Sources of seed mortality

Factors affecting the rate of seed mortality in the soil include (i) the action of seed predators, including vertebrates, invertebrates, fungi, and bacteria, (ii) physiological aging and exhaustion of reserves through respiration, and (iii) germination at depths in the soil or times of year that are unsuitable for emergence. Strictly speaking, the latter involves the death of seedlings rather than seeds, but it is customarily treated as a source of seed mortality. Several studies have partitioned the sources of weed seed mortality, and of the three factors listed above, inappropriate germination often causes the greatest reduction in seed density (Roberts, 1972). For example, Schafer & Chilcote (1970) found that after burial for 60 days at 10 cm depth, 11% to 13% of Lolium multiflorum seeds were nonviable whereas 40% to 64% had died after germination. Zorner, Zimdahl & Schweizer (1984a, 1984b) and Gleichsner & Appleby (1989) found that in situ germination was the largest source of mortality for deeply buried Avena fatua, Kochia scoparia, and Bromus rigidus (B. diandrus) seeds, but that loss of viability increased as a cause of mortality with shallower placement. Wilson (1972) noted (i) that A. fatua lost dormancy more quickly at the soil surface and (ii) that the seeds on the soil surface rapidly lost weight 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.

A problem with the above studies and observations is that the species investigated 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 proximity to the soil surface and lack of competition also suffer large mortality due to inappropriate germination remains to be determined. The technical problems 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 germinate inappropriately in any given time interval, and recovering tiny seedlings in the white thread stage is difficult.

Physiological aging of seeds involves loss of membrane integrity, deterioration 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 maintain seeds in a fully imbibed state. Eventually, even imbibed seeds die, presumably 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, insects, and fungi (Wilson & Cussans, 1972; Grant, 1983; Brust & House, 1988; Fellows & Roeth, 1992). Seed predation, and the manipulation of agricultural systems to increase predation on weed seeds, are discussed further in Chapters 5 and 8.

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

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