Agriculture Reference
In-Depth Information
for as long as carbohydrate reserves last. However, because the efficiency of this
process is much less than the efficiency of aerobic respiration—only 2mol of
ATP are produced per mol of glucose consumed compared with 38—the rate of
fermentation must increase sharply under anoxia if the cell energy supply is to be
maintained. This can lead to rapid exhaustion of plant reserves under prolonged
anoxia. In addition anoxic cells must contend with toxic products of fermenta-
tion, particularly ethanol and lactate. Hence the plant will go to some lengths to
avoid anoxia in its active tissues.
6.1.1 ADAPTATIONS TO ANOXIA
The most important adaptation plants make to anoxic soil conditions is the devel-
opment of highly porous tissue in the root cortex called aerenchyma (Figure 6.2)
(Jackson and Armstrong, 1999). The development of aerenchyma may occur
both through closely regulated separation and expansion of cells, or, more usu-
ally, through programmed cell death, also under tight regulation in response to
external stimuli. The result is a continuous pathway of gas channels between the
base of the root and the tip. This both permits gas transport between the plant's
aerial parts and respiring root tissues, and lessens the amount of respiring tissue
per unit root volume. In addition, the root wall layers become partially suberized
along part of the root length, resulting in decreased permeability to gases and
hence less loss of O
2
to the anaerobic soil outside.
The mechanisms by which the aerenchyma remains gas-filled rather than water-
filled are not fully understood but appear to involve metabolic control (Raven,
1996). The gas-filled state is favoured by inward gradients of water potential
created by evapo-transpiration and by barriers to water movement in the apoplasm
such as exodermis (van Noordwijk and Brouwer, 1993). Thus the root acts as a
moderately gas-tight pipe conveying O
2
down from the shoots to the elongating
and actively respiring tip and venting CO
2
and other respiratory gases in the
opposite direction. Figure 6.3 shows changes in root porosity and respiration rate
along the length of maize roots grown in anoxic media.
Metabolic adaptations in the root to provide alternative respiratory pathways
are far less important. Where these do occur, they are only of short-term use.
Indeed, in plants that tolerate prolonged soil submergence, root tissues are often
particularly sensitive to anoxia (Vartapetian and Jackson, 1997). Without mor-
phological adaptations and a continuous supply of O
2
to the root tip, survival
is limited. That said, some rice genotypes will survive several days of anoxia
resulting from complete submergence of the plant following flash flooding, which
is a widespread phenomenon in rainfed rice systems. The tolerance appears to
depend on (a) the water being sufficiently clear and with a sufficient dissolved
CO
2
content that the plants can continue to photosynthesize and produce carbo-
hydrates; (b) cessation of growth so as to preserve carbohydrates for maintenance
processes; and (c) increased alcoholic fermentation to maintain glycolysis, NAD
