seed production can be prevented by cytoplasmic male sterility and by regular harvesting of biomass before flowering;
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Plant Physiology and Development
Roguing and Genetic Identity
An important component of seed production is ensuring the desired genetic identity of the subsequent generation that will be used for propagation. A key practice in maintaining genetic purity is called roguing, where fields are carefully examined and off-type plants removed. Off-type plants occur as random mutations, as a result of segregation in open-pollinated crops, or due to inadvertent mixture with seeds of other crops or cultivars. Seed crops must be carefully rogued prior to flowering to remove all off-type plants whose pollen may contaminate the remainder of the crop. Guidelines for when to rogue and what to look for are specific to each crop and cultivar and can be obtained from a seed certifying agency or the breeder who developed the cultivar. Roguing is also essential for removing male-fertile plants from female rows in hybrid seed production. Seedborne diseases, such as lettuce mosaic virus, may also be removed by careful rouging.
The quality of the stock seed used for establishment determines the quality of the seed produced. Much more stringent criteria must be used for propagating the early generations than later ones. An advantage of the root-to-seed or bulb-to-seed production systems in biennial crops is that there is an opportunity for quality evaluation and roguing at the time that the roots or bulbs are dug and transplanted to the seed production field. In a seed-to-seed production scheme, where the plants are established in the final production field by seed and are not transplanted, there is no opportunity to rogue for root or bulb quality. In this case, the quality of the stock seed must be extremely high.
Seed production and dispersal are particularly important for plants that are annual weeds. Initial weed infestation is dependent on seed invasion but continued survival requires on site seed production. Weeds characteristically produce very large numbers of seeds but seed size is variable, striking a balance between large seeds and a high rate of seedlings survival and small seeds with a low seedling survival rate. Weed seed populations usually comprise a mixture of species and ages and the seed bank, the population of seeds in the soil, has a loose relationship to the composition of the weed population in previous years. Weed seed populations are dynamic and are affected by weather, seed predators and cultivation practices. An understanding of weed seed populations is important as this governs how weed are managed of a period of time.
Breeding, Genetics and Seed Corn Production
L.L. Darrah , . M.S. Zuber , in Corn (Third Edition) , 2019
Seed production requires special care. Hybrid plants with the same pedigree but from seed produced by two different companies or from two differing locations can vary as much as two different hybrids because of different production or conditioning techniques.
After the seed producer chooses the desired hybrid, parental seed stocks must be located. Many large companies produce their own seed stocks, but smaller producers may buy seed from foundation seed stock companies that specialize in developing inbred lines and producing seed of public inbred lines. Seed of parental inbred lines is sold on a thousand-viable-kernel basis. For example, if the germination is 90%, the number of seeds is adjusted to 1111 kernels to provide 1000 viable kernels.
Weed Reproduction and Dispersal
4 Vegetative or Asexual Reproduction
Perennial weeds reproduce vegetatively, an unfortunate aspect of weed management. Simple and creeping perennials also reproduce by seed, but the importance of seed production varies. I suppose a good example is water hyacinth, whose pretty flowers produce seed pods with up to 300 seeds that can live 5–15 years submerged in water. Vegetative reproduction alone can double the size of an infestation in open water in 10–15 days ( Leakey, 1981 ) to produce floating mats weighing up to 200 tons/acre. Transpired water losses from mats of water hyacinth will be three to five times the loss from an open water surface.
The reproductive organ, the depth to which it penetrates soil, and the importance of seed production for several important perennial weeds are shown in Table 5.17 . Seed production is not of great importance for Canada thistle, which is dioecious. Whereas the pappus is always produced, it does not always have viable seed attached. On the other hand, seed production is important for leafy spurge, nettles, and curly dock.
