Important biotic factors are competitors and nurse plants, predators, parasites, and symbionts eliciting antagonisms
and mutualisms. Competition between plants for light often is given by shading and also by filtering certain wavelengths out of the solar spectrum, especially in the red region, where they are important for photosynthesis and for signaling functions. In the soil, competition is for water and nutrients. Plants develop chemical defense mechanisms, so-called allelopathic reactions. Conversely, plants can also nurse other plant species allowing seedling establishment and early growth under their canopy, a phenomenon which like in ecology is also very important in agroforestry. Animal predators and microbial and animal parasites, of course, cause manifold stress situations which need not be enumerated here. Plants often develop characteristic responses against such parasites where, in a so-called hypersensitive reaction, they produce aggressive defense compounds, often ROS, sacrificing small parts of their own tissue and killing the intruders with it. Important plant/plant parasites are the hemipara-sites which are green and photosynthetically active but divert the xylem stream from their hosts parasitizing for water and nutrients, such as mistletoes and the agricultural weed Striga in the tropics. In plant/plant holoparasitism, the parasites are not active photosynthetically and tap both the phloem and the xylem of their hosts, obtaining assimilates together with water and nutrients. Mutualisms and important symbioses of plants are the mycorrhiza with fungi for obtaining water and nutrients in exchange against photosynthetic assimilates and the root nodules with bacteria fixing atmospheric nitrogen.
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