Adaptations to Anoxia

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, 

Pressurized flow could in principle occur in a non-throughflow root system, such as that of rice, driven by dissolution of respiratory CO2 produced from gaseous O2. However, Beckett et al. (1988) have shown that convection by this means will always be subordinate to diffusion in non-throughflow systems and will only ever have a minor effect. Hence diffusion is the principle means of gas transport. 

The effectiveness of the internal O2 transport by diffusion or convection depends on the physical resistance to movement and on the O2 demand. The physical resistance is a function of the cross-sectional area for transport, the tortuosity of the pore space, and the path length. The O2 demand is a function of rates of respiration in root tissues and rates of loss of O2 to the soil where it is consumed in chemical and microbial reactions. The O2 budget of the root therefore depends on the simultaneous operation of several linked processes and these have been analysed by mathematical modelling (reviewed by Armstrong 

To summarize, the main factors influencing the O2 budget of a non-throughflow 

Eor simplicity, the effects of lateral roots are not dealt with explicitly in Armstrong and Beckett s model, but they are dealt with in Section 6.2. 

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Taiz and Zeiger (2002) give a full account of this topic. Mineral ions absorbed from solution outside the root surface must be transported across the root to the main long-distance transport vessels in the xylem, through which they reach the shoot. This process is highly specific for different ions and molecules and is closely regulated. The regulation is in part a fnnction of the anatomy of the varions root tissues and in part a fnnction of active transport processes in root cells. The pathways and transport processes are affected by root adaptations to anoxia. 

Mendelssohn, I.A., McKee, K.L., and Patrick, W.H. (1981) Oxygen deficiency in Spartina altemiflora roots metabolic adaptation to anoxia. Science 439-441. 

Molecular oxygen regulates both aerobic and anaerobic microbial metabolisms (Bodelier, 2003). Because of intense competition for O2, aerobic rhizosphere bacteria may be adapted to low O2 concentrations and periods of anoxia. Competition for O2 among microbial species, and between biotic and abiotic processes, has received relatively little attention (Laanbroek, 1990) but is likely to influence rhizosphere oxidation rates. Aerobic processes that occur in the wetland rhizosphere are those found in other aerobic-anaerobic interface environments and... 

Buck LT, Pamenter ME. 2006. Adaptive responses of vertebrate neurons to anoxia—Matching supply to demand. Respir Physiol Neurobiol 154 226-240. 

Bickler, P.E., and L.T. Buck (1998). Adaptation of vertebrate neurons to hypoxia and anoxia maintaining critical Ca++ concentrations. J. Exp. Biol. 201 1141-1152. 

Bidder PE, Buck LT. 1998. Adaptations of vertebrate neurons to hypoxia and anoxia Maintaining critical Ca " concentrations. J Exp Biol 201 1141-1152. 

Figure 6. Variations on phosphoenolpyruvate (PEP) and pyruvate (PYR) metabolism in animals. In mammalian liver, pyruvate kinase (PK) and PEP carboxykinase (PEPCK) function in opposite directions to support glycolysis versus gluconeogenesis. In anoxia-tolerant mollusks, PEP is routed via PK when oxygen is present and via PEPCK in anoxia. Note that PEPCK is adapted for physiological function in opposite directions in the two situations.

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