HETEROPHYLLY IN PLANTS - O.M. NEDUKHA - 2011

CHAPTER ONE. MORPHOLOGICAL AND STRUCTURAL-FUNCTIONAL CHARACTERISTICS OF VEGETATIVE ORGANS IN HETEROPHYLLOUS PLANTS

1.7. LEAF EPIDERMIS IN HETEROPHYLLOUS PLANTS

1.7.2. Cuticle and Wax

The leaves of heterophyllous plants, as well as plants without distinct heterophylly, are covered with a cuticle. Cutin, the main component of the cuticle, is a polymer composed of cross-linked oxymonocarboxylic acids that are insoluble in organic Solvents. These oxymonocarboxylic acids contain 16 to 28 carbon atoms and two or three hydroxyl groups. The acid molecules are linked together into chains via ester bonds. The composition and ratio of oxycarboxylic acids in the leaf cuticle vary depending on the plant species. Furthermore, by altering its thickness and Structure, the cuticle plays a key role in forming the cuticular barrier against Water Transpiration (Holloway, 1982, a, b).

Submerged leaves of most heterophyllous plants lack Stomata. The outer Cell walls of the epidermis in such leaves are covered with a thin cuticle that facilitates gas transport. Recently, cuticular pores have been described in the epidermis of submerged leaves of hydrophytes, specifically in Potamogeton pectinatus, P. perfoliatus, and Myriophyllum spicatum L. (Nedukha, 2010, a). However, the physiological Functions of cuticular pores in submerged hydrophyte leaves remain largely unexplored.

The Composition and Structure of the cuticular layer in the epidermal cell walls of terrestrial plants have been characterized in detail, demonstrating that the cuticle performs transport functions for ions and water, as well as for various aqueous solutions (Schönherr, 1982; 2006). It has been established that cuticular functioning in terrestrial plant leaves depends on ambient air Temperature, humidity, and light intensity (Grammatikopoulos, Maneta, 1994; Yates, Hutley, 1995).

In certain species tolerant to complete submergence (Mentha aquatica, Oenanthe aquatica, Rumex palustris), older aerial leaves typically die off underwater, followed by The Development of new, highly dissected leaves with a different cellular structure (Bruni et al., 1996; Rascio et al., 1999; Mommer, Visser, 2005) and a modified cuticular architecture (Frost-Christensen et al., 2003; Frost-Christensen, Floto, 2007). In the cuticle of leaves and stems undergoing rapid elongation growth, accelerated Hydrolysis of cutin polymers takes place (Hoffmann-Benning, Kende, 1992).

The presence of a cuticular layer has been detected not only in the outer epidermal cell walls but also in the inner periclinal walls of epidermal Cells bordering the guard cells in the leaf epidermis of Cirsium horridum Michx. (Pesacreta, Hasenstein, 1999). The authors demonstrated that the cuticular layer in the periclinal walls is three times narrower than that in the outer walls of the upper and lower epidermis. A cuticular layer has also been described in the periclinal and anticlinal walls of the leaf epidermis in Clivia L. (sp.) species (Oladele, 1983). Measurements of water transport rates through the non-stomatized leaf epidermis showed that cuticular transpiration in the lower epidermis reached 3.6 × 10^-5 m s^-1, and in the upper epidermis, 3.1 × 10^-6 m s^-1, respectively. Meanwhile, stomatal water conductance on both surfaces was 11–12 times higher (particularly on the lower surface) (Riederer, Schreiber, 2001; Kerstiens, 2006).

Water conductance through cuticular pores increases with rising atmospheric humidity (Hoad et al., 1997; Schreiber et al., 2001). According to J. Schönherr, such variations depend on the degree of Hydration of polar groups located within the cuticular pores (Schönherr, 1982). Water on one side of the cuticle is in a liquid state, whereas upon emerging through the pores, it may exist in a vapor state depending on the ambient temperature (Becker et al., 1986). The rate of cuticular transpiration is thus determined by the physical state of water. At a temperature of 25^oC and standard pressure, the density of liquid water is several orders of magnitude higher than the saturation density of water vapor in the air. The exact mechanism of water passage through these pores remains unknown.

