Agriculture Reference
In-Depth Information
Tillage can directly induce inappropriate germination by scarification of
hard seeds and by providing a light flash. However, inappropriate germina-
tion due to scarification of hard-seeded species like Abutilon theophrasti and
Convolvulus arvensis during tillage is probably a small source of mortality since
(i) usually only a fraction of seeds in the seed bank are hard, (ii) most of these
probably escape scarification during any given tillage operation,and (iii) some
of the germinating seeds emerge as seedlings. If a seed of a light-sensitive
species receives a brief flash of light prior to reburial beyond the depth from
which emergence can occur,the seed may be stimulated to germinate inappro-
priately. This is more likely with tillage methods like moldboard plowing or
rototilling that move large amounts of soil vertically.The effect probably kills
a larger proportion of small-seeded than large-seeded species, since most
small-seeded species must be close to the surface for successful emergence,
and most large-seeded species lack a light requirement (see Chapter 2). The
role of both scarification and light flash in tillage-induced seed mortality
requires experimental investigation.
Tillage probably also promotes the inappropriate germination of weed
seeds by the same mechanisms that it stimulates appropriate germination,
namely, by increasing temperatures and the amplitude of temperature fluctu-
ations, modifying the soil atmosphere, and changing the chemistry of the soil
solution,particularly nitrate concentration.However,stimulation of inappro-
priate germination by tillage may be less than stimulation of appropriate ger-
mination, since the magnitude of all of these effects declines with increasing
depth, and seeds that germinate close to the surface often produce emerged
seedlings.The magnitude of inappropriate germination due to changes in the
soil environment induced by tillage needs to be examined.
Seed mortality often varies with depth in the soil (reviewed by Mohler,
1993),and as tillage moves seeds,it exposes them to changes in mortality risk.
However, in most studies to date, the seeds were confined in packets or con-
tainers that excluded most seed predators. Even more important, seedling
emergence could not be observed. Since germination is generally greater for
seeds closer to the surface due to light, warmer soil, and fluctuating tempera-
tures (see Chapter 2), the greater apparent mortality often observed near the
surface may have been due to death of individuals that would have emerged
had they not been restrained. For a few studies (Stoller & Wax, 1973; Dawson
& Bruns, 1975; Froud-Williams, Chancellor & Drennan, 1983; Moss, 1985 b ),
Mohler (1993) computed seed mortality by subtracting the number of emer-
gents from the number of seeds that disappeared. These data indicated that
survival of seeds not producing seedlings decreased with depth as often as it
increased (Mohler, 1993). However, several additional studies on Avena fatua
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