Processes in Roots and the Rhizosphere - The Biogeochemistry of Submerged Soils - page 163

6 Processes in Roots and the
Rhizosphere

Though wetland plants have the advantage of an assured water supply, they must
contend with various difficulties that their dryland counterparts are largely spared.
First, because of the very slow diffusion of respiratory gases through water and
submerged soil compared with dryland soil, the roots must aerate themselves
internally by forming internal gas channels to the air above. Second, the roots
must exclude toxic products of anaerobic metabolism in the soil, such as organic
acids and ferrous iron, or tolerate large concentrations of these toxins internally.
Third, they must contend with the altered forms and solubilities of nutrients in
the soil under anaerobic conditions, for example the predominance of ammonium
rather than nitrate as the plant-available form of nitrogen. That wetland plants
are capable of surmounting these difficulties is shown by the great productivity
and biodiversity of wetland systems. This chapter discusses the various processes
and mechanisms involved in this.

6.1 EFFECTS OF ANOXIA AND ANAEROBICITY ON PLANT ROOTS

Generally, in plant cells well supplied with O 2 , energy is provided for growth and
metabolism by the oxidation of glucose in the three stages shown in Figure 6.1:

(1) glycolysis, in which 1mol of glucose is converted to 2mol of pyruvate yield-
ing 2mol of ATP (the main form in which energy is transported and utilized
in plants) and 2mol of NADH 2 (reduced NAD which acts as a universal
reducing agent in non-green plant tissues);
(2) the Krebs cycle, in which 1mol of pyruvate is completely oxidized to CO 2
yielding 1mol of ATP and 5mol of NADH 2 ;and
(3) the mitochondrial cytochrome chain, in which 1mol of NADH 2 generates
3mol of ATP.

The net result is that complete aerobic respiration of 1mol of glucose yields
38mol of ATP.
However in the absence of O 2 , anaerobic glycolysis—fermentation—produces
only 2mol of ATP per mol of glucose consumed. In the absence of O 2 the mito-
chondrial cytochrome chain ceases to operate and as a result NADH 2 accumulates