The cuticle is a thin (0.1-10 µ m) layer covering the surface of all higher plant aerial parts; it is composed of the polymeric matrix (cutin), polysaccharides, and cuticular waxes [1,2]. The plant cuticle prevents uncontrolled water and ion losses and interferes with pathogenic microorganism penetration inside the plant [3]. The structure of the cuticle is a species-specific trait and, according to current knowledge, is not very variable. Modern classifications consider two groups of cuticles: lamellate and nonlamellate. Within these two groups, six to eight structural types are distinguished based on the distribution and orientation of lamellae and dendrites in the primary and secondary layers of the cuticular membrane [1,2,4]. A direct dependence of cuticle permeability on its structure was not observed. Diffusion is considered a basic mechanism of molecule transport across the cuticle [5]. The cuticle permeability is extremely low, although it varies depending on the plant species and environmental conditions [6]. On the other hand, plant specialized secretory systems are characterized by a high rate of liquid secret release through the cuticular membrane. Examples of such systems are nectaries, digestive, essential-oil, and salt glands.Sugars, the major components of the nectar, are transported to the nectary via phloem sieve elements [7]. Phloem unloading into the symplasm of secretory cells occurs passively along the concentration gradient, which is created due to the active energy-dependent sugar transport across the plasma membrane of these cells into the apoplasm. Because of the produced hydrostatic pressure gradient, water flows into the apoplasm. Thus, sugars and minor components are pushed along the nectary apoplasm as a continuous mass flow [8]. Nectar can be secreted from the nectary either through modified (always open) stomata or through the cuticle [9].Usually, in spite of stomata absence in nectaries, their cuticle does not display any specialized structures for transcuticular nectar secretion. For instance, the nectary cuticles of Aphelandra spp [10] and Lonicera japonica [11] did not show any electron-microscopic differences from the low-permeable leaf cuticle. Nevertheless, such cuticle, which did not show any visible damages, was highly permeable during both nectary secretory activity and nectar reabsorption [12]. Sometimes, nectar is released through ruptures of the cuticular membrane formed under the pressure of nectar accumulated in the subcuticular cavities [13]. In the nectaries of trichomes, special pores in the cuticles were described, which facilitated nectar secretion [14].We studied the ultrastructure of Helleborus foetidus and H. caucasicus (Ranunculaceae) floral nectaries devoid of stomata and observed an unusual type of the cuticle structure related to its high permeability for nectar.
Materials and methods.The structure of the nectary cuticles of H. foetidus L. and H. caucasicus A. Br. was compared with the cuticle structure of their leaves. For electron-microscopic investigat...
