SOIL ORGANISMS - Soil Ecology - page 247

a) mucilage secreted by Golgi organelles in the root cap cells;
b) hydrolysates of the polysaccharides of the primary cell wall between the
epidermal cells of the primary wall and sloughed root cap cells;
c) mucilage secreted by epidermal cells and root hairs. The development of
a secondary wall of epidermal cells proximal to the root tip stops the production
of secretions and mucilages;
d) mucilage produced by bacterial degradation of dead epidermal cells;
4. Mucigel is the gelatinous material at the surface of roots grown in non-sterile soil.
It includes plant mucilages, bacterial cells and their metabolites, together with colloidal
mineral and organic matter from the soil;
5. Lysates and sloughed cells from the epidermis and cortex.

Hale et al. (1981) and Curl and Truelove (1986) also consider leakages as compounds
of low molecular weight which diffuse into the apoplast (cell walls) and, via the apoplast,
move to the root surface or leak directly from the epidermal or cortical cells. Leakages
include a number of volatile compounds that have a wider sphere of influence on soil
micro-organisms, beyond the immediate environs of the root surface.
The term exudates is often used to designate the mixture of exudates sensu stricto and
secretions because of the difficulty of identifying the origin of these products.

3.2.3.2 Sites and mechanisms of exudation

The meristematic region behind the root cap is an area of active exudation, although it
may also occur along the whole length of the roots of herbaceous species, including parts
covered with root hairs (Figure III.25). Two principal features of the exudation occurring
in the active, non-woody roots of herbaceous and woody plants are (Smith, 1970; Dart,
1971; Greaves and Derbyshire, 1972; Bowles and Northcote, 1974; Hale et al., 1981):
(i) An intense release of sloughed cells and mucilage from the apices of lateral roots;
(ii) A diffuse exudation of soluble compounds all along the root axis, with a maximum
occurring immediately behind the root tip (Pearson and Parkinson, 1961)
Exudation involves both passive leakage and active secretion processes. The photo-
synthates produced in leaves are translocated to the roots via the phloem by a 'source-to-
sink' effect. They accumulate in a soluble pool and are transformed into other compounds.
They may be leached out by passive diffusion ( i.e., exudation sensu stricto ) along a
concentration gradient from the root to the surrounding medium. A pool of soluble
compounds has to be formed in the root prior to any release into the soil. McDougall
(1970) observed that fixed was translocated to roots as sucrose and that after 10 hours,
50 % of the sucrose had been transformed into other molecules (mainly other sugars and
amino acids); 33 % of the appeared as exudates after 24 hours.
Secretion implies the expenditure of metabolic energy to transport molecules across
cell membranes to the soil, occasionally against electrochemical or chemical potential
gradients. Morré et al. (1967) and Burke et al. (1974) have shown that the Golgi appa-
ratus of the outer root-cap cells of wheat and maize secretes polysaccharides which are
transported out of the cells and form viscous droplets at the root tips.