HETEROPHYLLY IN PLANTS - O.N. NEDUKHA - 2011

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

1.8. ULTRASTRUCTURAL FEATURES OF LEAF AND ROOT CELLS

The ultrastructure of Cells in submerged and floating leaves of plants characterized by heterophylly is largely similar to that of non-heterophyllous hydrophytes and flooded terrestrial plants. Therefore, it is appropriate to first outline the ultrastructural Features of the latter. The functioning of hydrophyte leaves underwater, or of the Organs of flooded terrestrial plants, induces ultrastructural changes primarily in the METABOLISM/14.html">Chloroplasts of photosynthetic submerged and floating organs, as well as in the Cell/35.html">Mitochondria and cell walls of ROOT Tissues (Pomeroy, Andrews, 1979; Sarkar et al., 2008). Numerous studies have focused mainly on the Functional Characteristics of the photosynthetic apparatus in submerged and floating leaves (see the following subsection). Data regarding cell ultrastructure under flooding conditions, however, remain scarce.

Data on the ultrastructural characteristics of leaves and stems subjected to flooding are diverse. Some authors report an absence of changes at the ultrastructural level. No changes were found in the ultrastructure of leaf chloroplasts and mitochondria 4 days after the flooding of Helianthus annuus L. plants, noting only an increase in the starch content within chloroplasts (Wample, Davis, 1983). Meanwhile, other researchers have revealed the significant role of Lipids in the cellular response to anaerobiosis. Specifically, experiments using labeled molecules demonstrated the incorporation of the label into phospho-, glyco-, and neutral lipids of Oryza sativa (rice) stems under anaerobic conditions. As a result, the content of cytoplasmic lipid droplets in the stem parenchymal cells of rice increased significantly (Vartapetian et al., 1978). During flooding, the accumulation of metabolites occurs in the above-ground organs of plants: a 3-4 fold increase in ethanol content is observed in the leaves of winter rye and wheat (Beletskaya, 1977; Crawford, 1977), many grasses (Beard, Martin, 1970), and the leaves of several agricultural crops (Kawase, 1972), which is accompanied by impaired mitochondrial function.

Short-term winter flooding caused certain Changes in the cell ULTRASTRUCTURE OF THE apical meristem in seedlings of wheat Triticum aestivum L. and barley Hordeum vulgare L. In non-flooded seedlings, cells contained a large Nucleus, numerous mitochondria, and ER cisternae. After three days of flooding, against a Background of reduced growth, The formation of concentric structures by The Endoplasmic reticulum was observed; Plastids elongated or assumed an irregular shape, and large loop-like structures formed from the nuclear envelope membranes within The Nucleus. After three weeks of flooding, destructive changes became more pronounced (Pomeroy, Andrews, 1979).

Short-term (up to six hours) waterlogging of Cucurbita pepo (pumpkin) seedlings caused the destruction of mitochondria in coleoptile cells: mitochondria initially swelled, followed by The breakdown of organelle membranes. These Organelles lost their capacity for Oxidative Phosphorylation, while both ATP content and the ATP to ADP ratio increased in the cells. When aeration conditions normalized, readjustment occurred: mitochondrial Structure was restored, and their functioning normalized (Vartapetian et al., 2003).

A characteristic feature of submerged leaves in heterophyllous plants and hydrophyte leaves is the presence of chloroplasts in epidermal cells, which are smaller in size than parenchymal cells. Contact between photosynthetic epidermal cells and the external environment is mediated through the formation of substantial Plasmalemma invaginations in the periplasmic space, similar to those described in Elodea canadensis (Rascio et al., 1991) and Vallisneria spiralis (Rascio, 2002).

Of particular note are studies on the ultrastructure of submerged leaves in Eleocharis retroflexa (Poir.) Urb. and the terrestrial forms of this plant (Ueno et al., 1998). In terrestrial plants, the leaves were upright and exhibited an unusual Kranz anatomy at the Light Microscope level. Bundle sheath cells lacked chloroplasts and structurally resembled parenchymal cells. In submerged leaves, Kranz cells were smaller than those of terrestrial plants, although stomatal dimensions did not differ between submerged and terrestrial forms. In submerged leaves, some epidermal cells contained chloroplasts. Ultrastructural analysis of Kranz cells in terrestrial forms showed that chloroplasts were characterized by well-developed grana, and mitochondria (diameter 0.57 ± 0.12 µm) were larger than those in mesophyll cells (0.35 ± 0.05 µm). Both mesophyll and bundle sheath cells also contained chloroplasts with well-developed grana. A peripheral reticulum was observed in the chloroplasts of both Kranz and mesophyll cells. The density of chloroplasts and other organelles was lower in the cells of submerged leaves. Thus, anatomical and ultrastructural characteristics were similar in the examined cells of terrestrial and submerged leaves of this species (Ueno et al., 1998).

Studies on the leaf ultrastructure of the freshwater hydrophyte Elodea canadensis have shown that chloroplasts feature a well-developed granal system, with typical plastoglobules and small starch grains present in the stroma. Mitochondria possess tubular cristae, granular ER, and small dictyosomes (Esposito et al., 2007).

Chinese researchers conducted a thorough investigation into the formation of sclereids in the mesophyll of floating leaves of Nymphoides coreana H. Lev. & Vaniot and Nuphar schimadai Hayata from lakes on Taiwan Island (Kuo-Huang et al., 2000). Sclereids are Structural elements of stems and leaves composed of stone cells containing calcium oxalate. Researchers observed the formation of sclereids at the boundary with air spaces and within the air lacunae of the spongy mesophyll at the light microscope level. Ultrastructural analysis of sclereids in floating leaves of N. coreana revealed that at the onset of development, their cells possessed a large vacuole and a thin layer of dense Cytoplasm. Structurally, the cells resembled typical parenchymal cells of a young expanding leaf: small plastids with poorly developed lamellar structure, granular ER, numerous dictyosome clusters, and condensed-type mitochondria. In N. schimadai, the ultrastructure of sclereids was similar, except that crystals—evidently composed of calcium oxalate, as they dissolved in Hydrochloric acid—formed on their outer Cell wall. These crystals originated between the primary wall of the parent sclereid cells and their cytoplasmic membrane. Under the Electron microscope, these sclereid crystals appeared electron-lucent, eventually rupturing The Cell wall and emerging into the intercellular spaces of the mesophyll. Following crystal formation, a thick secondary cell wall was laid down. Thickening proceeded centripetally until the secondary wall completely enveloped the sclereid. Secondary walls eventually became lignified (Kuo-Huang et al., 2000). The mechanism initiating calcium crystallization remains unknown. Similar calcium crystals have been described in other plant species. The authors consider calcium oxalate crystals as nutrient reserves for apoplastic Nutrition. Their presence has been reported in the central vacuoles of Phaseolus cells (Kuo-Huang, Zindler-Frank, 1998) or Lemna minor L. cells (Franceschi, 1987). Sclereids are only occasionally observed in other aquatic plants (Chiang, Huang, 1984).