Abiotic and biotic stresses and changes in the lignin ... - ResearchGate

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12 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 UV-B (280-320nm) In recent years, there has been increased interest in the biological effects of UV radiation on living organisms, in part due to the reduction of the atmospheric ozone layer caused by anthropogenic actions, leading to an increase in UV rays reaching the Earth’s surface. Plants and animals both undergo morphological, anatomical, physiological and biochemical changes following exposure to UV rays. Among the changes caused by UV-B radiation are inhibition of growth, increased thickness of leaf blades, reduced biomass, damage to the photosynthesis apparatus, altered membrane composition and modifications to the balance of phytohormones (Zagoskina et al., 2003). Structural damage to the DNA is also observed, reflected in the formation of cyclobutane pyrimidine dimers (CDPs) and pyrimidine-(6-4)-pyrimidone (Yamasaki et al., 2007). Thus, to protect themselves from UV damage, plants produce mechanical barriers, such as thicker cuticles and trichomes; they also synthesize protective substances, such as secondary compounds (Yamasaki et al., 2007). UV-B affects the production of several secondary compounds such as flavonoids, tannins and lignin, which are thought to have played an important role in the transition of aquatic plants to land, when they became more exposed to UV-B radiation (Rozema et al., 1997). UV-B radiation affected the morphogenesis of trichomes in the cotyledons of Cucumis sativus L. and induced the accumulation of lignin in these structures (Yamasaki et al., 2007). In a study with two types of Camellia sinensis L. cell cultures that differed in their capacity to biosynthesize the phenolic compounds ChS1 and ChS- 2, the latter had greater capacity for production of phenolic compounds (Zagoskina et al., 2003). It was observed that under exposure to UV-B radiation, ChS2 demonstrated
13 274 275 276 277 278 279 280 281 282 283 284 greater accumulation of lignin in the cell walls of the parenchyma of tracheid elements and in the intercellular space, in addition to the formation of a layer of lignin-likematerial on the surface of the callus. The ChS2 lineage was also more resistant to UV- B, suggesting the involvement of lignin in cell protection against this type of stress. Cotyledons of Chenopodium quinoa exposed to UV-B radiation exhibited increased epidermal cell wall thickness; this was associated with a large accumulation of lignin due to increased peroxidase activity (Hilal et al., 2004). Additionally, changes in the ultrastructure of chloroplasts rendered them similar to the chloroplasts of shaded plants. It was suggested that such alterations in the chloroplast were caused by reduced light penetration in the cells of the mesophyll, due to the protective effect of lignin deposition. Therefore, lignin in epidermal tissue may attenuate radiation. 285 286 287 288 289 290 291 292 293 294 295 296 297 298 Mechanical injuries Injuries induce specific reactions in the wood which can be restricted to the bark or might extend to the cambium and xylem (Frankenstein et al., 2006). Several studies, if not most of them, have focused on characterizing histological and anatomical wood changes produced by a particular type of injury, and the obtained information is then used to explain the roles of these changes in adaptation and resistance to stress. In general, attention has been paid to the changes occurring after insect attack or after microorganism infection. Some species have defence mechanisms that range from physical barriers including cuticle formation, lignification, spines and trichomes to the biosynthesis of toxic compounds such as alkaloids and tannins (Delessert et al., 2004). Electron microscopy has demonstrated that injuries to the stem of Populus spp. induce the thickening of the xylem fibre cell wall in three ways: synthesis of an additional S2 layer, thickening of the existing cell wall or synthesis of a sclereid-like
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