Table 5.17 . Characteristics of Important Perennial Weeds ( Roberts, 1982 )
|Species||Reproductive Parts and Overwintering State||Depth of Vegetative Reproductive Parts a||Importance of Seed Reproduction|
|Bermuda grass||Creeping rhizomes, decumbent stems spread laterally||Shallow||Moderate|
|Bracken fern||Rhizomes; leaves die||Deep||Reproduces by spores|
|Canada thistle||Creeping roots overwinter; shoots die||Deep||Occasionally produced|
|Coltsfoot||Rhizomes; leaves die||Very deep||Important|
|Common nettle||Rhizomes; short green shoots overwinter||Very shallow||Very important|
|Creeping bent grass||Aerial creeping stems overwinter||Aboveground||Unknown|
|Creeping buttercup||Procumbent stems; a few leaves overwinter||Aboveground||Very important|
|Curly dock||Taproots; rosette of leaves overwinter||Very shallow 7–10 cm||Very important|
|Dandelion||Fleshy taproot; few leaves overwinter||Shallow||Important|
|Field bindweed||Creeping roots overwinter; shoots die||Very deep||Important|
|Field horsetail||Rhizomes with tubers that overwinter||Deep||Reproduces by spores|
|Hedge bindweed||Rhizomes overwinter; shoots die||Deep||Rarely produced|
|Hoary cress||Creeping roots; small rosettes of leaves overwinter||Deep||Important|
|Japanese knotweed||Rhizomes, dormant underground buds; shoots die||Shallow||None produced|
|Leafy spurge||Creeping roots overwinter||Very deep||Very|
|Oxalis sp. (wood sorrel)||Bulbils, taproots, and rhizomes; leaves||Shallow||Important in some|
|Perennial sow thistle||Creeping roots; shoots die||Very deep||Important|
|Quack grass||Rhizomes with dormant underground buds; shoots overwinter||Shallow||Moderately|
|Red top||Rhizomes with dormant underground buds; shoots overwinter||Shallow||Very important|
|Roughstalk bluegrass||Short stolons; a few leaves overwinter||Aboveground||Very important|
|Slender speedwell||Stems creeping on surface||Aboveground||None produced|
|Wild onion||Offset bulbs and bulbils overwinter||Aerial or very shallow||Rarely produced|
|Yarrow||Stolons; terminal rosettes of leaves overwinter||Very shallow||Very|
Many methods of vegetative reproduction are found among weeds. Stolons or creeping aboveground stems are found in creeping bent grass, and yarrow. Rhizomes are found in Bermuda grass, quack grass, red top, hedge bindweed, and field horsetail. Bulbs and aerial bulblets are found in wild onion and wild garlic. Goldenrod has corms: thickened, vertical, underground stems that are reproductive organs. Tubers are produced by yellow and purple nutsedge and Jerusalem artichoke. Vegetative reproduction of simple perennials such as dandelion is from their tap root.
A seedling of a perennial species growing from seed has not yet assumed perennial characteristics (especially the ability to regenerate vegetatively) when it first emerges from soil and can be controlled more easily than after it assumes these characteristics. It is generally considered that quack grass assumes perennial characteristics within 6–8 weeks of emergence and johnsongrass after only 3–6 weeks. Field bindweed becomes a perennial when it has about 20 true leaves, and yellow nutsedge 4–6 weeks after it emerges from seed. These young plants can be controlled by tillage or hoeing before they assume perennial characteristics.
Seed production of perennials may be unimportant relative to vegetative reproduction, but it should not be neglected. In April 1990, one field bindweed seed was planted in a small planter, and on April 25, the two-leaf seedling was transplanted to a 2 × 4 × 16–ft box. The plant was harvested on October 19 by opening the box and washing all of the soil away with water. The seedling had colonized the entire box. Vertical roots (197), each about 4 ft long, grew a total of 788 ft. Horizontal root runners from the taproot (34) averaged 4 ft long and were 136 ft long. They had produced 141 new plants. The creeping roots of field bindweed can grow up to 1.5 yards in a little more than 3 months ( Frazier, 1943 ). One little seed produced a major new weed. 5
A similar experiment was conducted in Colorado 6 with Canada thistle. One seed was planted in a 2 × 4 × 8–ft box of soil in April 1994. In July 1995, the plant was harvested. If the height of all 142 shoots was added, the plant would have been 157 ft tall. There were 331 flowers on 60 shoots. Vegetative buds producing new shoots were found up to 4 ft below the soil surface. Total root length was estimated to be 1700 ft. Canada thistle roots have been reported to spread up to 5 yards in a single season ( Bakker, 1960 ).
