The composition and spatial arrangement of cuticular waxes on the leaves of Prunus laurocerasus were investigated. In the wax mixture, the triterpenoids ursolic acid and oleanolic acid as well as alkanes, fatty acids, aldehydes, primary alcohols and alcohol acetates were identified. The surface extraction of upper and lower leaf surfaces yielded 280 mg m -2 and 830 mg m -2, respectively. Protocols for the mechanical removal of waxes from the outermost layers of the cuticle were devised and evaluated. With the most selective of these methods, 130 mg m -2 of cuticular waxes could be removed from the adaxial surface before a sharp, physically resistant boundary was reached. Compounds thus obtained are interpreted as 'epicuticular waxes' with respect to their localization in a distinct layer on the surface of the cutin matrix. The epicuticular wax film can be transferred onto glass and visualized by scanning electron microscopy. Prunus laurocerasus epicuticular waxes consisted entirely of aliphatic compounds, whereas the remaining intracuticular waxes comprised 63% of triterpenoids. The ecological relevance of this layered structure for recognition by phytotrophic fungi and herbivorous insects that probe the surface composition for sign stimuli is discussed.

The seasonal development of adaxial Prunus laurocerasus leaf surfaces was studied using newly developed methods for the mechanical removal of epicuticular waxes. During epidermal cell expansion, more than 50 g leaf Ϫ1 of alkyl acetates accumulated within 10 d, forming an epicuticular wax film approximately 30 nm thick. Then, alcohols dominated for 18 d of leaf development, before alkanes accumulated in an epicuticular wax film with steadily increasing thickness (approximately 60 nm after 60 d), accompanied by small amounts of fatty acids, aldehydes, and alkyl esters. In contrast, the intracuticular waxes stayed fairly constant during development, being dominated by triterpenoids that could not be detected in the epicuticular waxes. The accumulation rates of all cuticular components are indicative for spontaneous segregation of intra-and epicuticular fractions during diffusional transport within the cuticle. This is the first report quantifying the loss of individual compound classes (acetates and alcohols) from the epicuticular wax mixture. Experiments with isolated epicuticular films showed that neither chemical conversion within the epicuticular film nor erosion/evaporation of wax constituents could account for this effect. Instead, transport of epicuticular compounds back into the tissue seems likely. Possible ecological and physiological functions of the coordinate changes in the composition of the plant surface layers are discussed.

Corn (Zea mays L.) root adaptation to pH 3.5 in comparison with pH 6.0 (control) was investigated in long-term nutrient solution experiments. When pH was gradually reduced, comparable root growth was observed irrespective of whether the pH was 3.5 or 6.0. After low-pH adaptation, H ؉ release of corn roots in vivo at pH 5.6 was about 3 times higher than that of control. Plasmalemma of corn roots was isolated for investigation in vitro. At optimum assay pH, in comparison with control, the following increases of the various parameters were caused by low-pH treatment: (a) hydrolytic ATPase activity, (b) maximum initial velocity and Michaelis constant (c) activation energy of H ؉ -ATPase, (d) H ؉ -pumping activity, (e) H ؉ permeability of plasmalemma, and (f) pH gradient across the membranes of plasmalemma vesicles. In addition, vanadate sensitivity remained unchanged. It is concluded that plasmalemma H ؉ -ATPase contributes significantly to the adaptation of corn roots to low pH. A restricted net H ؉ release at low pH in vivo may be attributed to the steeper pH gradient and enhanced H ؉ permeability of plasmalemma but not to deactivation of H ؉ -ATPase. Possible mechanisms responsible for adaptation of plasmalemma H ؉ -ATPase to low solution pH during plant cultivation are discussed.Acid soils make up to 40% of the worldЈs arable land (Kochian, 1995). Plant growth and development on acid soils may be affected by high levels of Al and Mn, as well as by limited availability of various nutrients (Adams, 1981). On the other hand, low pH (high H ϩ activity) in root medium (pH e ) may directly inhibit plant growth (Islam et al., 1980; Schubert et al., 1990). Mechanisms of Al toxicity have been studied extensively during the last decade (Kochian, 1995), whereas the understanding of H ϩ toxicity in plants remains poor. It has been observed that root growth rate was related to net H ϩ release, which may be restricted at low pH e . Therefore, it has been suggested that H ϩ homeostasis of plant root cells may be influenced by low pH e , resulting in the reduction of root growth rate (Yan et al., 1992). Net H ϩ release results from H ϩ efflux driven by plasmalemma H ϩ -ATPase activity and from H ϩ influx following the plasmalemma H ϩ gradient. Reduced net H ϩ release may be attributed to a decrease in H ϩ pump activity, an increase in plasmalemma H ϩ permeability, or both. Because of its overall importance in physiological processes, the plasmalemma H ϩ -ATPase has been investigated extensively during the last two decades. This enzyme has been found to respond to a number of environmental factors, such as saline stress (Braun et al., 1986; Ayala et al., 1996), nutrient supply (Kuiper et al., 1991; Santi et al., 1995; Schubert and Yan, 1997), high-O 2 treatment (Pinton et al., 1996; Xia and Roberts, 1996), mechanical stimulation (Bourgeade and Boyer, 1994), and fusicoccin, a fungal toxin (Marré, 1979). Although there are reports in the literature describing a response of yeast H ϩ -ATPase to low pH e (Eraso and Gancedo, 198...

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