Principles and Practices of Seed Selection

learning objectives• Define breeding systems.

• Categorize seed-propagatedcultivars and species.

• Define procedures to controlgenetic variability.

• Describe systems of seedselection and production.

• Define legal controls ongenetic purity.

5Principles and Practices of Seed SelectionINTRODUCTIONMany annual and biennial crop, forage, vegetable, and ornamental selec-tions are produced by plant breeding to be propagated by seed (7, 8, 29,34). Breeding involves selection of parents, specific breeding procedures,and genotype stabilization (1, 3, 47). The last process is sometimesreferred to as “fixing the genotype.”Seed is used to reproduce most woodyperennial plants in forestry as well as inthe landscape. Propagation of manyornamental, fruit, and nut trees utilizesseedlings for rootstocks that are thengrafted (49, 58). However, characteristicsimportant in agriculture, horticulture, and forestry may not be consis-tently perpetuated into the next seedling generation unless appropriateprinciples and procedures are followed. This chapter deals with seedselection and the management of genetic variability in seedling popula-tions in both herbaceous and perennial plant species for the purposes ofpropagation.

“fixing” The processof stabilizing thegenotype of a seedlingpopulation to make ithomozygous so that itwill “breed true.”

BREEDING SYSTEMSThe main objective of a breeding program is to use the observed variabilityavailable within a particular genus or species to create new, stable popula-tions with improved plant characteristics. Variability in seed-propagatedplants can be described both at the pheno-type (appearance) and genotype (genetic)levels. Seedlings that are phenotypi-cally very similar in appearance to eachother are termed homogeneous, whilethose that are dissimilar are described

as heterogeneous. When morespecific information is knownabout the seedling popula-tion’s genetic makeup, they can

be described as homozygous orheterozygous. Homozygous

populations share many com-mon paired alleles (genes) ateach chromosome loci and

breed true-to-type offspring. Hete-rozygous populations have dissimilar

homogenousA population of seedlingsthat are phenotypicallysimilar.

heterogeneousA population of seedlingsthat are phenotypicallydissimilar.

homozygousA population of seedlingswhose genotypes arevery similar.

heterozygousA population of seedlingswhose genotypes aredissimilar.

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principles and practices of seed selection chapter five 141

(a)

Flower Fruit Seeds

(b) (c)

Figure 5–1A cleistogomous flower in violet. The flower remainsunderground and never opens, forcing self-pollination. (a) Unopened cleistogomous flower. (b) Fruit with developingseeds. (c) Fruit with ovary wall removed to show the seeds.

paired alleles at manychromosome loci andgenerally lead todiverse genetic off-spring. These charac-teristics are determinedby the breeding sys-tem, characteristics ofthe crop species, andmanagement conditionsunder which seedpopulations are grown(1, 3, 22). Three impor-tant considerations fordetermining a plantbreeding system arewhether the plantsreproduce primarilyfrom self-pollination,cross-pollination, andapomixis (22).

Self-PollinationSelf-pollination occurs when pollen germinates onthe stigma and the pollen tube grows down the style tofertilize the same flower or a flower of the same plantor clone. Self-pollination is a natural condition insome species because of flower structure. The extremecase is when pollination occurs before the flower opens(Fig. 5–1). This type of behavior is called cleistogamyand occurs in some crop plants like peanuts (Arachis).A wonderful example of this reproductive strategy isfound in several types of violets (Viola). Violets canproduce two types of flowers. Chasmogamous (open)flowers are produced in the spring or summer whenpollinators are plentiful and active. Chasmogamous

flowers open to permit cross-pollination between flow-ers and produce offspring (seeds) with generousgenetic diversity. These same plants also produceunderground cleistogamous flowers in the autumnthat never open and self-pollinate. Although thisrestricts genetic diversity, it does not require thesame level of plant resources for seed production andprovides insurance against poor seed production fromearlier out-crossing flowers.

The degree to which self-pollination occurs canvary among species. Some are highly self-pollinated(i.e., less than 4 percent cross-pollinated) such ascereal grains [barley (Hordeum), oats (Avena), wheat(Triticum), rice (Oryza)], legumes [field pea (Pisum),and garden bean (Phaseolus)], flax (Linum), and somegrasses. There are also those that are self-fertile but cancross-pollinate at more than 4 percent, including cot-ton (Gossypium), pepper (Capsicum), and tomato(Solanum). Self-pollination is not typically found inmost woody plant species, but some exceptions occur,such as peach (Prunus) (58).