Tillage can worsen the problem after plants become perennial. Canada thistle spreads by creeping roots and pieces as small as 0.25 inches have produced new plants. Field bindweed spreads by creeping roots, and although they seldom emerge from greater than 4 ft, they can emerge from 20 ft. Pieces as small as 1 inch that contain a bud can produce a new plant. Most quack grass plants, developed from rhizomes, emerge from the top 12 inches of soil. Therefore, deep plowing may be a control method if rhizomes can be permanently buried. Permanent burial below 12 inches is highly unlikely because plowing is rarely that deep and mixing, not burial, occurs. However, most quack grass roots are 2–4 inches below the surface and shoots do not emerge from deep in soil, so it is possible. The ability of root segments to produce new plants varies with the season and is highest in spring and lowest in fall ( Swan and Chancellor, 1976 ). Many root segments produced shoots, but regeneration of roots was largely from vertical roots.
Leafy spurge roots penetrate up to 20 ft. Over 56% of the total root weight is in the upper 6 inches of the soil profile and most leafy spurge shoots originate from buds in the top foot of soil. Shoots emerge freely from 1.5 ft deep and some emerge from as deep as 6 ft.
Vegetative buds are not killed by winter freezes. Studies in Iowa on the winter activity of Canada thistle roots showed that buds on horizontal roots continued to develop new shoots until soil was frozen 50 cm deep ( Rogers, 1929 ). When the soil finally froze, the shoots were killed but the root bud was not. In January, when soil was still frozen, the latent buds on large roots were larger than they had been in December. By mid-January, these buds had developed thick, vigorous shoots up to 20 mm long. By February, shoots were 4–7 cm long and each had roots 10–20 cm long. When the soil thawed, root growth increased rapidly and green shoots appeared by mid-April. Rogers noted that the cycle of bud and root formation in field bindweed and skeleton leaf bur sage was similar to that described for Canada thistle.
Safflower (Carthamus tinctorius L.; family Asteraceae)
Seed production in safflower is directly related to the success of pollination because the plants show SP in the absence of pollinators ( Knowles, 1969 ). Classen (1950) reported 0–100% CP. In most of the plants, CP ranged from 5–40%. Pollinators contribute to various degrees of pollination of the flower ( Kadam and Patankar, 1942; Levin and Butler, 1966; Butler et al., 1966; Levin et al., 1967 ). Safflower is usually considered to be a self-pollinated crop. Insects, particularly bees, are the major agents of pollination ( Boch, 1961; Eckert, 1962; Rubis et al., 1966 ). Temperature and humidity affect the seed setting of bagged flowers ( Patil and Chavan, 1948 ). Pandey and Kumari (2007) found that there was 85.99% seed setting in open pollinated heads followed by 38.15% (in muslin cloth) and 35.54% (in butter paper) bagged conditions ( Table 17.12 ).
Table 17.12 . Self-Incompatibility Test in Different Experimental Conditions
|Types of pollination||Total number of flowers||Number of filled seeds||Number of unfilled seeds||Percentage of seeds setting|
|Naturally pollinated heads||54.75+7.70||47.08+13.78||7.67+6.07||85.99|
|Butter paper bagged head||54.76+5.50||19.46+8.88||35.3+3.38||35.54|
|Muslin cloth bagged head||51.53+5.35||19.66+4.67||31.87+0.68||38.15|
Source: Pandey and Kumari (2007) .
