SOIL ORGANISMS - Soil Ecology - page 251

and exudation increases from potentials of 0 to -0.19 MPa, decreases between -0.19 and
-0.55 MPa and increases again below the latter figure. Finally, accidental defoliation
(Smith, 1972) also leads to increased exudation of specific compounds ( i.e ., fructose,
cystine, glutamine, lysine, phenylalanine and tyrosine).
Interactions with soil organisms are a particular feature of root exudation since plants
tend to produce more exudates in the presence of the soil microflora than under axenic
conditions. Heulin et al. (1987), e.g., have observed a 16 to 37 % increase of exudation
following the inoculation of axenic rice seedlings with strains of Azospirillum. The soil
microflora acts as a sink for root exudates and may stimulate production by up to 100 %
(Barber and Martin, 1976; Prikryl and Vancura, 1980). Thus, extrapolating results from
plants grown in axenic solutions to soil conditions may well be misleading.
Exudation decreases following attacks by parasites such as nematodes and by
pathogenic fungi (Jalali and Suryanaryana, 1971; Claudius and Mehrotra, 1973)
3.2.3.4 Quantitative estimates
The quantitative production of root exudates has been measured in a number of
situations. The variety of the methods used (plants grown in axenic nutritive solutions
or the tracking of -labelled photosynthates), the units employed (percentage of
biomass or percentage of photosynthates fixed by the leaves) and plant species
considered (trees or cultivated herbaceous plants) make comparisons difficult.
Overall, this work indicates that a large part of plant production, ca. 10 % of
net production and 33 % of the assimilates translocated to roots may be exuded into
the rhizosphere annually. This is an overall input of several Mg a vitally-
important flux of readily-assimilable resources for the soil organisms (Table III.10).
Insoluble mucilages and cell debris are the main constituents of exudates. Their pro-
duction has been estimated at 80 % of the total carbon released into the soil by wheat roots
(Guckert, 1985). Maize mucilage contains 94.4 % of polysaccharides, including 21.9 %
uronic acids and 3.6 % total protein (Guckert et al., 1975).
The production of sloughed cells is an important component of the release of organic
materials into soils since the functional life of cells from the root cap on the plant may
be as short as 22 to 33 hours (Clowes, 1976). For example, Griffin et al. (1976) have
calculated values of 1.5 mg dry root per week, i.e., 0.15 % of the root biomass for
groundnuts ( Arachis hypogaea ). In some cases, the production of cell debris may exceed
that of secretions (Holden, 1975; Martin, 1977).