SOIL ORGANISMS - Soil Ecology - page 246

3.2.2.1 Short-term mechanisms

Under conditions of moderate water-stress, root elongation may initially be favoured
(Sharp and Davies, 1985) and root production is enhanced (Comeau and Kimmins, 1989).
In this situation, a continuous proliferation of roots into unexplored regions of the soil
profile may help to avoid the effect of rapid water depletion at the soil-root interface
(Newman, 1969). With persistent water stress, short-term resistance results from the
accumulation of amino acids (particularly proline) and soluble reducing sugars ( e.g.,
sucrose, glucose) due to limitations in the allocation of translocated photosynthates to
growth and in the hydrolysis of starch. As a result, osmotic pressure is increased which
helps to prolong turgor and delays wilting (Vartanian, 1977). In roots of Sinapis alba
submitted to water stress, the total carbohydrate contents of the roots did not change
although the ratio soluble sugars:polysaccharides increased up to six times. Similar
observations have been made on Carex spp. (sedges) and Gossypium sp. (cotton) plants
(Hubac, 1978) although, after some time, the accumulation of soluble sugars and amino
acids may inhibit enzymatic activities and impede root recovery.

3.2.2.2 Medium-term mechanisms

When transpiration starts to decline due to water stress, some plants develop short
tuberous roots which neither grow in length nor produce root hairs. Cortical cells swell
and accumulate starch reserves. When moisture conditions again become favourable,
these roots extend rapidly and produce active root hairs (Sabatier and Vartanian, 1983;
Vartanian and Chauveau, 1986).

3.2.3 EXUDATION AND RHIZODEPOSITION

3.2.3.1 Definition

Part of the carbon fixed in the leaves as photosynthates and translocated to the roots via
the phloem is transferred to the soil as a mixture of soluble and insoluble substances,
together with sloughed cells, through the process of rhizodeposition. Rhizodeposition is
important because of the huge flux of carbon and nutrients involved (up to 10-25 % of
carbon fixed by photosynthesis and 30-40 % of the photosynthates translocated to the
roots). Through this process, a wide variety of chemical compounds is added to the soil
and this is of critical importance to the organisms close to the roots.
Rhizodeposition gives the soil-root interface its characteristically high metabolic
activity and roots are therefore the major regulators of biological activities and nutrient
cycling in their sphere of immediate influence, the 'rhizosphere' (see Chapter IV.3).
Several constituents may be distinguished (Rovira et al., 1979; Hale et al., 1981) fig III.25:
1. Exudates sensu stricto are water-soluble, low molecular weight compounds leached
from the roots without metabolic control by the plant;
2. Secretions include compounds of low molecular weight released by metabolic
processes;
3. Plant mucilages are insoluble organic compounds of four different origins: