Removal of the lower epidermis from cowpea and tobacco leaves inoculated with tobacco mosaic virus (TMV) or tobacco necrosis virus (TNV) resulted in reduction of local lesion numbers. The reduction was time-dependent and was greatly influenced by darkening the plants 24 h before inoculation. In both darkened and non-darkened plants the period of contact between epidermis and mesophyll required for lesion formation to occur differed markedly. For cowpea inoculated with TMV these periods were 1.5 and 8 h respectively, and in the combination cowpea-TNV they were 1 and 6 h respectively. For Xanthi nc tobacco inoculated with TMV these periods were 1 and 8.5 h respectively, and in the tobacco-TNV system they were 1 and 6 h respectively. More plasmodesmata were observed in superficial cells of dark-treated plants than in those of non-dark-treated plants. The significance of this observation in relation to the passage of infectious virus units from the epidermis into the mesophyll is discussed.


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