G. Kerstiens suggests that water molecules transported through the lipophilic phase of the cuticle by diffusion, forming droplets on the opposite outer surface, acquire the energy required to detach from the liquid donor-water phase (Kerstiens, 1996; 2006). Energy activation occurs via the Cleavage of Hydrogen Bonds in the water molecule to facilitate the formation and diffusion of its liquid phase (Schreiber et al., 2001; Kerstiens, 2006). The pores consist of polar components, and their molecules possess temporary dipoles known as polar molecules. Three to five water molecules bind to K⁺ and Na⁺ ions, whereas six water molecules associate with Ca⁺⁺. All dipolar molecules (C, H, NO₂, COOH) can interact with water molecules—specifically with hydrogen—requiring an Energy Expenditure of approximately 20–100 kJ mol^-1 (Israelachvili, 1999). Notably, the binding of water molecules to monovalent cations within the cuticle occurs extremely rapidly, in about 10^-9 s, with Ca⁺⁺ and Mg⁺⁺ ions in 10^-8 and 10^-6 s, respectively, whereas with trivalent ions, it ranges from several seconds to hours (Schönherr, 2006). In other words, hydrated ions—those bound to water molecules forming a hydration shell—permeate through the aqueous cuticular pores. Using a polymer matrix and a cuticular membrane as a model system to investigate cuticular pore function, J. Schönherr established that increasing humidity from 20 to 100% enhanced water penetration through the cuticle (Schönherr, 2006).

Depending on the water-vapor pressure, the cuticle can swell and form an aqueous phase, thereby creating water-filled pores. This occurs provided the cuticle contains temporary dipolar molecules, such as amino, hydroxyl, and carboxyl groups capable of adsorbing water. Cuticular water pores form exclusively in the Presence of water, and their localization can be visualized using fluorescent Dyes. These water pores are predominantly situated at the base of trichomes and along the cuticular margins above the anticlinal epidermal walls (Schönherr, 2006). Pore size varies depending on the species and plant organ: in the epidermal peel of onion bulbs, the cuticle contained pores with a radius of about 0.41 nm; in tomato and pepper fruits, from 0.87 to 1.18 nm (Beyer et al., 2005; Schönherr, 2006); whereas in artificial cuticular membranes, it was 1.5–2.5 times larger (Schönherr, 2006). Unusual, exceptionally large "giant" cuticular pores, with an aperture diameter of about 1 µm and a height of up to 5 µm, have been described in the leaves of Eidothea zoexylocarya growing in the Australian tropics (Carpenter et al., 2007). The density of these pores reached 1.2 × 10⁵ mm^-2 of The Cell wall area. The authors hypothesize that these giant pores do not directly affect water conductance; instead, their function is likely related to moderating solar radiation by transporting (and deploying) hydrophobic epicuticular wax through the pores to protect leaves from ultraviolet radiation during the dry season (Carpenter et al., 2007).

The surface of emergent (or floating) leaves in aquatic heterophyllous plants, as well as those of terrestrial heterophyllous species, is covered not only with a cuticle but also with a wax layer composed of Unsaturated Fatty acids containing 16 to 50 carbon atoms. The content and ratio of unsaturated fatty acids in plant Waxes vary across different species. In addition to fatty acids, the waxes include unsaturated alcohols, β-diketones, hydrocarbon acids, and unsaturated esters (Sitte, Reiner, 1963; Schönherr, 2006).

The wax coating acts not only as a protective barrier against transpiration and biotic stressors (plant pests) but also helps regulate Light absorption by the leaf blade surface. The upper leaf epidermis responds to light absorption, thereby influencing Photosynthesis (Nishio et al., 1994). This can occur through The formation of the wax layer and modifications in its Morphology (Gorshkova, Zvereva, 1988; Nishio et al., 1994). Structured as saucer-shaped (concave or convex) deposits, the wax layer can gather (focus) or scatter the photon flux into the epidermal and upper parenchymal cells, or induce photoinhibition in a fraction of METABOLISM/14.html">Chloroplasts, thus regulating the rate of photosynthesis (Nishio et al., 1994). The composition of epidermal wax changes dynamically during ontogeny.

Epidermal cells contain UV-absorbing compounds that shield the mesophyll from harmful short-wave radiation (Smith et al., 1997; Mazza et al., 2000). Most epidermal cells have a convex shape, which helps them partially regulate The amount of light entering the cells (Vogelmann, 1993). When light strikes a completely smooth epidermal surface, it is largely reflected (Brodersen, Vogelmann, 2007).

Studies on English ivy (Hedera helix L.) leaves—investigating cuticular transpiration and wax composition from the 4th day of leaf development up to the 202nd day of functioning using gas Chromatography and mass spectrophotometry—have revealed that cuticular wax consists of polar and nonpolar monomeric fractions, as well as a polar oligomeric wax fraction composed of primary alcohols and acids esterified predominantly with C₁₂-, C₁₄-, and C₁₆-ω-hydroxy fatty acids (Hauke, Schreiber, 1998). It was demonstrated that the maximum concentration of the nonpolar fraction occurs on the 30th day of leaf ontogeny, gradually declining over the following 6 months of development. Meanwhile, the content of the polar wax fraction peaked on the 40th day and remained stable throughout the rest of ontogeny. Cuticular transpiration rates also fluctuated during development: dropping progressively During the first 30 days and remaining virtually constant over the subsequent 180 days. Consequently, it was observed that the rate of cuticular transpiration correlates with the presence of nonpolar wax monomers within the cuticular matrix (Hauke, Schreiber, 1998).