Histology of chrysanthemum roots exposed to salinity stress and Phytophthora cryptogea

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Rooted cuttings of Chrysanthemum morifolium 'Paragon' grown in nutrient solution were given a 24-h pulse exposure to salinity by amending the solution with 200 mequiv. NaCl/L. The cuttings were then returned to nonsalinized solution; one half were inoculated with motile zoospores of Phytophthora cryptogea Pethybr. & Laff, and the other half were maintained as non-inoculated controls. Nonstressed cuttings also were inoculated with P. cryptogea, and roots from all treatments were sampled at various intervals thereafter for light and electron microscopy. Penetration of nonstressed roots by P. cryptogea was frequently limited to three or four cell layers 6–12 h after inoculation. Formation of appositions adjacent to hyphae, increased wall staining density, and accumulation of osmiophilic material in cell vacuoles were associated with sites of limited penetration. In contrast, hyphae of P. cryptogea rapidly colonized salinity-stressed roots, causing extensive necrosis within 12–24 h after inoculation. The cytological responses to infection seen in nonstressed roots were rarely observed in stressed roots, indicating that salinity stress inhibits active root defense mechanisms.