220.127.116.11 Seed storage, handling and grading
Seed production in many countries is carried out in regions distinct and geographically separate from those mainly used for ware (e.g. British Potato Council, 2006 ), with seed often stored in the area of production until shortly before planting. For ware producers with limited facilities for seed storage, co-ordinating delivery of seed with planting requires effective logistics, and the ability to store seed locally is an advantage in unpredictable climates where planting may be interrupted. Seed is often stored for long periods after the natural dormant period has ended, and sprout growth must be prevented by keeping the seed cold (typically 2–4°C) unless sprouting is desired and suitable systems implemented (see Section 18.104.22.168 ). Unintended sprouting of seed in bulk often causes difficulty in handling and planting, and damage to the fragile sprouts causes variable emergence and is associated with loss of yield and increased susceptibility to some diseases (e.g. Erwinia spp.).
Some preparation of seed for ware production by size grading may be carried out prior to storage, or seed may be stored initially ungraded and then be prepared during the storage period or immediately prior to dispatch for planting. Grading may be used to divide seed into two or more fractions for better control of seed rates, and the process also provides an opportunity to remove diseased or damaged tubers and to apply fungicidal seed treatments. Inspection of seed may be required for certification prior to sale and labels or plant passports issued to identify seed lots when prepared for transport. Seed may be transported in bulk, large wooden boxes or polypropylene bags (often 1t) or smaller hessian bags (often 25–50 kg).
WEEDS | Weed Seed Biology
Seed production by annual weeds is crucially important to their survival. It is no less important in perennial species where seed production as a consequence of sexual reproduction is important for the reassortment of genes and creation of new biological variability. Thus, although in annual species seed production takes place just once (but may be over an extended period), in perennial species seed production can take place on more than one occasion over an extended period of years. The seed is an important biological feature for dispersal of a species. In perennial species, vegetative reproduction may be important for consolidation and growth of a population at a single location. For perennials also, the production of seeds provides for new genetic individuals and the possibility of long-distance dispersal.
The production of flowers, fruits, and seeds can vary widely between related species, between populations of the same species at different sites, and between years at the same site. The number of seeds produced is a phenotypic character reflecting the interaction between genetic seed production potential and the environment experienced by the individual plant during its life. Local weather conditions and the crop management imposed by growers influence the number of plants achieving reproduction and the likelihood of pollination. Environmental conditions during seed development can influence the proportion of fertilized ovules which are retained, the amount of reserves they accumulate, and the degree of dormancy. Nevertheless, the most successful annual weeds generally have a large seed output which is maintained even in relatively adverse conditions.
For annual weeds, which possess many of the features of pioneer plants, the individual plants may be some considerable distance apart, more than the area over which pollen can easily be spread by the wind. Self-compatibility ensures that pollen from the same plant can fertilize ovules, thus ensuring some successful seed production at a site even when neighboring plants are scarce. In contrast, in perennial species, in which more energy is devoted to vegetative means of propagation, there are frequently mechanisms to ensure outcrossing by restricting the compatibility of pollen to fertilize ovules on the same plant.
One measure of the reproductive success of a weed is the number of seeds produced ( Table 1 ). Different species have different capacities for seed production. In addition there is great phenotypic variation within species. Within one population, individual plants of blackgrass (Alopecurus myosuroides) produced from 4 to 54 inflorescences and bore a total of from 14 to 7614 seeds. This suggests that the potential for population growth is great.
