Ultrastructure and development of the outer epidermal wall of spruce (Picea abies) needles

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Abstract
The outer epidermal wall of Picea abies needles was investigated throughout its life-span. Several cytochemical procedures at the light and electron microscope level including enzyme–gold affinity labeling and elemental mapping by electron energy loss spectroscopy were applied to gain detailed information about its fine structure. The adult outer epidermal wall averages 9 μm in thickness and occupies half the cell height. It is composed of five main layers and additional epicuticular waxes. In the cuticular membrane a thin homogeneous cuticle proper is clearly distinguishable from the cuticular layer, which is composed of an exterior striated, a middle reticulate, and an interior arborescent sublayer. Cellulose and mannans were localized and identified among other polysaccharides in the cuticular layer in situ. Arborescent structures extend to an adjacent, usually lignified layer, containing numerous calcium crystals. Beneath this is an electron-dense layer, formed mainly of hemicelluloses. The innermost wall layer comprises several alternately arranged lamellae formed of several types of polysaccharides and differing mainly in their polyphenol content. During ontogeny of the outer epidermal wall, lasting a whole year, five phases were distinguished: a protodermal phase, one of even growth, one of uneven growth, and one of lignification and further growth. With the fifth phase the outer epidermal wall of current needles achieves maturity in late autumn. No differences in ultrastructure and development of the outer epidermal wall between apparently healthy and damaged needles were observed. Key words: Picea abies, outer epidermal wall, light and electron microscopical cytochemistry, enzyme–gold technique, polysaccharides, calcium crystals.

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