ULTRASTRUCTURE OF THE CUTICLE OF OHELO (VACCINIUM RETICULATUM)

When viewed by electron microscopy of thin sections, the inner two-thirds of the petiolar cuticle of Vaccinium reticulatum appears dense and is penetrated by a channel-like reticulum which emanates from the primary cell wall. The outer one-third of the cuticle stains lightly and is homogeneous. This arrangement is compared with the ultrastructure of cuticle previously reported for other plant species. THE ultrastructure of leaf and petiolar cuticle has been studied by electron microscopy of thin sections in several species of plants (Chafe and Wardrop, 1973; Fisher and Bayer, 1972; Hallam, 19 70; Hilkenbaiimer, 1958; Jarvis and Wardrop, 1974; Ledbetter and Porter, 1970; Maier, 1968; O'Brien, 1967). In most cases the inner one-half to two-thirds of the cuticle is amorphous and penetrated by an electron-dense reticulum composed of fibril-like (O'Brien, 1967) or channellike components (Chafe and Wardrop, 1973). This reticulum appears to be continuous with the matrix of the cell wall. The reticulum is absent from the outer portion of the cuticle where distinct periclinal, electron-dense lamellations are often found (Chafe and Wardrop, 1973; Fisher and Bayer, 1972; Jarvis and Wardrop, 1974). During our studies of the ultrastructure of ohelo leaves (Vaccinium reticulatum) we have been able to obtain thin sections of the cuticle of this plant. We present these micrographs which add to our knowledge of cuticle ultrastructure and compare its arrangement with that previously reported for other plant species. MATERIALS AND METHODS-Samples of young petioles were collected from the ohelo plant, Vaccinium reticulatum Smith, growing off the Saddle Road above Hilo, Hawaii. Petiolar samples were fixed in 3% glutaraldehyde in 0.05 M sodium cacodylate buffer for 1 hr at room temperature and for the next 15 hr at 4 C. Samples were then washed with several changes of the buffer, post-fixed in 2% osmium tetroxide in 0.05 M sodium cacodylate buffer and subsequently washed in the buffer. Samples were dehydrated in an ascending series of acetone and embedded in Spurr low-viscosity resin. Ultrathin sections, stained with uranyl acetate and lead citrate, were 1 Received for publication 17 April 1978; revision accepted 20 June 1978. This study was supported in part by an N.I.H. Minority Biomedical Support Grant No. RR08073. We thank Mrs. La Vanda Warren for her secretarial assistance. examined and photographed in a Zeiss 9 S-2 electron microscope. OBSERVATIONS AND DISCUSSION-The petiolar cuticle of Vaccinium reticulatum is approximately 2.6 gm thick and can be divided into two layers according to the density of staining. The inner two-thirds is denser than the outer portion and contains a reticulum of fibrils or channels which appear to emanate from humps of wall material projecting from the primary wall into the cuticle (Fig. 1, 3). The reticulum components are largely directed toward the cuticle surface although other elements can occasionally be seen near the primary wall within the cuticle running parallel to the surface of the wall or forming anastomosing networks (Fig. 2, 3). This reticulum is a consistent structure seen in almost all cuticles studied, but its composition and function are still matters of conjecture. O'Brien (1967) presented evidence that this reticulum is made up of polyuronides and that it corresponds closely to the pectin lamella identified by early histologists. He states that cuticular transpiration and transport of water soluble substances may occur along these routes. Chafe and Wardrop (1973), however, showed that the reticulum has varying diameters and appears more like channels than fibrils and they postulated that these channels may be avenues through which cuticular precursors diffuse and epicuticular wax is secreted. In our preparation the reticulum stains with the approximate density of the primary walls and does appear fibrillar in the outer cuticle. Nearer the primary epidermal wall, however, the reticulum often anastomoses to form a channel-like network (Fig. 2). Our micrographs, especially those like Fig. 3, give the impression that the cuticular reticulum is formed by a flow of materials into the cuticle from distinct sites within the cell wall, although pores even of the thinnest diameter were not observed in the cell wall. The outer border of the dense layer has ob-