Table 1 . Examples of seed production in some weed species. The values (taken from a wide variety of sources) represent some of the highest values recorded for each species
|Species||Common name||Seeds m −2|
|Avena fatua||Wild oats||250|
|Capsella bursa-pastoris||Shepherd’s purse||40000|
|Cardamine hirsuta||Hairy bitter-cress||640|
|Chamaenerion angustifolium||Rosebay willowherb||76000|
|Chenopodium album||Fat hen||72000|
|Cirsium arvense||Creeping thistle||5200|
|Matricaria discoidea||Pineapple weed||7000|
|Papaver rhoeas||Corn poppy||17000|
|Ranunculus repens||Creeping buttercup||100|
|Raphanus raphanistrum||Wild radish||2250|
|Rumex crispus||Curled dock||60 000|
|Veronica hederifolia||Ivy-leaved speedwell||45|
The number of seeds produced by an individual plant or by a weed population is not the only measure of reproductive success. It may be better to take into account the success of the seeds in producing new seedlings at a later date, or the success of an individual plant in producing further plants. Sometimes the growing weed has been regarded merely as the mechanism whereby one seed produces further seeds.
For seed production , onion bulbs are grown under conditions conducive to proper bulbing. At maturity, the foliage of the plant collapses and dries. Occasionally, the onions may be undercut to hasten maturity. After bulbing, the onion requires a cool dormant period to induce the formation of floral initials. The length of time required under 15 °C to induce flowering is different for various onion populations. A majority of bulbs stored at 7–10 °C for 60-100 days will flower. However, highly dormant material may not be induced to flower after months of storage below 10 °C.
Hundreds of perfect flowers are present in each onion umbel. The flowers consist of a single superior pistil with six locules, therefore producing a maximum of six seeds per capsule. Outcrossing is encouraged by protandry. The inner whorl of three stamens are the first to shed pollen, followed by the outer whorl of three. Self-pollination is possible because the individual flowers of the onion umbel mature at different times. 12 Therefore, receptive stigmas will be present when pollen is being shed by flowers in the same umbel.
Manual emasculation and pollination of onion are tedious. Although the stamens are easily removed from a single flower, the size, number and continuous maturity of the many small flowers per umbel make hand pollination unattractive. As a result, onion breeders have developed techniques to manipulate insect pollinators. House flies or blowflies are commonly used for self-pollinations or intercrosses of less than five plants. 55 In the USDA onion breeding program, onion inflorescences are covered with small cages constructed from three metal rings supported by metal strips. The cages are covered with pillow tubing, 50 cm wide, made from a 65% polyester, 35% cotton blend. A small plastic tube is inserted in the top of the cage for introducing the fly pupae and the pillow tubing is tied shut. The cages are draped over the plants and tied tightly under the flowers with small paper-coated wires. The cage must be tied tightly enough that fly pupae do not fall out of the bottom. The bags are attached to wire supports at the top and bottom of the cage to avoid movement in the wind. After addition of the pupae, a cork is placed in the tube. Flies will emerge from the pupae and pollinate as they visit the flowers searching for nectar. Fly pupae can be raised or obtained from commercial sources.
Production of open-pollinated (OP) populations and experimental hybrids is completed in large mesh cages using bees. Mother bulbs are grown, harvested, cured, vernalized, selected for desirable traits, and planted in rows. Wilson 114 reported that transverse cutting of stored bulbs induced a greater number to flower. In the USDA onion breeding program, the top one-third to one-half of the bulb is cut to stimulate even bolting. After the scapes have emerged, the plots are covered with a mesh cage and bees introduced. For field production of seed, convention dictates that over 4 km should separate seed-production fields of different colored onions (N. Molenaar, personal communication). For hybrid seed production, a ratio of eight seed-parent rows (see below for description of cytoplasmic male sterility) to two pollen-parent rows is commonly used. A problem with seed-to-seed production of hybrids is the simultaneous flowering of inbred lines. To a certain extent, flowering can be manipulated by changing the planting date of inbreds. However, for large-scale production, harvesting and planting of mother bulbs is too expensive and seed-to-seed production is required. Desirable inbreds can be subjected to selection to shift the flowering date to flower simultaneously with the other inbred. Breeders will often select for upright, straight scapes which help to reduce losses of mature seed. Researchers have also noted differences in the seed from self-pollination of individual plants. 54 No information is available on the basis of these differences in seeding ability. Morse (cited in Jones 47 ) reported that yellow and brown cultivars are the heaviest seeders of the OP cultivars of that time.
