Weeds: Seed Banks and Seed Dormancy

Introduction..................................................................................................129

Characteristics of the Soil Seed Bank........................................................129

Periodicity and Intermittent Germination................................................130

Weed Seeds and Depth of Burial................................................................130

Persistence of Weed Seeds in Soil...............................................................130

Environmental Factors That Affect Seed Dormancy................................131

Temperature • Light

Lynn Fandrich Conclusion.....................................................................................................131

Oregon State University References.......................................................................................................132

Introduction

The relationship between seed dormancy and success of a plant as an agricultural weed is significant. Holm et al.111 estimated that there are about 250 significant weeds in world agriculture. If the top 40 weeds classified as “serious” are selected by ranking them according to the number of countries in which they are considered “serious” weeds, it can be shown, without exception, that each one of the 40 species has dormancy. If weed seed were forever dormant, or simply nondormant, the complexities of weed management would be greatly reduced. However, weed seeds vary widely with respect to degree, duration, and source of dormancy. The existence of large populations of weed seed with varying degrees and states of dormancy is the basis for the annual weed problem. Dormancy allows a weed seed to avoid germination when conditions are not appropriate to complete its life cycle and prolongs population survival in the form of a soil seed bank.

Characteristics of the Soil Seed Bank

All of the viable seed present on the surface and in the soil is usually described as the soil seed bank. The number of seeds in the seed bank is determined by the rate of input in the seed rain, minus the losses resulting from disease, predation, and germination. The seed bank consists of new seeds recently shed by a parent plant, as well as older seeds that have persisted in the soil for several years. All of these seeds will vary in depth of burial and state of dormancy. Only a small proportion of seeds, varying between and among species, germinate when conditions are favorable. It is generally assumed that annual species contribute up to 95% of the seeds in the agricultural seed bank and only 2 to 10% of weed seeds emerge as seedlings each year.

Because most weeds are capable of producing a high number of seeds, especially compared to cultivated species, a few uncontrolled plants can rapidly increase the number of seeds in the soil seed bank. Data collected from plants grown in monoculture showed that wild oat (Avetia fatua) produced 372-623 seeds,[1] common lambsquarters (Chenopodium album) produced 75,600-150,400 seeds,01 and Palmer amaranth (Amaranthus palmeri) produced 200,000 to 600,000 seeds per plant.|4I It should be noted that biotic (inter- and intraspecific competition) and abiotic (climate) factors affect seed production, and most plants in agricultural situations do not often approach these high seed production values.

Large numbers of seeds are dispersed both horizontally and vertically in the soil. Plowing, chiseling, disking, and other forms of tillage are major ways that seeds are moved and covered with soil. After a number of years of soil disturbance, a relatively stable vertical distribution of seeds is reached in the tillage zone. Early estimates of seed numbers in agricultural soils exceeded 100,000 seeds per hectare in the tillage layer. Samples of soil on well managed farms in Minnesota averaged 16 million seeds per hectare, and as many as 45 million per hectare have been counted.

  • [1] It is quite common for seeds to be dormant when they are fully mature on the parent plant and for dormancy to be lost after they are shed. The term “after-ripening” has frequently been used to denote theinteraction between environment and seed over time that leads to a loss of dormancy.181 The length of theafter-ripening period varies between and among species and by seed location on the plant. Most weedspecies show one or two periods of high percent germination in seed produced during any given year,and the percent germination in subsequent years can be quite low. The mean length of the dormancyperiod in populations drives turnover in the seed bank. Grime191 makes a distinction between “transient” (complete turnover in less than one year) and “persistent” seed banks (some seed remains in thebank longer than one year). The term “aging” is sometimes used incorrectly to describe after-ripening.Aging reduces seed viability and seedling vigor.
 
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