|Descripción||Waterlogging and submergence<br><br>The number of floods has increased in recent decades (Figure 18.1), and the severity of floods is expected to increase further in many regions of the world. floodplains, wetlands, salt marshes). A spectacular example of an important natural ecosystem shaped by flooding is the Amazon Floodplain forests, in which seasonal floods are deep and prolonged (Figure 18.2).<br><br>Tolerance of plants to soil waterlogging, and to shoot submergence, varies greatly; ranging from many very sensitive 'dryland' species (including most of our crops) to highly tolerant species such as rice and other wetland species. In addition, aquatic and marine plant species have adopted submerged lifestyles under water. Knowledge of tolerance mechanisms will underpin future breeding of more robust crops, and understanding plant responses to flooding will aid management of plant communities in flood prone environments. Recent breakthroughs in submergence tolerance research on rice resulting in new varieties will help sustain a growing world population (see Case Study 1), and has improved knowledge of plant adaptive mechanisms to flooding stress.<br><br>This chapter summarises the adverse conditions faced by plants when water is in excess, and acclimations and adaptations to flooding stress. We consider the situations for roots in waterlogged soils, and for shoots submerged by overland floods. Four case studies highlight important developments in plant flooding research. The chapter demonstrates that interdisciplinary research in plant sciences has improved knowledge of plant flooding tolerance, with applications in crop breeding.<br><br>In drained soils, diffusion in the gas phase of the <a href="http://www.ismybag.nl/" target="_blank" style="text-decoration: none;">hermes birkin buy fake</a> bulk soil sustains the O2 supply needed for roots to respire at optimal rates. Soil flooding impedes O2 movement into soils, and so roots experience hypoxia (sub optimal O2) and anoxia (absence of O2). O2 is the terminal electron acceptor of mitochondrial electron transport, so anoxia inhibits respiration and the resulting energy deficit has major implications for roots. In addition, decreases in soil redox potential result in significant changes to the soil elemental profile. The sequence of events following soil flooding are listed in Table 18.1, which also shows the rates of change measured in two soil types differing in temperature and organic matter contents. The impeded gas exchange during soil waterlogging leads to root hypoxia or anoxia, high CO2 in the root zone, and phytotoxins in reduced soils, all with consequences for root metabolism, nutrient acquisition, and growth of roots and shoots.<br><br>As soon as O2 is depleted, NO3 is used by some soil microorganisms as an alternative electron acceptor in their respiration; NO3 is reduced to NH4+, so it becomes the main form of mineral nitrogen in waterlogged soils. In the rhizosphere of roots with radial O2 loss (ROL), however, NH4+ can be converted back to NO3 , with both these forms of mineral nitrogen absorbed by roots. Manganese oxides are the next electron acceptors used by anaerobic microorganisms, followed by iron oxides resulting, respectively, in elevated concentrations of Mn2+ and Fe2+ in the soil solution; these soluble forms often increase to levels that are toxic to plants. Further decrease in the redox potential results in the reduction of SO42 to H2S, which is also potentially toxic. In addition to these inorganic phytotoxins (Fe2+, Mn2+, or H2S), various short chain fatty acids <a href="http://www.ismybag.nl/category/cheap-hermes-bags/" target="_blank" style="text-decoration: none;">hermes jpg birkin copy</a> can also accumulate in waterlogged soils.<br><br>High concentrations of both Mn2+ and Fe2+ are considered to be major constraints for growing sensitive cultivars of wheat in waterlogging prone areas <a href="http://www.ismybag.nl/" target="_blank" style="text-decoration: none;">herm??s birkin bag fake</a> of Australia (Khabaz Saberi et al. 2010); these elemental toxicities also limit rice yields in many flooded areas around the globe. acetic acid, butyric acid, propionic acid) produced as a result of anaerobic metabolism by microorganisms in waterlogged soils. The types and amounts of these organic compounds depends upon the fermentative character of the microorganisms, the organic matter in the soil, and on soil conditions such as pH and temperature. activity of various membrane transporters, membrane permeability) and, ultimately, shoot growth (Shabala 2011).<br><br>Of particular interest is the finding that the function of root plasma membrane transporters may be affected by these phytotoxins or secondary metabolites in waterlogged soils. Transporters located at the root rhizosphere interface would be exposed to these toxins in waterlogged soils. Ion flux kinetics for plant roots changed rapidly upon exposure to secondary metabolites (Pang et al. 2007), and uptake of phytotoxins per se may be mediated by membrane transporters. Whether wetland plant roots, as compared with waterlogging sensitive crops, posses membrane transporters more resistant to these toxins is an important question for future research, with possible implications for improving waterlogging tolerance in crops.<br><br>Abdelbagi M. Ismail, International Rice Research Institute, The Philippines<br><br>The genus Oryza constitutes about 24 species, 20 of which <a href="http://www.ismybag.nl/category/cheap-hermes-bags/" target="_blank" style="text-decoration: none;">birkin hermes bag faux</a> are wild and only O. sativa and O. glaberrima are cultivated. O. sativa is grown worldwide, whereas O. glaberrima is restricted mostly to West Africa. Ecological and geographic distribution of these species is largely determined by temperature and water availability. O. sativa is cultivated on about 144 million ha worldwide, from 50 N in North China to 35 S in Australia (New South Wales) and in Argentina. It is also grown from 3 m below sea level in Kerala, India, to as high as 3000 m in Nepal and Bhutan. Two broad categories are generally identified within O. sativa, with some overlaps; japonica varieties are mostly grown in temperate regions, while indica varieties are grown in tropical and subtropical areas. A third category tropical japonica is mostly grown in the uplands of the tropics and subtropics. Japonica types are known for their better tolerance of low temperatures compared with indica types, and japonica types also have shorter, thicker grains that are softer and stickier when cooked.