Epistomatal Chambers in the Needles ofPinus strobusL. (Eastern White Pine) Function as Microhabitat for Specialized Fungi

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Abstract
The stomatal complex of eastern white pine (Pinus strobus L.) possesses a feature of uncertain function: the epistomatal chamber. The chambers are formed by the subsidiary cells that overarch the guard cells and are normally occluded with epicuticular wax. Morphologically distinct fungi resembling black yeasts are frequently observed within the epistomatal chambers submerged in the wax occlusions. Histochemistry shows that the fungal hyphae are viable and sheathed in a complex polysaccharide matrix. Experimental covering of the branches prevents colonization of the stomata, indicating that the inoculum originates exogenously. Preliminary investigations into the distribution of the fungus in Ontario, Canada, show an uneven distribution. Ecologically, the epistomatal chamber is proposed to be an ecotone between the phylloplane and the interior of the needle, one that possesses the survival advantages of both the phylloplane and the interior with few of the disadvantages associated with these two habitats. The presence of stomatal fungi may influence the response of host trees to aerial pollutants, may affect transpiration, and may prevent pathogen ingress.