Plant breeding and seed production
11.6.7 Herbage seed
All herbage seed production in the UK is overseen by NIAB. Only Basic seed is used for C1 generation production. In some cases it is permitted for seed crops to be taken from the same crop for two seasons, but no C2 generation multiplication is permitted. All seed crops are inspected at around the time of ear emergence to establish variety type and to check for off-types, weeds and isolation.
In total about 8000 hectares of grass are grown for seed in the UK at present. Of this over 6500 hectares is ryegrass of various types. European Union seed production aid is available at the time of writing for Italian, hybrid and perennial ryegrasses. The reader is advised to check with DEFRA the current status of this aid. Whereas it is possible to achieve good gross margins from seed production, the vagaries of the weather, the difficulties of combining seed crops at high moisture levels and very low prices in the late 1990s have brought about substantial reductions in the area grown.
Seed production from grass requires considerable planning as well as skill and perseverance. Good yields are dependent on good growing conditions and dry weather during the critical harvesting period. Grass seed is grown most successfully in low rainfall areas such as in the south and east of England, and usually on chalk or limestone based soils. The Italian and hybrid ryegrasses are harvested for one season only, but most of the perennials will produce seed crops for two seasons. Grass for seed production will, in addition, provide threshed hay and some grazing.
It is very important that there are no grass weeds in the field – especially blackgrass, couch, rough stalked meadow-grass, sterile brome and other grasses with similar sized seed. Docks too are an important contaminant of grass seed. Every effort should be made to rid the field of these weeds prior to growing a seed crop. High weed infestations can lead to a failure of the crop to pass field inspection. It is also important that the field is not contaminated with volunteer plants of the same species; NIAB stipulate up to four years of arable cropping prior to sowing a grass seed crop. The herbicide ethofumesate is regularly used on establishing seed crops to reduce the incidence of grass weeds. Broad-leaved weeds can be easily controlled by a range of post-emergence treatments.
Where there is a danger of cross-pollination (i.e. when neighbouring grass crops contain the same species), their flowering period overlaps and they are of the same ploidy (e.g. both diploids), then a gap of at least 50 m between the seed crop and its neighbour should be established before flowering begins. If this is not possible then, when flowering is over, a discard strip must be cut out of the seed crop adjacent to the neighbouring field. This material must be discarded and no seed may be used from it. It is important to remember that grass seed crops can be laid down for several years and careful planning is required to avoid cross contamination. Even areas such as waste ground and motorway verges can be sources of pollen.
The ryegrasses and fescues are usually undersown in spring cereals, either in narrow rows or broadcast, whereas cocksfoot and Timothy are usually sown direct in wide rows (about 50 cm). Seed rates are often substantially below (as little as 12.5 kg/ha for Italian ryegrass) those which would be considered normal for forage production. Italian and hybrid ryegrasses are usually cut early for a high quality silage crop or grazed prior to being laid up for the seed crop. This must be accomplished before the end of April. Moderate levels (about 70 kg/ha) of nitrogen fertilisers are then applied and the seed crop harvested usually in July or August. Perennial ryegrasses receive nitrogen at similar times to the Italians and hybrids, but no cutting or grazing can be carried out. A good ryegrass crop will usually ‘lodge’ about a fortnight before harvest and this will reduce losses by shedding. The crop may be combined direct or from windrows. The seed must be carefully dried (usually by prolonged blowing with ambient air) and cleaned. Yields of cleaned seed range from 1 to 1.75 t/ha but can be extremely variable and considerable skill is needed to achieve high yields.
Red clover. The USA and Canada are the main world centres of production of red clover seed. Only very small areas have been sown for seed in the UK in recent years, but the substantial increase in interest in this legume creates a new opportunity for herbage seed producers.
