Fruit and Seed Production

What are seeds and fruits?

A seed develops from the fertilized ovule and contains an embryonic plant surrounded by a protective seed coat. It also contains nutritional reserves in the form of either endosperm or cotyledons. In angiosperms, seeds may be dispersed within a fruit formed from the flower ovary or receptacle. A fruit may contain one or many seeds. Fruits are divided into two basic types: fleshy and dry. Fleshy fruits include peaches, tomatoes and figs; acorns, rice and beans are dry fruits.

Seed set

Since a plant has a limited amount of resources for reproduction, there is a trade-

Seed number

The number of seeds produced by an individual plant will depend on the number of ovules produced, their rate of fertilization, and on how many fertilized ovules survived to become mature seeds. What determines the actual number of seeds produced? First, there are the genetic constraints over the number (and size) of seeds a species can produce. Orchids produce thousands of dust-sized seeds but cannot produce coconut-size seeds; coconut trees cannot produce as many seeds as orchids do. Within these constraints, seed number is influenced by the availability of resources and by the environmental conditions during pollination and seed development. For example, the number of seeds produced by redroot pigweed (Amaranthus retroflexus) decreases as light level decreases (McLachlan et al., 1995).

The benefit of producing many seeds is that they may have more opportunities for colonization because of the sheer numbers of seeds produced and reduced losses from seed herbivores that cannot usually find and destroy all the seeds.

Seed size

Seed size has many repercussions for dispersal and seedling establishment. The main benefit of having large seeds is that resultant seedlings are usually more competitive because they have more nutrient reserves and can survive harsher conditions for longer periods of time. Seedlings from large seeds are better able to withstand drought, defoliation, shade, litter and competition from established vegetation or concurrently emerging seedlings from relatively small seeds (Westoby et al., 1996; Leishman, 2001). Larger seeds, however, require more energy to produce and are more likely to be consumed by seed herbivores in search of easy to find and nutritious meals (Reader,

1993; Thompson et al., 1993; Rees, 1996). There are advantages to having small seeds. Individuals with small seeds produce many more of them than individuals with large seeds and can do so because small seeds require less energy to produce (Leishman, 2001). Though small seedlings (from small seeds) are more likely to die before reaching maturity, they make up for the loss in sheer numbers.

While it was once thought that seed size was a genetically stable trait within a species (Harper, 1977), it is now recognized that environmental variation often causes seed size to vary greatly within species, populations and individuals (Michaels et al., 1988). For example, the seed size of the annual weedy cucumber (Sicyos deppi) is dependent on the environment in which the fruit develops (Orozco-Segovia et al., 2000). This weed is a vine that climbs up the stems and trunks of the other vegetation in fields and disturbed forests of Mexico. As a result, some fruits develop in full sunlight while others develop in the shade. Seeds that develop in full sunlight are larger and heavier than shaded seed, but seed viability is the same. Alternatively, the size of redroot pigweed seeds do not vary with light level (McLachlan et al., 1995)

Because seed size is genetically controlled and environmentally influenced, selection pressures could lead to a change in seed size. For example, seed size of the weedy gold-of-pleasure (Camelina sativa) has diverged over time, depending on the type of flax crop (Linum spp.) the weed grows in. Weed seeds became similar to crop seeds because both pass through the winnowing machine at the same time and thus weed seeds similar in size and weight to flax are selected for. In fibre flax, gold-of-pleasure seeds are flat; in flax grown for oil production, the weed seeds are smaller and plumper.

Examples of the trade-off between seed number and seed size

Producing a very few, nutrient-rich, seeds can be disastrous if all of the seeds die because they end up in unsuitable habitats or are destroyed (e.g. eaten). Likewise, producing many nutrient-poor seeds can be equally disastrous if the seedlings are not able to survive the biotic and abiotic stresses of their environment. So how will a plant resolve this trade-off? Eriksson (2000) proposed a model to explain the dispersal and colonizing ability of a species based on seed

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Fig. 6.2. A model of the relationship between seed size, seed number, recruitment and dispersal and colonizing ability (redrawn from Eriksson, 2000).

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Fig. 6.2. A model of the relationship between seed size, seed number, recruitment and dispersal and colonizing ability (redrawn from Eriksson, 2000).

size (Fig. 6.2). Here, seed size decreases with increased seed production. The number of seedlings that survive to become adults ('recruited') increases as seed size increases but this reaches a limit, i.e. a maximum recruitment threshold. As a result, the maximum combined dispersal and colonizing ability is at an intermediate level of seed size. Eriksson (2000) recognized that this relationship is dependent on habitat and community type. The peak of the dispersal and colonizing ability curve moves leftward in disturbed sites because the benefits of increasing seed number outweigh those of increasing seed size. Additionally, these maxima will vary with short- and long-term environmental variation.

Fruit and seed polymorphisms

In some cases, one plant may produce two or more types of seeds that differ in morphology. Species with two types of seed ('morphs') are dimorphic and those with more are polymorphic. Seed morphs may have different sets of germination requirements or different dispersal mechanisms associated with them. Having two or more morphs is a form of 'bet-hedging'. By producing seeds with different germination and/or dormancy requirements, the plant is likely to have at least some seeds germinate. This strategy is advantageous in environmentally variable habitats. The trade-off with bet-hedging is that while it increases the chance of at least some seed germinating in most conditions, fewer seeds will germinate in optimal conditions.

Seeds of common lambsquarters (Chenopodium album) are dimorphic for two characters: seed wall and seed texture. There are thin-walled brown seeds that germinate immediately, and thick-walled black seeds that are dormant. In addition, both brown and black seeds have smooth coat and textured coat morphs. The proportion of each of the four seed types varies among populations (Harper et al., 1970). Seed polymorphisms are common in the daisy (Asteraceae), goosefoot (Chenopodiaceae), grass (Poaceae), and mustard (Brassicaceae)

families; these families also contain many weed species (Harper, 1977).

Wingpetal (Heterosperma pinnatum) is a summer annual that has different types of 'achenes' (a dry, single-seeded fruit) within each flowering head; the polymorphisms ensure that some of these short-lived (1 year) seeds will germinate each year. Venable et al. (1995) divided these into three morphology types (central, intermediate and peripheral) based on length/width ratio, and the presence of a beak and/or wing (Fig. 6.3). Central achenes are awned (winged) and tend to disperse further. They lose dormancy earlier than other morphs and germinate in the spring. Peripheral achenes do not disperse as far but tend to have higher germination under harsh conditions. Intermediate achenes are longer and skinnier than peripheral achenes but are not awned. The relative proportion of these morphs differs among population; this is a result of selection under different environments. For example, populations in disturbed habitats (Mirador population) have a higher proportion of central achenes as these are more likely to dispersal away from a habitat that may be eliminated. In populations receiving heavy early rains (Tula population), there are more peripheral achenes that germinate late and can withstand this environment.

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