Aerenchyma Formation

FIGURE 7.9 Micrographs of gas spaces in emergent plants. (Courtesy Hans Brix, University of Aarhus, Denmark.) 

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

Serves as pathway for oxygen diffusion from atmosphere to roots Provides oxygen for aerobic respiration in root Enhances oxidation of rhizosphere around the root Buffers plant against soil toxins (ferrous iron and sulfide) 

Provides favorable habitat for microorganisms Reduces living cells, and hence reduces oxygen demand of root 

Serves as conduit for gaseous or volatile metabolites to move from the plant roots to the atmosphere (e.g., ethanol, acetaldehyde, methane, and carbon dioxide) 

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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. 

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. 

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. 

Jackson MB, Armstrong W. 1999. Formation of aerenchyma and the processes of plant ventilation in relation to soil flooding and submergence. Plant Biology 1 274-287. 

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. 

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. 

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