A DEVELOPMENTAL STUDY OF SILICIFICATION IN THE ABAXIAL EPIDERMAL CELLS OF SUGARCANE LEAF BLADES USING SCANNING ELECTRON MICROSCOPY AND ENERGY DISPERSIVE X-RAY ANALYSIS

A developmental study of the accumulation of silicon and other elements in the abaxial epidermis of sugarcane (Saccharum officinarum L.) leaf blades using scanning electron microscopy and energy dispersive x-ray analysis showed that accumulation of silicon progresses at different rates in each epidermal cell type. In basal cells of two-celled microhairs and in prickles there is accumulation of silicon while the leaf is immature and still enclosed within the spindle cluster of leaves and not involved in transpiration. After transpiration begins, all epidermal cells rapidly accumulate silicon. However there are differences in the rate of silicon accumulation and in the maximum amount of silicon accumulation among the various cell types. This may relate to differences in their physiology or structure. THE PROCESS of silicification of the aboveground portions of grasses and sedges diffiers for cell type, tissue, organ, species, and amount of silicon available in the culture medium. Of the major types of epidermal cells found in leaf blades-silica cells, cork cells, long cells, trichomes, guard cells, subsidiary cells, and bulliform cells-only silica cells have been studied extensively. However, silica cells diffier from other epidermal cells in that loss of cytoplasm and cell death occurs just prior to the rapid accumulation of silicon (Blackman, 1969Kaufman, Petering and Smith, 1 970aKaufman, Petering and Soni, 1970b). The location of the silicon in most silica cells is also unique in that the silicon occurs within the cell lumen (Kaufman et al., 1969; Hayward and Parry, 1975; Kunoh and Akai, 1977). As most other epidermal cells are living at maturity and the silicon is deposited within their cell walls it would be of interest to compare developmentally the silicification of these other epidermal cells with the silica cells. The present study uses energy dispersive x-ray analysis to determine the sequence of silicon deposition ' Received for publication 9 January 1980revision accepted 1 1 September 1984. We wish to acknowledge support from NSF Grant GD3427 for purchase of the SEM and NSF Grant GU-2755 through a University of Hawaii Pacific Biomedical Research Grant for purchase of the energy dispersive x-ray analyser. This study was partially supported by an AID 21 ld Grant csd-2833 and was completed while WSS was on sabbatical leave with the Department of Botany, Hawaii Institute for Tropical Agriculture and Human Resources University of Hawaii, Honolulu, HI. We thank M. Nagao for critical review of the manuscript. in the abaxial epidermis of sugarcane leaf blades. The abaxial epidermis was chosen because it allowed for study of the effiects of transpiration on silicon deposition-the developmental sequence involves an earlier unexposed stage (enclosed within the leafblade of an older leai) and later stages in which the abaxial epidermis is exposed and actively transpiring. MATERIALS AND METHODS-Leaf samples were collected from field-grown plants of sugarcane (Saccharum officinarum L.) var. 597209 at the Kunia Substation of the Hawaiian Sugar Planter's Association. The soil was of the Molokai series, a member of the clayey, Kaolinitic, isohyperthermic family of Typic Torrox. Both the soil and the iingation water have high levels of silicon (Fox et al., 1967a b). Numbenng of leaves follows the standard procedure for tissue analysis as described by Clements and Ghotb (1968). Leaves were numbered in order counting the outer leaf of the spindle cluster as Leaf 1. The criteria for defining the spindle cluster was such that the leaf sheath of Leaf 3 was mature. Leaf-1 of this study was the second leaf inside of the No. 1 leaf. Leaf samples were taken about a third of the way up the blade from the leaf sheath. The samples were then cut paradermally or into 3-mm squares. The material was frozen in Freon-22 in a liquid nitrogen bath followed by vacuum sublimation using a Vitris 10-010 Automatic Freeze-Dryer. The material was mounted on carbon stubs using collodion and carbon coated by vacuum evaporation using a Denton DV-502 Vacuum Evaporator. For