Stomatal plugs and their impact on fungal invasion in Agathis robusta

The hypothesis that stomatal wax plugs in Agathis robusta (C. Moore ex F. Muell.) F. Muell. (Araucariaceae) protect leaves against fungal invasion by preventing hyphae entering the stomatal pore was tested. Leaves with intact wax plugs and leaves from which wax plugs had been experimentally removed were inoculated with either Botrytis cinerea (De Bary) Whetzel or Alternaria solani (Ellis & G. Martin) L. R. Jones & Grout. Interactions of fungal hyphae with stomata were subsequently analysed by scanning electron microscopy. Wax plugs blocked the penetration of fungal hyphae into stomata of A. robusta in 100% of encounters. In contrast, hyphae readily penetrated stomata of leaves from which wax plugs had been removed or where the wax plugs were damaged. On rare occasions, hyphae were also observed to penetrate directly through the cuticle. Florin rings around the external surface of stomata also acted to deflect fungal hyphae from the stomatal opening. Formation and degradation of wax plugs was also examined to assess the period over which they offer protection from fungal invasion. Wax plugs formed in young leaves in spring and were complete by summer. There was subsequent degradation of wax plugs in the following winter, and they then regenerated again in spring. However, regeneration of wax plugs appeared to decline after the leaves reached 2 years of age. The data are discussed with reference to the adaptive significance of wax plugs in conifers and other plants.