Homozygosity in a self-pollinated herbaceouscultivar is “fixed” by consecutive generations of self-fertilizations (Table 5–1) (1, 22, 47). To produce a“true-breeding” homogeneous and homozygous culti-var, plant breeders will start with a single plant andthen eliminate the off-type plants each generation for aperiod of six to ten generations. If one assumes a moreor less homogeneous population with individuals pos-sessing homozygous traits, self-pollination will result ina population of individuals that will remain homoge-neous and homozygous. If a mutation occurs in one ofthe alleles and is recessive, the genotype for that traitbecomes heterozygous. Then the next generation willproduce homozygous plants that are similar in appear-ance but genetically heterozygous for the mutant allele.The proportion of homozygous individuals with thetwo traits will increase in consecutive generations,while the proportion with heterozygous genotypes willdecrease by a factor of one-half each generation. Thegroup of descendants of the original parent will segre-gate into a heterogeneous mixture of more or less true-breeding lines.

Cross-PollinationIn nature, many, if not most, species are naturally cross-pollinated, a trait that seems to be desirable both forthe individual and its population. Not only does theincreased heterozygosity provide the opportunity for evolutionary adaptation within the populationconfronted with environmental change, but plant vigor

self-pollinationA breeding system in which the plant flower ispollinated by itselfbecause of flowerstructure or isolation.

cross-pollinationA breeding system in whichthe plant is pollinated bypollen from a separategenotype either becauseof flower structure orartificial control duringpollination.

apomixis A breedingsystem in which theembryo is apomictic (i.e., produced from avegetative cell and not as a result of reductiondivision and fertilization).


142 part two seed propagation

also tends to be enhanced. Enforced self-pollination ofnaturally cross-pollinated plants through consecutivegenerations may result in homozygous plants and a

homogeneous popula-tion (inbred line), butvigor, size, and produc-tivity may be reduced,a condition describedas inbreeding depres-

sion. If, however, two inbred lines are crossed, thevigor of the plants of the resulting population may notonly be restored but may show more size and vigor

than either parent, aphenomenon knownas heterosis or hybridvigor. In this case, theindividual plants willbe heterozygous, but

the population is likely to be homogeneous and haveuniform characteristics.

Many species have also developed morphologicalor genetic mechanisms to prevent self-pollination andpromote cross-pollination.

Here are four illustrations of morphological adap-tations to facilitate cross-pollination (6):

• Dioecy. Dioecious plants have pistillate (female) andstaminate (male) flowers present in separate plants,such as asparagus (Asparagus), pistachio (Pistacia) and

holly (Ilex) (Fig. 5–2).Plants with only femaleflowers are called gynoe-cious, and those withonly male flowers are

androecious. This type of flower arrangement usuallyforces cross-pollination.

• Monoecy. Monoecious plants have pistillate (female)and staminate (male) flowers in separate flowers on the same plant. Thissystem occurs in cucurbits(Cucurbita), corn (Zea),walnut (Juglans) (Fig. 5–3),oak (Fagus), and manyconifers. Although thisfacilitates cross-pollination,

Table 5–1EFFECT OF SELF-POLLINATION AND ROGUING FOLLOWING CROSSING OF A TALL (DD) PEA AND DWARF

(dd) PEA (SEE FIG. 2-14). “Fixing” of the two parental phenotypes can be observed in succeedinggenerations in the proportion of tall and dwarf plants. Continuous roguing for the recessive trait neverquite eliminates its segregation from residual heterozygous individuals.

A. Continuing self-pollination proportions

Percent homozygous

B. Roguing of all dwarfed plants

DD Dd Dd Tall Dwarf %dd

P1 1 1 100F1 1 0 allF2 1 2 1 50 3 1 25F3 3 2 3 75 14 1 7.1F4 7 2 7 87.5 35 1 2.8F5 15 2 15 93.75 143 1 0.7F6 31 2 31 96.88 535 1 0.2F7 126 2 126 98.44 2143 1 0.05

(a) (b)

Figure 5–2Holly (Ilex) plants are dioecious, producing female (a) andmale (b) flowers on separate plants, forcing cross-pollination.Many flowers in dioecious plants produce remnant femaleand male parts that are usually non-functional. Note the non-functional male stamens present in the female flowers.

inbred lineA population of seedlingsthat produced aconsecutive series of self-pollinations.

hybrid vigor Vigorexpressed by a seedlingpopulation that exceeds that of either of the parents.

dioecious Plant trait inwhich male and femaleflowers are producedon different plants.

monoecious Planttrait in which themale and femaleparts are in differentflowers but on thesame plant.



self-pollination is usually possible in monoeciousplants unless another barrier to self-pollination ispresent.

• Dichogamy. Dichogamy is the separation of femaleand male flower function in time (50). There are

two types of dichogamydepending on whetherthe female becomesreceptive before themale sheds pollen (pro-togyny) or the male

sheds pollen before the female is receptive (protandry).There are numerous examples of this type of floweringincluding carnation (Dianthus) (Fig. 5–4). Dichogamy

does not ensure cross-pollination but reduces the ratioof self- to cross-pollinated flowers (40).

