Roots aerenchyma

It has been postulated that CH4 in the gaseous form or dissolved in water enters into root aerenchyma, which forms by degeneration of cortical cells between the exodermis and the vascular bundle, where the dissolved CH4 is gasified and moves by diffusion from the root aerenchyma through the restrictive transition zone into the aerenchyma of the culm and then... 

Smits, A. J. M., Laan, P., Thier, R. H., and van der Velde, G. (1990). Root aerenchyma, oxygen leakage patterns and alcoholic fermentation ability of the roots of some nymphaeid and isoetid macrophytes in relation to the sediment type of their habitat. Aquatic. Botany. 38, 3 — 17. 

Lu Y, Wassmann R, Neue HU, Huang C. Impact of phosphorus supply on root exudation, aerenchyma formation and methane emission of rice plants. Biogeochemistry. 1999 47 203-218. 

Figure 6.2 Cross-sections of primary rice roots, (a) Radial section close to tip showing interceUnlar spaces (I), central cylinder (CC), and rhizodermis (RH). (b) and (c) Radial sections of yonnger (39 days) and older (72 days) basal parts showing exodermis (E), schlerenchymatons cylinder (SC), parenchymatons or cortical cells (P) and aerenchyma (AE). (d) and (e) Axial sections of matnre root (72 days) showing break through of lateral roots (Butterbach-Bahl et al., 2000). Reproduced by permission of verlag...

Transport of gases through the aerenchyma may occur by diffusion and, where pressure gradients develop, by convection. Pressurized flow is important in wetland plants with root systems permitting a throughflow of gases, but is insignificant in other plants (Beckett et al., 1988 Skelton and Alloway,... 

Figure 6.3 Aerenchyma development and changes in respiration rate along the length of maize roots grown in anoxic media (adapted from Armstrong et al., 1991a). Reproduced by permission of Backhuys publishers...

The extent of aerenchyma development by the degradation of the primary root cortex. 

Rates of respiration in different root tissues. The formation of aerenchyma decreases the respiratory O2 demand per unit root volume because there is less respiring root tissue. Also, some plants can tolerate a degree of anoxia in parts of the root, which substantially reduces the O2 demand per unit root volume. 

Because very large concentrations of dissolved CO2 develop in submerged soil, in spite of root respiration the CO2 pressure outside the root may be greater than that inside it, resulting in a flow of CO2 from the soil to the atmosphere through the aerenchyma. Net removal of CO2 by the root decreases the concentration of the acid H2CO3 near the root, and this may offset the acidity produced in oxidation and excess cation uptake. 

Plants have adapted to the harsh anaerobic conditions of wetland soils. Development of aerenchyma tissnes permit oxygen pumping to the roots, to support root respiration and aerobic bacteria in the root zone. 

Methane and carbon dioxide produced in soils are transported into the atmosphere by diffusion and mass flow via two pathways (1) the aerenchyma tissues of plant roots and stems and (2) flux from soil to the overlying water column (Figure 5.61). Gas exchange in plants is discussed in detail in Chapter 7. Carbon dioxide is highly soluble and undergoes various chemical reactions, and it may be difficult to estimate flux accurately without considering aU associated reactions. Because of the potency (on molecule-to-molecule basis, methane absorbs 25 times as much infrared radiation as carbon dioxide) of methane as greenhouse gas, we will focus our discussion on methane emissions from wetlands. 

Ninety-five percent of the methane produced in the anaerobic zone is either taken up by aerenchyma of plants and released as gas or oxidized back to carbon dioxide in the water column and aerobic root zone. 

Functions of aerenchyma in wetland plants and oxidized root zone are as follows ... 

Acceleration of airspace formation is attributed to production of ethylene and increased cellu-lase activity in the tissue (Kawase, 1981). The sequential processes in aerenchyma development are presented by McLeod et al. (1987). They suggest that flooding first results in soil oxygen depletion, followed by depletion of root oxygen. This results in ACC (1-aminocyclopropane-l-carboxylic acid) production that requires ATP. Ethylene is produced from ACC, and this process requires oxygen and is sensitive to temperature. Ethylene produced accelerates cellulase activity that softens tissue, resulting in the formation of aerenchyma tissue. 

Increase in aerenchyma formation that parallels the increase in porosity in response to reducing soil conditions may not be sufficient to satisfy respiratory needs of roots for oxygen. Such conditions impact nutrient uptake and carbon assimilation in wetland plants including morphological, anatomical, and metabolic characteristics. 

Wetland plants growing in flooded soil possess well-developed aerenchyma (airspace tissue) systems that act as conduits for the diffusion of oxygen from the atmosphere through the plant leaves and stems to the roots. The root rhizosphere contains an oxidized area where reduced sulfide can be oxidized to snlfafe or elemenfal snlfnr (Fignre 11.20). Sulfide can be oxidized either chemically or microbiologically to elemental snlfnr. Beggiatoa organisms that oxidize H2S to elemental sulfur are abundant in the root zone. 

The low solubility of methane in water limits its diffusive transport in the flooded soil, and most methane is oxidized to carbon dioxide. The aerenchyma of plants mediates the transport of air (oxygen) to the roots and methane from the anaerobic soil to the atmosphere. The flux of gases in the aerenchyma depends on concentration and total pressure gradients and internal structure, including openings of the aerenchyma (see Chapter 7 for details). 

Approximately 90% of the methane transport from soils to the atmosphere in rice paddies and freshwater marshes is through aerenchyma portion of roots and stems of the plants. Gases are transported according to their concentration gradient, not only for CH4 but also for N2O (Yu et al.,... 

Jackson, M. B., T. M. Penning, and W. Jenkins. 1985. Aerenchyma (gas space) formation in adventitious root of rice (Oryza sativa L.) is not controlled by ethylene or small partial pressure of oxygen. J. Exp. Bot. 36 1566-1570. 

Konings, H. and H. Lambers. 1991. Respiratory metabolism oxygen transport and the induction of aerenchyma in roots. In M. B. Jackson, D. D. Davies, and H. Lambers (eds.) Plant Life under Oxygen Deprivation. SPB Academic Publishing, The Hague, The Netherlands, pp. 247-265. 

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