is not considered to be a form an allelopathic. The chemical released by one species may inhibit species of plants other than the one releasing it (allow inhibition) or may inhibit more strongly plants of the producer species itself (auto inhibition). Toxic substances may be converted into active substances by some micro- organisms (functional allelopathic). The type quantity of allelochemical produced will vary depending on the environment and genetic make up of the plant.
Some allelochemical may be produced by the aerial portion of the plant and may reach the ground through raindrops, falling leaves or insects, inhibition the growth of the species growing underneath. Allelochemical produced from the leaves of Eucalyptus globules drastically reduced the germination of mustard (Brassica spp.) seed sown underneath (Trenbath, 1976).
Many plants exude organic substances from their roots and some of these roots exudates act as allelochemicals inhibiting the growth of the neighboring species living roots of walnut ( juggles nigra ) cucumber ( curcumas sativa) and path ( prunes Persia ) are known to exude toxic substances which inhibit the growth of the plants growing near them.
Annidations: Annidations is complementary use of resources by exploiting the environmental supplies in different ways by the component of a community (Lugwig, 1959). Or it refers to complementary interaction which occurs both in space and time.
a) Annidations in Space: The leaf canopies of component crops may occupy different vertical layers with taller component tolerant to strong light and high evaporative demand and charter component favoring shade and high relative humidity. Thus, one component crop helps the outer. Multistoried cropping in coconut and planting of shade tress in coffee, tea and cocoa plantation use this principle (Nair, 1949). Similarly, root system of component crops exploits nutrients from different layers of soil and thus utilizing the resources efficiently. Generally, one component with shallow root system and another with deep root system are selected for intercropping as in set aria (shallow red gram (deep) intercropping system.
b) Annidations in Ttime: When two crops of widely varying duration are planted, their peak demands for light and nutrients are likely to occur at different periods, thus reducing competition. When the early maturing crops are harvested, condition becomes factorable for the late maturing crops to put forth its full vigor. This has been observed in sorghum + red gram, groundnut + red gram and maize + green gram intercropping system.
c) Other Complementary Effect in Intercropping Systems: In an intercropping system, involving as legume and non-legume, part of the nitrogen fixed in the root nodule of the legume may become available to the non-legume component. The numerous reports of such beneficial effect of legumes on non-legumes are available in literature (IARI, 1975, palaniappan et al 1976, Merchant et al 1977, soundarajan and palaniappan 1979). The presence of rhizosphere micro flora and mycorrhiza, one species may lead t mobilization and greater availability of nutrients not only to the species concentrated, but also to the associated species (christen et al 1974). Another example is the provision of physical support by one species to the intercropped is climbing species may improve the yield of the climber. E.g. coconut + pepper and maize + beans. The taller component acts as wind barrier protecting the short crop as in maize+ groundnut, onion + castor, turmeric + castor.