PLANT HETEROPHYLLY - O.M. NEDUKHA - 2011

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

1.7. LEAF EPIDERMIS IN HETEROPHYLLOUS PLANTS

1.7.5. Surface structure of leaves in heterophyllous plants

1.7.5.2. Sagittaria sagittifolia

Emergent leaves of a single specimen of *Sagittaria sagittifolia* (common arrowhead) at the budding stage exhibited varying morphologies (Fig. 1.3.3, a-c; see subsection 1.3 and insert IV). Each plant sampled for the study bore emergent leaves of two distinct shapes (sagittate and oblong), whereas the submerged leaves were greatly elongated and linear in shape. The number of submerged leaves per plant ranged from five to nine, while emergent leaves varied from three to ten.

Emergent leaves. Sagittate leaves. Ultrastructural Analysis of the upper and lower surfaces of sagittate leaves revealed that this leaf morphotype is amphistomatic (Fig. 1.7.5.2.1, a, c). Common features of both epidermises include the stomatal type (paracytic), stomatal density, stomatal shape (ovate-elongated), the presence of a cuticular ridge along the perimeter of all Cells, and a wax coating on the outer surface of each Cell (Fig. 1.7.5.2.1, b, d). Structural differences between the upper and lower epidermises were observed in the length of the long axis of epidermal cells and the dimensions of wax tubercles. Cells of the lower epidermis were longer compared to the upper ones, whereas the size of wax deposits was smaller on the lower surface (Table 1.7.5.2.1). The wax tubercles were oval or round, with an average size of 1.49 ± 0.13 µm, and the density of the wax coating per cell averaged 30.7 ± 2.4. The stomatal index was 13.6% on the upper leaf surface and 16% on the lower surface.

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Fig. 1.7.5.2.1. Structure OF THE upper (a, b, d, e) and lower (c, d, e, g) surfaces of emergent Leaves of Sagittaria sagittifolia with different lamina shapes: a-d - leaf with a sagittate lamina; e-g - leaf with an oblong lamina.

Wax deposits are indicated by arrows (b, d, f, g).

Oblong leaves. Ultrastructural analysis of the upper and lower surfaces of oblong leaves showed that, similarly to sagittate leaves, they belong to the amphistomatic type (Fig. 1.7.5.2.1, e, f). Stomata on both epidermises are of the paracytic type. On the upper epidermis, they are elongated-oval, usually oriented along the lamina or occasionally arranged haphazardly, with an average stomatal density of 74 ± 3 per 1 mm². The anticlinal wall contours of the fundamental epidermal cells of the upper epidermis are nearly straight, and cell shapes vary from round or oval to pentagonal (Fig. 1.7.5.2, e, g), covered by a prominent cuticular ridge 5.4 ± 0.2 µm in height. The surface of a portion of these epidermal cells (~43%) is covered with oval or circular wax deposits. The stomatal index on the upper surface is 9.3%. Stomata on the lower epidermis are longitudinally oval, with stomatal pores oriented in nearly the same direction. Structural parameters of the cells on the upper and lower epidermal surfaces of oblong leaves are presented in Table 1.7.5.2.1.

TABLE 1.7.5.2.1. Cell Structure parameters of the upper and lower epidermises of emergent and submerged leaves of Sagittaria sagittifolia according to scanning Electron Microscopy data

Note: * P ≤ 0.05 (comparing traits of emergent and submerged leaves)

The contours of the anticlinal walls of the lower epidermal cells are straight, and cell shapes are diverse, ranging from round or oval to pentagonal, much like those of the upper epidermis, and are covered by a prominent cuticular ridge. Up to 24% of the epidermal cell surface is covered with a wax coating, featuring oval or round wax tubercles. Thus, these leaves have a significantly lower number of wax-covered cells compared to sagittate leaves, and the number of wax tubercles is halved relative to the epidermal cells of sagittate leaves. The stomatal index on the lower surface is 10.9%.

Submerged leaves. The structure of the upper and lower epidermal surfaces of submerged common arrowhead leaves differed markedly from that of emergent leaves, notably by the complete absence of stomata on both surfaces. The cells were elongated in shape (Fig. 1.7.5.2.2), with their long-axis dimensions exceeding those of emergent leaves. Cell outlines were framed by a very thin cuticular rim (ridge); the height and width of the cuticular ridge were three times smaller than in emergent leaves (Table 1.7.5.2.1). Furthermore, the wax coating was entirely absent On the surface of submerged leaves. Algal cells were frequently attached to The Cell surfaces.

Fig. 1.7.5.2.2. Structure of the upper (a, b) and lower (c, d) surfaces of submerged Sagittaria sagittifolia leaves.

Stomata are absent from the epidermis; unicellular Algae are attached to the upper and lower epidermises.

Thus, a comparative analysis of the epidermal structure of submerged and emergent arrowhead leaves demonstrated that a wax coating is present on emergent leaves regardless of their lamina shape. However, The amount of wax per cell was greater in sagittate laminae compared to oblong ones. The percentage of cells bearing a wax coating in oblong laminae was lower than in sagittate ones: 2.5 times lower on the upper epidermis and four times lower on the lower epidermis.

The lower wax content in oblong leaves compared to sagittate leaves may be attributed to both exogenous and endogenous factors. This phenomenon is likely an adaptive response aimed at mitigating direct solar radiation on the oblong emergent leaves, given that sagittate leaves project higher above the Water surface than oblong ones.

Additionally, the stomatal index on both epidermal surfaces of emergent oblong leaves was lower than that of emergent sagittate leaves. Consequently, in sagittate leaves, which rise higher above the water surface than oblong leaves, the stomatal index is elevated, resembling the pattern observed in terrestrial plants (Vovk, 1984).

It is well established that epicuticular wax formed on the outer side of epidermal cells restricts Transpiration and reflects ultraviolet light (Kattittukudy, 1996; Kerstiens, 1996). In common arrowhead, wax content was substantially higher in sagittate leaves than in emergent oblong leaves. Considering these literature data alongside our findings, it can be hypothesized that cuticular transpiration is significantly more intensive in oblong leaves—which reside closer to the water surface—than in sagittate leaves. The presence of convex wax deposits on the epidermis of both emergent leaf types evidently AIDS in maintaining water status and regulating the absorption of specific light photon fractions by the wax (Landsberg, 1986).

The mechanism driving the increase in Abundance and density of the wax coating on the surface of sagittate emergent leaves can also be explained by endogenous factors. It is known that wax-enriched epidermal cells, particularly in the leaves and stems of terrestrial plants, and especially in succulent leaves, contain lipid transfer Proteins that participate in the transport and secretion of Phospholipids into the periplasmic space (Clark et al., 1992; Clark, Bohnert 1993; Clark, Bohnert 1999). Recently, genes (*Ltp*, *LTPs*, and *WAX9*) responsible for the METABOLISM/31.html">Transcription of these lipid transfer proteins have been identified. Upon expression of the corresponding genes in epidermal cells, an accumulation of the respective mRNA has been detected (Clark, Bohnert 1993; 1999). Although the precise regulation of wax synthesis within the epidermis remains unclear, we can surmise that differences in wax deposition between emergent arrowhead leaves and its absence in the epidermis of submerged leaves may result from genetic plasticity, leading to the inhibition of wax precursor synthesis (C₁₂-, C₁₄-, and C₁₆-ω-hydroxy Fatty acids) and wax synthase (fatty acyl-coenzyme A: fatty alcohol acyltransferase), whose substrates include integral Membrane Proteins (Hauke, Schreiber, 1998; Lardizabal et al., 2000).