White clover. New Zealand, the USA and some South American countries are the primary producers of white clover for seed. In Europe, the largest producer is Denmark. At present, the only UK white clover crops for seed are very small areas of the Kent wild white local variety.
Preventive Weed Management in Direct-Seeded Rice
Adusumilli N. Rao , . David E. Johnson , in Advances in Agronomy , 2017
2.1.2 Prevention of Reproduction During Fallows
Prevention of seed production during the fallow period is also potentially a low-cost and valuable approach in preventing the build-up of the seedbank or perennial vegetative structures under DSR. This can be done during the long fallow period after rice harvest in the rice–fallow system in a single cropping system or during the short fallow period between harvest of rotational crop and planting of succeeding rice crop in doubling cropping systems.
In situations where immature weeds or perennating structures are present following harvest, tillage, or herbicide applications may prevent seed maturation or vegetative expansion ( Diallo and Johnson, 1997; Haefele et al., 2000 ). Greater monitoring of potential seed production during this postharvest interval and the evaluation of potential methods for preventing weed seed maturation during this time would be helpful in guiding growers in their decision making. For example, cover cropping has been found effective in suppressing weeds during fallow periods in tropical, subtropical, and temperate regions ( Akobundu et al., 2000; Brainard et al., 2011; Kumar et al., 2008, 2011; Teasdale et al., 2007 ). In addition, cover crops can be beneficial in improving soil health and the productivity of succeeding crops, especially if legumes are used. Saito et al. (2006) observed increased yields of direct-seeded upland rice and reduced weed growth in the subsequent rice growing season with the replacement of the natural fallow vegetation with stylo (Stylosanthes hamata), established as a relay crop with upland rice, in the short-term fallow systems. However, farmers may not be aware of the longer-term impact of this approach on weed populations or may not be willing or able to pay for the additional costs of weed management during this period. A clean fallow period has been identified as an important strategy for drawing down weed seedbanks in temperate cropping systems ( Mohler, 2009; Nordell and Nordell, 2007 ) but the economic and biological implications of this strategy in DSR have not been extensively explored.
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Seed Production seed production can be prevented by cytoplasmic male sterility and by regular harvesting of biomass before flowering; Related terms: Pollen Pollinator
How are seeds produced
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- seed – Children’s Encyclopedia (Ages 8-11)
- seed – Student Encyclopedia (Ages 11 and up)
Seed, the characteristic reproductive body of both angiosperms (flowering plants) and gymnosperms (e.g., conifers, cycads, and ginkgos). Essentially, a seed consists of a miniature undeveloped plant (the embryo), which, alone or in the company of stored food for its early development after germination, is surrounded by a protective coat (the testa). Frequently small in size and making negligible demands upon their environment, seeds are eminently suited to perform a wide variety of functions the relationships of which are not always obvious: multiplication, perennation (surviving seasons of stress such as winter), dormancy (a state of arrested development), and dispersal. Pollination and the “seed habit” are considered the most important factors responsible for the overwhelming evolutionary success of the flowering plants, which number more than 300,000 species.
The superiority of dispersal by means of seeds over the more primitive method involving single-celled spores, lies mainly in two factors: the stored reserve of nutrient material that gives the new generation an excellent growing start and the seed’s multicellular structure. The latter factor provides ample opportunity for the development of adaptations for dispersal, such as plumes for wind dispersal, barbs, and others.
Economically, seeds are important primarily because they are sources of a variety of foods—for example, the cereal grains, such as wheat, rice, and corn (maize); the seeds of beans, peas, peanuts, soybeans, almonds, sunflowers, hazelnuts, walnuts, pecans, and Brazil nuts. Other useful products provided by seeds are abundant. Oils for cooking, margarine production, painting, and lubrication are available from the seeds of flax, rape, cotton, soybean, poppy, castor bean, coconut, sesame, safflower, sunflower, and various cereal grains. Essential oils are obtained from such sources as juniper “berries,” used in gin manufacture. Stimulants are obtained from such sources as the seeds of coffee, kola, guarana, and cocoa. Spices—from mustard and nutmeg seeds; from the aril (“mace”) covering the nutmeg seed; from the seeds and fruits of anise, cumin, caraway, dill, vanilla, black pepper, allspice, and others—form a large group of economic products.
