Agriculture Reference
In-Depth Information
containing earthworms to pots containing no earthworms and concluded that PGI substances were
released into the soil by all three species, but the authors did not speculate further on the nature of
these substances.
Earthworms may liberate PGRs or PGIs themselves (Atlavinyte and Daciulyte 1969; El Harti
et al. 2001a,b), or their production may be mediated by interactions with microorganisms in the
drilosphere in a process that is not fully understood.
It is clear that microorganisms are capable of producing PGR and PGI substances such as
hormones, auxins, gibberellins, cytokinins, ethylene, and abscisic acid (Arshad and Frankenberger
1993; Frankenberger and Arshad 1995). Many microorganisms commonly found in the rhizosphere
can produce PGR substances.
Krishnamoorthy and Vajranabhaiah (1986) showed, in field experiments involving large earth-
worm populations, that seven species of earthworms could promote the production of cytokinins
and auxins in soils. They also demonstrated significant positive correlations (
r
= 0.97) between
earthworm populations and the levels of cytokinins and auxins present in ten different field soils
and concluded that earthworm activity was linked strongly with PGR production. They reported
that auxins and cytokinins produced through earthworm activity could persist in soils for up to 10
weeks although degraded in a few days if exposed to sunlight. For a more in-depth discussion of
the role of earthworms in producing PGR substances through promoting populations and activity
of microorganisms, see
Chapter 18
this volume.
4. R
OOT
A
BRASION
AND
I
NGESTION
OF
L
IVING
P
LANT
P
ARTS
BY
E
ARTHWORMS
Because earthworms burrow and cast near or within the rhizosphere, the soil disturbance and
abrasion may affect plant roots negatively, particularly the small, fine roots or the root tips, which
have not yet produced a protective cortex and are more susceptible to physical disturbance. This
abrasion may also break up the mycorrhizal hyphal network (mechanism 1), decreasing root
colonization and the many potential benefits of these fungi to plants.
Several authors have reported damage by earthworms to rice crops in Southeast Asia (Stephen-
son 1930; Otanes and Sison 1947; Chen and Liu 1963; Inoue and Kondo 1962, cited in Lee 1985;
Pradhan 1986; Barrion and Litsinger 1996), which may be caused by root abrasion if the earthworm
population is large, although other factors such as excessive casting on the rice tillers, soil loosening,
water drainage, and increased water turbidity have been proposed as the main factors responsible
for the damage (Kale et al. 1989; Stevens and Warren 2000).
Some authors have proposed that earthworms (mainly lumbricid species) can feed on living
plant roots (Stephenson 1930; Carpenter 1985; Baylis et al. 1986; Sackville-Hamilton and Cherret
1991; Cortez and Bouch 1992; Gunn and Cherrett 1993; Hameed et al. 1993), although only in
a few instances was this associated with decreased plant productivity. This phenomenon does not
seem to be widespread because studies on the crop, gizzard, or gut contents of over 30 earthworm
species revealed that roots form a very minor component of the ingested materials in most species
(see
Brown
et al. 1999). The extent of root feeding by earthworms, the identification of the species
involved, the conditions encouraging this to happen, and its possible damage to plant productivity
still need further evaluation.
Other negative effects, probably mostly caused by anecic earthworm species, involve the burial of
living plant leaves (Darwin 1881; Zicsi 1954) or damage to germinating seedlings (Walton 1928; Olson
1929; Trifonov 1957; Patel and Patel 1959; Lee 1985; Shumway and Koide 1994). For instance, Darwin
(1881) noted that the end of a
Triticum repens
leaf, still attached to the plant, had been pulled into the
burrow of an anecic earthworm species and had dried and turned dark brown; although the rest of the
leaf remained fresh and green. He attributed this to the fluids secreted by the earthworm mouth, which
rapidly stained the plant tissues, causing cortical cell discoloration and disintegration. Edwards and
Bohlen (1996) reported that
L. terrestris
destroyed a large part of a lettuce crop when soil containing
large numbers of the earthworms was taken into a greenhouse.
