Resistance of Healed Citrus Exocarp to Penetration byPenicillium digitatum

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Abstract
Germination of P. digitatum was delayed on healed citrus exocarp. Hyphae were unable to penetrate healed tissue and were confined within cell walls or collapsed cells at the injured surface. Contents of these hyphae were electron dense and the cell walls were distorted. The hyphae that successfully penetrated unhealed tissue contained normal organelle constituents and cell walls. During healing of injured cells, phenolic substances and lignin-like materials were identified with various histological stains at 16 and 30 h after injury. The inability of hyphae to penetrate healed tissue could not be attributed to lack of sufficient nutrients or water for growth or to increased physical strength of the lignified, healed tissue. Resistant tissue that had healed for 30 h was as easily macrerated as fresh tissue by exopolygalacturonase produced by P. digitatum, but tissue healed for 72 h was less susceptible to such maceration. Release of reducing groups from healed tissue upon maceration was less than from freshly injured exocarp. The accumulation of phenolic and lignin-like materials in cell walls of injured exocarp cells appeared to be the major factor involved in resistance of healed tissue to penetration by P. digitatum. These materials may inhibit fungal growth, interfere with enzymatic degradation and/or lignify penetrating hyphae.