Agriculture Reference
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whereas seeds that had been cultivated into the soil did not. He hypothesized
that seeds on the soil surface physiologically initiated germination, but died
prior to emergence of the radical. Many Abutilon theophrasti seeds on the soil
surface imbibe and the seed coat breaks, but germination does not proceed
further due to subsequent desiccation (C. L. Mohler, personal observation). In
contrast, most A. theophrasti seeds buried 1 cm deep emerge successfully. Seed
movement by tillage implements and seed survival at different depths in the
soil profile are considered further in Chapter 4.
Aproblem with the above studies and observations is that the species inves-
tigated lack strong tillage-cued germination mechanisms and, with the
exception of K. scoparia , all are relatively large-seeded. Whether the many
small-seeded species that rely on environmental cues to inform them of prox-
imity to the soil surface and lack of competition also suffer large mortality due
to inappropriate germination remains to be determined. The technical prob-
lems of investigating causes of seed loss in small-seeded species with great
longevity in the soil are substantial: few seeds are likely to lose viability or ger-
minate inappropriately in any given time interval, and recovering tiny seed-
lings in the white thread stage is difficult.
Physiological aging of seeds involves loss of membrane integrity,deteriora-
tion of organelles, and accumulation of damage to DNA (Abdalla & Roberts,
1968; Villiers, 1973; Roberts, 1988). These aging processes proceed most
rapidly when seeds are in warm conditions with seed moisture in the 8% to
15% range (Abdalla & Roberts, 1968; Villiers & Edgcumbe, 1975). In contrast,
fully imbibed dormant seeds are metabolically active and apparently capable
of repairing structural and genetic damage (Villiers & Edgcumbe, 1975; Elder
& Osborne, 1993). These observations probably explain why mortality due to
loss of viability increases toward the soil surface: conditions near the surface
are warmer than deep in the soil profile and are periodically too dry to main-
tain seeds in a fully imbibed state. Eventually, even imbibed seeds die, pre-
sumably due to accumulation of lethal levels of damage to membranes and
DNA (Osborne, 1980; Villiers, 1980). The extent to which depletion of food
reserves is involved in the aging process is poorly studied.
Seed predators consume significant numbers of weed seeds in some agroe-
cosytems. Prior to dispersal from the parent, predation is primarily by host-
specific natural enemies. Pre-dispersal seed predators may occasionally
consume a substantial proportion of the seeds produced (Forsyth & Watson,
1985), but particularly in annual crops, they may have difficulty in locating
their host plants,as explained in the section “Survival after emergence”below.
After seeds have dispersed from the parent, they are attacked by a range of
generalist seed predators including birds, small mammals, earthworms,
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