SOIL ORGANISMS - Soil Ecology - page 250

Growth factors may be present together with a range of chemically-undefined substances
that possess demonstrated stimulatory effects on micro-organisms, including plant
pathogens and such parasites as nematodes. However, not all exudates are stimulatory in
nature as evidenced by the presence of hydrocyanic acid and saponins. No relationships
have been observed between total sugar content of root exudates and the degree of VA
mycorrhizal infection of annual plants (Azcon and Ocampo, 1984).
Allelochemicals (principally phenolic acids) have also been identified (see, e.g., Hoagland
and Williams, 1985; Pope et al., 1985). Ions are generally absent from exudates with
the exception of phosphorus which has been found in the exudates of wheat seedlings.
Volatile organic compounds may also be produced. Most compounds found in exudates
have low molecular weights and in axenic cultures of wheat, 98 % of exudates had
molecular weights below 25,000 Da and 90 % of that fraction was below 800. An important
proportion of these compounds is water soluble: 17-25 % for three-week old wheat plants
(Barber and Martin, 1976), ca. 30 % for maize ( Zea mays ) (Guckert and Balandreau,
1981) and 44 % for axenic cultures of oats ( Avena saliva ) (Trofymow et al., 1987).
Most exudates have a high C:N ratio ( ca. 30) due to a predominance of polysaccharides
(80 %) and low protein content (1-5 %) (Lynch, 1982). As excreted into the soil, they
are mixed with substantial amounts of water and this is of great importance in relation
to the reaction of the rhizosphere communities.
Factors influencing the composition of exudates
The composition of exudates varies depending on the species, age of the plant and
general environmental conditions. Exudate production is sensitive to a variety of
external factors such as light intensity, temperature, aeration, nutrient availability and
soil characteristics (Lespinat et al., 1975). Defoliation, as with any adverse effect on
plant metabolism (pesticides, insect attacks), may also affect exudation.
Qualitative and quantitative variations
Seeds also produce exudates that differ from those produced by seedlings. Seedlings,
in turn, often produce quite different exudates than the unsuberised root-tips of mature
plants; exudates from the unsuberised root tips of mature sugar maple ( Acer saccharum )
trees had less abundant and diversified carbohydrates than seedlings. Nonetheless, mature
trees produce more abundant and diversified amino acids than seedlings, and six times
greater amounts of organic acids (Smith, 1970). These results indicate the difficulties in
extrapolating results from seedlings to mature plants.
Lighting, through its control of photosynthesis, is logically a major factor in the amount
of root exudates produced and Rovira (1959) observed that shading significantly
modified the composition of tomato root exudates. Exudation from maize plants still
occurs during the night although certain pigments are only produced during light
periods (Richter et al., 1968).
Stress generally leads to increased exudate production (Lespinat et al., 1975).
Low temperatures (Vancura, 1967), high carbon dioxide concentrations in the
rhizosphere (Rittenhouse and Hale, 1971), nutrient deficiencies (Shay and Hale, 1973)
and the alternation of dry and moist periods have all been shown to increase exudation
(Vancura and Garcia, 1969). However, the response to progressive drying is not simple