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5 Light-mediated Germination

Gallagher, R.S., Editor CAB International PDF

5

Light-mediated Germination

Thijs L. Pons*

Department of Plant Ecophysiology, Institute of Environmental

Biology, Utrecht University, Utrecht, the Netherlands

Introduction

The light response of seeds can control the time and place of germination of a seed, a crucial factor in the survival of the resulting seedlings, and the growth and fitness in subsequent developmental stages. The ultimate effect of light on seeds depends on genotype, and on environmental factors during ripening of the seeds, during dormancy and during germination itself. These environmental factors may include light, or factors other than light such as soil temperatures and soil chemical factors (see Chapter 6 of this volume). The picture is further complicated by the fact that the light climate itself has various aspects that have different effects on seeds, such as irradiance, spectral composition and duration of exposure of the seeds. All the above-mentioned factors can interact in one way or another in their effect on seeds. Moreover, the factors are not constant in time and are difficult to characterize at the seed’s position in the soil, thus complicating further the analysis of what is actually happening with a seed in a natural situation and the interpretation of a possible ecological significance of light responses.

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4 Seed Predators and Plant Population Dynamics

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4

Seed Predators and Plant

Population Dynamics

Michael J. Crawley*

Department of Biology, Imperial College London,

Ascot, Berkshire, UK

Introduction

The enormous seed production of most plants, coupled with the general paucity of seedlings and saplings, is vivid testimony to the intensity of seed mortality. The degree to which this mortality results from seed predation (the consumption and killing of seeds by granivorous animals) is the subject of the present review (for earlier references, see Crawley, 2000). To people who are unfamiliar with Darwinist thinking, it appears obvious that seed mortality is so high, because ‘plants need only to leave one surviving offspring in a lifetime’. In fact, of course, every individual plant is struggling to ensure that its own offspring make up as big a fraction as possible of the plants in the next generation, and for each individual plant there is a huge evolutionary gain to be achieved by leaving more surviving seedlings, i.e. more copies of its genes, in the next generation. The mass mortality of seeds is part of natural selection in action, and the group selectionist argument that plants only need to replace themselves is simply wrong. Since the numbers of seeds produced are so large, it only takes a small percentage change in seed mortality to make

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8 The Chemical Ecology of Seed Persistence in Soil Seed Banks

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8

The Chemical Ecology of Seed

Persistence in Soil Seed Banks

Robert S. Gallagher,1* Mark B. Burnham2 and E. Patrick Fuerst3

Independent Agricultural Consultant, Clinton, South Carolina, USA;

2

Department of Biology, West Virginia University, Morgantown,

West Virginia, USA; 3Department of Crop and Soil Science,

Washington State University, Pullman, Washington, USA

1

Introduction

The endogenous chemical regulation of seed persistence in the soil seed bank is often assumed to occur by many seed and plant ecologists, but the subject receives only sparse direct coverage in the scientific literature. Priestly (1986) reviewed the experimental evidence to that date for seed persistence in the soil, but gives little mention to potential chemical regulators of that persistence. Baskin and Baskin (1998) provide a very comprehensive review of seed dormancy and germination, but their discussion of chemical regulators of seed dormancy is largely limited to plant hormones as germination inhibitors. Shirley (1998) reviewed the role of flavonoids in seeds, outlining their potential role in preventing pathogen infection, reducing lipid peroxidation (i.e. antioxidant activity) and promoting seed dormancy, but focused primarily on seeds of agronomic crops. Based on the insight from Shirley (1998), Gallagher and

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2 Fruits and Frugivory

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2

Fruits and Frugivory

Pedro Jordano*

Integrative Ecology Group, Estación Biológica de Doñana,

CSIC-EBD, Sevilla, Spain

Introduction

The pulp of fleshy fruits, with the soft, edible, nutritive tissues surrounding the seeds, is a primary food resource for many frugivorous animals, notably mammals and birds, but also reptiles and fish, which are able to obtain energy and nutrients from it

(Howe, 1986). These animals either regurgitate, defecate, spit out or otherwise drop undamaged seeds away from the parent plants; they are the seed dispersers that establish a dynamic link between the fruiting plant and the seed/seedling bank in natural communities. Therefore, frugivory is a central process in plant populations where natural regeneration is strongly dependent upon animal-mediated seed dispersal.

Early conceptual contributions to the study of frugivory emphasized dichotomies in frugivory patterns and fruit characteristics that presumably had been originated by co-evolved interactions (Snow, 1971;

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10 The Functional Role of the Soil Seed Bank in Agricultural Ecosystems

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10

The Functional Role of the Soil Seed

Bank in Agricultural Ecosystems

Nathalie Colbach*

INRA, UMR1347 Agroécologie, EcolDur, Dijon, France

Introduction

In many countries with temperate climate, landscapes are highly anthropized, and arable crops constitute the major part of these landscapes. In these habitats, weeds consisting of both ‘real’ wild species and volunteers originating from lost crop seeds constitute the main component of wild plant biodiversity. Cropped fields differ from natural habitats by frequent disturbances (e.g. tillage, herbicides, harvest) and the presence of one

(or sometimes two) dominant plant species

(i.e. the crop) that usually changes every year. Though these disturbances can appear as stochastic and unpredictable from the weed’s point of view, they result from the farmer’s operational logic, and many of them specifically aim at controlling weeds because the latter are very harmful for agricultural production (Oerke et al., 1994). In this chapter, I propose to focus on the importance of the soil seed bank for plant regeneration in this particular habitat, to identify the relevant biophysical processes interacting with agricultural practices, and, finally, to determine what kind of weed species and traits are selected in different cropping systems.

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