Oxygen Deficiency Anaerobiosis and Hypoxia

Nelumbo nucifera, the lotus flower, is a typical swamp plant in the monsoon climate of northern Australia. The rhizomes grow in the oxygen-deficient mud and are supplied with oxygen via an aerenchyma. The flow of air in the aer-enchyma is driven by thermo-osmosis. Because of crocodiles, researchers in this area should exercise caution. Ka-palgam Northern Territories. Photo E.-D. Schulze

Recommended Literature

• Blom CWPM, Voesenek LACJ (1996) Flooding: the survival strategies of plants. Trends Ecol Evol 11:290-295

• Chang C, Bleeker AB (2004) Ethylene Biology. More than a Gas. Plant Physiol 136:2895-2899 (and subsequent articles in this issue)

• Drew MC (1997) Oxygen deficiency and root metabolism: injury and acclimation under hypoxia and anoxia. Annu Rev Plant Physiol Plant Mol Biol 48:223-250

Common soils consist of four components: soil particles, water, air and organisms including plant roots (Fig. 1.4.1). Freely draining soils can only retain water in pores with diameters smaller than 10-60 (.im. Even at water saturation up to the field capacity (see Chap. 2.2) the air-filled pore volume is 10-30% of the total soil volume. However, in partially or permanently waterlogged soils, there are almost no air-filled pores, as the air dissolves in the water.

Gas exchange in well-aerated soils is mainly through diffusion in the continuum of the air-filled pores, but is accelerated by a number of active processes in the soil and thus becomes a relatively fast process. For example, if oxygen is consumed by the respiratory activity of microorgan isms and plant roots, oxygen from the atmosphere flows quickly into the soil following the concentration gradient. As a result, the partial pressure of 02 in the soil air, at least in pore-rich soils, remains in the range of 15-20%. Similarly, C02 that accumulates in the soil pores quickly leaks out from the soil. The situation is completely different when gas exchange is via the water-filled pores of waterlogged soils. Pick's first law of diffusion describes the amount of gas diffusing per unit of time, i.e. the net gas flux, as depending on the diffusion coefficient D, on the size of the exchange area, and on the concentration gradient. At the same temperature, the diffusion coefficient of oxygen in water is about 10,000 times (exactly 11,300 times) smaller than in air. Oxygen has also a very low solubility in water (0.03 ml 02 l-1 H20). Thus, gas exchange in waterlogged soils is very slow and oxygen becomes one of the limiting factors for growth and the development of plants. Oxygen supply to roots is enhanced by a temperature gradient as well as by water flow in the soil. For roots, the critical oxygen concentration is generally 510%, although some plants are able to grow at lower 02 concentrations, e.g. cotton grass (Erio-phorum angustifolium) roots grow at 2-4% 02 (Armstrong and Gaynard 1976). The apparent tolerance of these plants is easily explained by the large intracellular channels extending from the

Root cortex Rhizodermis

Root hair

Root cortex Rhizodermis

Root hair

Fig. 1.4.1. Four-component system: Root/soil organism, soil particle, soil water (solution), soil air

Air filled soil pore 15-20 %02

Soil particle (clay or humus)

Capillary bound or plant-available water Water shell of soil colloids (chemically bound water)

Fig. 1.4.1. Four-component system: Root/soil organism, soil particle, soil water (solution), soil air

1 At saturated * I water content | (maximum -1 water capacity)

1 At double > saturated J water content increase in 02 consumption by addition of organic matter from

Submerged

30 60

Days

Fig. 1.4.2. Development of redox potential of a loamy clay soil as influenced by the water content and the amount of organic matter. (After Amberger 1988)

1 At saturated * I water content | (maximum -1 water capacity)

1 At double > saturated J water content increase in 02 consumption by addition of organic matter from

Submerged

30 60

Days

Fig. 1.4.2. Development of redox potential of a loamy clay soil as influenced by the water content and the amount of organic matter. (After Amberger 1988)

shoot and leaves into the roots maintaining a sufficiently high oxygen concentration in the root tissue. A similar aeration tissue (termed aerenchy-ma) is found in deep-water rice, the roots of which are able to grow at 0.8% oxygen (Armstrong and Webb 1985). In such cases, a corky exodermis is often produced, forming a gas-tight outer cell layer of the root that renders the escape of gases from the interior of the root into the soil very difficult. If such a diffusion barrier tissue is missing, oxygen leaks out of the aerenchyma to the surrounding soil, where the heavy metal ions in the immediate proximity of roots are oxidised forming "rusty spots and root channels" in pseu-dogley and are, thus, "detoxified" for soil organisms.

Regarding the relationship between oxygen concentration and metabolism, a situation, where biochemical reactions are not limited by partial oxygen pressure, is called normoxia. If mitochondrial ATP synthesis is affected, but not completely inhibited by low 02, it operates under hypoxia. In the absence of oxygen (anoxia), oxidative phosphorylation in the mitochondria is negligible, compared with ATP synthesis by glycolysis and fermentation.

Long-term waterlogged soils have a negative redox potential because of the low oxygen partial pressure (see Box 1.7.1), i.e. they exhibit reducing properties. Oxygen entering such soils (e.g. through root or earth worm channels) is readily consumed by soil organisms.

The redox potential of soils decreases dramatically already after a few days of flooding (Fig. 1.4.2) and microaerophilic and anaerobic microorganisms start to grow. They mainly live on the organic matter of the soil as energy source, but require ions as electron acceptors that can be reduced. If nitrate is used as electron acceptor, giving rise to nitrite, N20 and finally N2 (deni-trification), the process is termed nitrate respiration and, accordingly, in sulfate respiration sulfide is formed from S04~ (see Chap. 3.3.3). Similarly, three-valent iron and four-valent manganese can be reduced to two-valent ions. In addition, C02 may be used as electron acceptor, resulting in the production of methane. Table 1.4.1 shows the sequence of redox reactions occurring in the soil when the redox potential decreases. Such reactions often consume protons, i.e. result in an alkalinisation of the soil.

However, reduced heavy metal ions are toxic. Thus, the growth of roots is not only inhibited by the lack of oxygen, but also by toxic ions in the vicinity of roots (Fig. 1.4.3). This applies particularly to the very sensitive symbiosis of plant roots with mycorrhizal fungi: Plants growing on waterlogged soils are very sensitive to pathogens and rarely form mycorrhizae; therefore, their capability of nutrient acquisition and growth is usually very limited (Table 1.4.2).

Two-thirds of the earth's land mass is flooded, at least occasionally (e.g. the monsoon regions of Southeast Asia or the areas at the lower

| Table 1.4.1. Sequence of soil-bound redox reactions. The redox potential provides important information about the reactions in the soil, as these reactions take place in the sequence listed [i.e. sulfate is not reduced if iron(lll) ions are still present]. (After Marschner 1986)

Redox reaction

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