Index >> Rhizosphere and Phyllosphere >> Biochemical Reactions in the Phyllosphere

The fungal spores produce a chemical substance or substances which are active in inducing the production of phytoalexins by the host as a defence reaction. The fungal metabolite, even in the absence of spores, may also independently induce phytoalexin for­mation.

However, the concept of phytoalexin has been brought under a wide coverage so as to include all chemical compounds contributing to disease resistance in response to injury, physiological stimuli, the presence of infectious agents and their products. In keeping with this view, some of the known phytoalexins are summarized in.

Epiphytic microorganisms are known to synthesize indole acetic acid. A more interesting and useful biological function is the fixation of nitrogen by microorganisms inhabiting the leaf surface. Bacteria of the' genera Escherichia, Brevibacterium, Bacillus, Diplococcus, Pseudomonas, Flexibacterium, Rhizobium, Beijerinckia, Azotobacter, Xanthomonas and Micrococcus have been isolated from the phyllosphere of maize, cowpea, sugarcane and gram and some of them have proved to be potential
nitrogen fixers.

Notwithstanding all the observations and implications of phyllosphere microorganisms in plant disease manifestation, the exact role of these microorganisms in the nutrition of plants has remained largely conjectural in spite of the demonstration that many epiphytic bacteria grow on nitrogen-free media and fix atmospheric nitrogen to varying degrees.

Ex periments have also been done under laboratory conditions to demonstrate nitrogen fixation in the phyllosphere of several plants by the use of 15N and the quantitative data obtained are so divergent that one is led to believe that fixation of nitrogen is a very variable phenomenon on the plant surface.