There are many varieties of rice and they
differ in their water requirements. Most of
the rice in south-east Asia is grown in
unusual conditions for a cereal plant. It is
grown partly submerged in water in paddy
fields.
The fields are flooded and then ploughed. Young rice plants are planted in the
rich mud formed in these paddy fields. The oxygen concentration of this mud
fails rapidly after the paddy field has been flooded. The top ten centimetres or
so retains some oxygen because it is able to diffuse in but below this depth
anaerobic conditions exist and there is little or no oxygen present.
Rice plants have a number of adaptations which enable them to grow well in
these conditions.
Rice stems contain a large number of air spaces (hollow aerenchyma) running
the length of the stem and into the roots. This allows oxygen (some formed in
the plant from photosynthesis) to penetrate through to the roots which are
submerged in water, supplying oxygen for aerobic respiration. This decreases
the chance that anaerobic respiration will occur.
When fields in which a cereal such as wheat is growing are flooded for any
length of time, the plants die. The oxygen concentration of the waterlogged soil
falls rapidly. The root cells are unable to get the oxygen they need in order to
respire. In these conditions they can carry on respiring without oxygen. This is
called anaerobic respiration and results in ethanol being formed as a waste
product. Unfortunately, this substance is poisonous so a plant can only respire in
this way for a short time before the ethanol concentration builds up and kills it.
Cells in the roots of rice plants have been shown to be extremely tolerant of
ethanol, much more so than cells from the roots of other cereals. They can
therefore respire anaerobically for longer periods. When oxygen levels fall too
low, the young rice plants can respire anaerobically, producing ethanol. Ethanol
is normally toxic to cells, but the root cells of rice have an unusually high
tolerance to it – they have a high concentration of the enzyme alcohol
dehydrogenase in their cells. Adult plant roots are as intolerant of flooding as
any other crop.
There are two advantages of growing rice in paddy fields. Flooding brings about
chemical changes in the soil which increases the supply of soil nutrients required
by the rice plants. It also reduces weeds. Rice does not grow well when it has to
compete with weeds for the resources that it needs.