The Leaf Epidermis: Its Ecophysiological Significance

Abstract

The epidermis is the outer cell layer of the primary plant body. Concentrating on photosynthesis as obviously the main function of leaves has brought the usually chlorophyll-free epidermal cells of higher plants out of focus for many plant biologists. Nevertheless, a considerable number of publications provide information on the tissue-specific role of the epidermal cells and have deepened our insight into the various functions of the epidermis as a barrier interconnecting and separating leaves from the environment, and their importance in the response of plants to external stimuli. To a significant extent this progress was due to methods which were frequently employed to analyze leaves and not primarily to characterize the leaf epidermis. Instead, these approaches gave information on the epidermis more as a side product. This will be illustrated in the section on new methods. To put the subject of this review in concrete terms: stomatal functioning will not be treated here, since it has been concisely reviewed recently (Raschke et al. 1988; Tallman 1992). This review will also not deal with specialized epidermal cells or cell descendants such as idioblasts, hairs, glands, absorption scales, and papillae (Wagner 1991; Romberger et al. 1993). However, it can be hypothesized that the latter structures developed by advancing and optimizing functions which were already inherently associated with normal epidermal cells of cormophytes. This will be evident for ion compartmentation, synthesis of secondary plant compounds, or protection of shoot surfaces (Romberger et al. 1993). The scheme of Fig. 1 summarizes ecophysiologically important functions of the leaf epidermis either in the direct interaction with environmental factors or indirectly as part of the response of plants to changes in growth conditions such as salinity.