The effect of silicon (Si) supply on the infection and spread of Pythium aphanidermatum was studied in the roots of tomato [Lycopersicon esculentum (=Solanum lycopersicum), an Si excluder] and bitter gourd (Mormodica charantia, an Si intermediate accumulator). Individual roots were mounted into PVC compartmented boxes which allowed the application of Si and zoospores to defined root zones. Two days after inoculation, root growth was recorded, and P. aphanidermatum colonization of individual root sections was determined by ELISA. In tomato as well as in bitter gourd the root tip was the root section most sensitive to P. aphanidermatum infection. Application of Si did not affect severe root-growth inhibition by P. aphanidermatum in either species. However, continuous Si supply significantly inhibited the basipetal spread of the pathogen from the infected root apex in bitter gourd but not in tomato. Si application to the roots only during pretreatment or only during/after the infection of the roots failed to inhibit the spread of P. aphanidermatum. Determination and compartmentation of Si in the roots of bitter gourd revealed that apoplastic Si was not, but symplastic Si was, associated with the ability of the plant to reduce the spread of the fungus in roots. It is concluded that accumulation of Si in the root cell walls does not represent a physical barrier to the spread of P. aphanidermatum in bitter gourd and tomato roots. The maintenance of elevated symplastic Si contents is a prerequisite for Si-enhanced resistance against P. aphanidermatum.
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