The nature of seeds
In the typical flowering plant, or angiosperm, seeds are formed from bodies called ovules contained in the ovary, or basal part of the female plant structure, the pistil. The mature ovule contains in its central part a region called the nucellus that in turn contains an embryo sac with eight nuclei, each with one set of chromosomes (i.e., they are haploid nuclei). The two nuclei near the centre are referred to as polar nuclei; the egg cell, or oosphere, is situated near the micropylar (“open”) end of the ovule.
With very few exceptions (e.g., the dandelion), development of the ovule into a seed is dependent upon fertilization, which in turn follows pollination. Pollen grains that land on the receptive upper surface (stigma) of the pistil will germinate, if they are of the same species, and produce pollen tubes, each of which grows down within the style (the upper part of the pistil) toward an ovule. The pollen tube has three haploid nuclei, one of them, the so-called vegetative, or tube, nucleus seems to direct the operations of the growing structure. The other two, the generative nuclei, can be thought of as nonmotile sperm cells. After reaching an ovule and breaking out of the pollen tube tip, one generative nucleus unites with the egg cell to form a diploid zygote (i.e., a fertilized egg with two complete sets of chromosomes, one from each parent). The zygote undergoes a limited number of divisions and gives rise to an embryo. The other generative nucleus fuses with the two polar nuclei to produce a triploid (three sets of chromosomes) nucleus, which divides repeatedly before cell-wall formation occurs. This process gives rise to the triploid endosperm, a nutrient tissue that contains a variety of storage materials—such as starch, sugars, fats, proteins, hemicelluloses, and phytate (a phosphate reserve).
The events just described constitute what is called the double-fertilization process, one of the characteristic features of all flowering plants. In the orchids and in some other plants with minute seeds that contain no reserve materials, endosperm formation is completely suppressed. In other cases it is greatly reduced, but the reserve materials are present elsewhere—e.g., in the cotyledons, or seed leaves, of the embryo, as in beans, lettuce, and peanuts, or in a tissue derived from the nucellus, the perisperm, as in coffee. Other seeds, such as those of beets, contain both perisperm and endosperm. The seed coat, or testa, is derived from the one or two protective integuments of the ovule. The ovary, in the simplest case, develops into a fruit. In many plants, such as grasses and lettuce, the outer integument and ovary wall are completely fused, so seed and fruit form one entity; such seeds and fruits can logically be described together as “dispersal units,” or diaspores. More often, however, the seeds are discrete units attached to the placenta on the inside of the fruit wall through a stalk, or funiculus.
The hilum of a liberated seed is a small scar marking its former place of attachment. The short ridge (raphe) that sometimes leads away from the hilum is formed by the fusion of seed stalk and testa. In many seeds, the micropyle of the ovule also persists as a small opening in the seed coat. The embryo, variously located in the seed, may be very small (as in buttercups) or may fill the seed almost completely (as in roses and plants of the mustard family). It consists of a root part, or radicle, a prospective shoot (plumule or epicotyl), one or more cotyledons (one or two in flowering plants, several in Pinus and other gymnosperms), and a hypocotyl, which is a region that connects radicle and plumule. A classification of seeds can be based on size and position of the embryo and on the proportion of embryo to storage tissue; the possession of either one or two cotyledons is considered crucial in recognizing two main groups of flowering plants, the monocotyledons and the eudicotyledons.
Seed, the characteristic reproductive body of both angiosperms and gymnosperms. Essentially, a seed consists of a miniature undeveloped plant (the embryo), which, alone or in the company of stored food, is surrounded by a protective coat. Learn more about seed characteristics, dispersal, and germination.