<br><br>Rice is grown on a variety of soils, but the physical ability of the soil to hold water is an important property, so medium and heavier textured soils are typically favoured over light textured sandy soils. Various types of models were used to classify rice types based on field ecologies. The most widely used classification distinguishes four broad categories; upland, irrigated lowland, rainfed lowland and flood prone ecosystems (Maclean et al., 2002). Characters of varieties suitable for each ecology are mostly determined by local hydrology, and in some cases multiple systems co exist based on the toposequence (Figure 2).<br><br>Upland rice is grown in aerobic unbunded soils with topographies ranging from undulating and steep sloping lands with high runoff, to low laying valleys and well drained flat lands. Soils vary considerably in texture, fertility and water holding capacity; from poor highly leached soils of West Africa, to fertile soils in Southeast Asia. About 13% of the world rice is grown in uplands, but with low yields of about 1 t ha 1, and farmers are among the poorest. Upland varieties are mostly short maturing, with deeper roots (drought avoidance) and with higher tolerance of acid soils.<br><br>Irrigated ecosystem is the largest rice production system, covering 55% of the world rice area and producing over 75% of world rice grains. Fields have assured water supply and rice is grown in puddled soil in bunded fields with water depths of 2.5 10 cm through most of the season, and with 1 3 crops per year depending on location and farming systems. Dwarf high yielding varieties that are responsive to high use of fertilizers are predominant, and yields are usually high, averaging over 5 t ha 1.<br><br>Rainfed lowlands constitute about one quarter of rice world lands and contribute about 18% of rice production. These areas are generally densely populated with poor communities, and are prone to both drought and submergence because of lack of water control, besides adverse soils, inhibiting adoption of high yielding varieties and use of high cost fertilizer inputs. Local landraces with yields of less than 2 t ha 1 still dominate in most areas; however, new high yielding varieties tolerant of prevailing abiotic factors are becoming available over recent years and are gradually replacing existing local landraces.<br><br>Flood prone rice ecosystems are subjected to uncontrolled floods, ranging from transient flash floods causing complete submergence, to longer term floods of 0.5 m to over 4.0 m for most of the season, and sometimes associated with excess salinity, acid sulfates and drought. Over 15 million ha in South and Southeast Asia are annually affected by uncontrolled floods. Yields are low, averaging 1.5 t ha 1, and yet these areas support over 100 million people. Traditional varieties still dominate because they are better adapted to water fluctuations than modern varieties. Recently, varieties that tolerate complete submergence are becoming available through the incorporation of the SUB1A gene (see also main text). These varieties tolerate 1 2 weeks of complete submergence and considerable yield benefits have been achieved in farmers' fields, with yield advantages of 1 to over 3 t ha 1 (Mackill et al., 2012).<br><br>The extreme diversity in adaptation to various ecological and hydraulic conditions made rice one of the most widely grown cereal crops worldwide; and an interesting model for crop improvement research. Currently, rice is the most important food crop in developing word and the stable food for over half of the world population.<br><br>Further reading on this topic:<br><br>Maclean JL, Dawe DC, Hardy B, Hettel GP (2002) Rice Almanac. Adv Agron 115: 303 356.<br><br>Gas exchange of submerged plants is greatly impeded by their environment. Although the distances across diffusive boundary layers (DBL) around leaves are the same order of magnitude in water and air, the 10,000 times slower diffusion in water results in the high resistance to gas exchange across the DBL in water. Consequently, submerged aquatic plants have developed adaptive features of their leaves that reduce the DBL, to facilitate exchange of O2 and CO2 with the surrounding water (Table 18.3).<br><br>Other leaf features/properties can also differ between terrestrial wetland plants and submerged aquatic plants, such as: venation, lignification, stiffness, surface topography, differences between adaxial and abaxial surfaces, and in the case of some halophytic wetland species presence of salt bladders and glands (Table 18.3).<br><br>In addition to the slow diffusion, the solubility of O2 in water is poor. One litre of air contains approximately 33 fold more O2 than one litre of water at 20 C at sea level (101 kPa). Temperature affects the solubility of O2; the solubility decreases with increasing temperature (Figure 18.3). Imagine a kettle that heats up; the water starts bubbling long before it reaches the boiling point because the solubility of gaseous N2 and O2 steeply decreases as the temperature rises.<br><br>Salinity also affects the solubility of O2 in water. Sea water contains 35 ppt (parts per thousand) salt, which is approximately 550 mM NaCl, and at 20 C contains only 231 mol O2 L 1 as compared to freshwater that holds 290 mol O2 L 1.<br><br>Like for O2, the solubility of CO2 also decreases with increasing temperature and salinity. However, the chemistry of CO2 in water is more complicated than for O2, as CO2 reacts with water in the following pH dependent equilibria (Figure 18.3 and its caption). CO2 reacts with water (H2O) and forms carbonic acid (H2CO3). However, H2CO3 dissociates immediately into a proton (H+) and bicarbonate (HCO3 ) so the dissolution of CO2 into water causes pH to drop. At high pH, HCO3 can further dissociate into a second H+ and carbonate (CO32 ). The sum of CO2, (H2CO3), HCO3 and CO32 is referred to as dissolved inorganic carbon (DIC) and the relative distribution of the three main forms of inorganic carbon is determined by pH (Figure 18.4).<ul>
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