• Polymorphism. Floral polymorphisms refer to differ-ent arrangements of flower parts in flowers from thesame or different plants within the same species. Manyof these adaptations are designed to alter the ratio ofself- to cross-pollination. A range of flower structures isillustrated in asparagus (Asparagus) (Fig. 5–5, page 144).These types of polymorphisms were of particular inter-est to Darwin (14) as he described the different flowerforms in primrose (Primula) referred to as heterostyly.Plants exhibiting heterostyly have two or three differentflower morphologies where the style of the female andthe filaments of the male are produced at differerntlengths (Fig. 5–6, page 144). In addition to the differ-ent heterostylous morphologies, each style and filamentlength combination may be linked to a sexual incom-patibility system to limit which flowers can cross witheach other (23).

Sexual incompatibility (10, 15) is a generalterm that describes the inability of plants that are notgenetically related to cross and produce offspring.Self-incompatibilityis a form of sexualincompatibility thathas evolved to preventself-pollination withinclosely related speciesand has been found inover 250 plant generafrom at least 70 fami-lies. Some horticulturally important plants showingself-incompatibility include lily (Lilium), cabbage(Brassica), Petunia, almond (Prunus dulcis), apple(Malus), cherry, and plum (Prunus).

Self-incompatible crosses are characterized by a lackof pollen germination or arrested pollen-tube growth(53). Self-incompatibility is a genetic mechanism con-trolled by a single gene locus (in diploids) with several dif-ferent S alleles. It is controlled by protein-to-proteinrecognition determined by the type of S allele in the maleand female partners. The two most common forms ofself-incompatibility are gametophytic and sporophytic(Fig. 5–7, page 145). Gametophytic self-incompatibilityis the most common form of self-incompatibility, and theinteraction between the male and female partners isdetermined by a single S-allele derived from the haploidgenetics within the pollen grain. Recognition only occursafter pollen germination and tube growth. When themale and female share a common S-allele genotype, thereis a protein-to-protein interaction that stops pollen-tube

(a) (b)

Figure 5–3Some nut-producing tree species have pollination systems thatensure cross-pollination. Walnuts (Juglans) are monoeciouswith female (a) and male (b) flowers produced separately onthe same plant.

an

ans s

Figure 5–4Sweet William carnation (Dianthus) flowers show dichogamy.Note how the flower on the left has anthers (an) sheddingpollen before the style (s) has fully developed and the floweron the right that has fully receptive female parts after theanthers have withered.

dichogamy Genetictrait in which male andfemale flowers on thesame plant bloom atdifferent times.

sexual incompatibilityGenetic trait in whichthe pollen either fails togrow down the style ordoes not germinate onthe stigma of a plantwith the sameincompatibility alleles.


chandrasekar

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Please check the word.


1 2 3 4

Figure 5–5Range of flower structure types expressed in different asparagus flowers ofindividual plants. Type 1. Completely female. Dioecious. Flowers contain only thepistil; stamens (male) are reduced and nonfunctioning. Type 2. Completely male. Dioecious. Flowers only contain stamens. The pistil is reduced andnonfunctioning. Type 3. Both male and female structures are functioning. Perfect.Type 4. Both male and female structures are nonfunctioning. Sterile. Commercialseed production of asparagus results from growing Type 1 and Type 2 plantstogether to enforce cross-pollination and produce the desirable hybrid plants.Courtesy Bryan Benson.

s

s

a

s

an

an

Pin Thrum

Pin Thrum

(a)

(c)

(b)

Figure 5–6Examples of heterosyly in primrose (Primula).On the left are “pin” flowers where thestigma (s) is elevated above the corolla andthe anthers (an) held on a short filament. Onthe right are “thrum” flowers with elongatedfilaments exposing the anthers above thecorolla, and a shortened style, keeping thestigma within the corolla tube.



growth. In some families (i.e., Papaveraceae), the pollentube stops growing soon after initial germination, whilein others (i.e., Solanaceae, Rosaceae) the pollen tube willgrowth a considerable distance down the style before itsgrowth is arrested (21). A unique breeding technique to

bypass incompatibility in lily (Lilium) is to remove theupper two-thirds of the style (including the stigma)before applying pollen. This allows time for the pollentube to reach the embryo sac before being arrested by theincompatibility reaction.

Sporophytic self-incompatibility differs fromgametophytic self-incompatibility because it is thediploid S-allele pair from the male and female parentsthat determines compatibility (32). Protein typesdetermined by different S alleles are deposited on thesurface of the pollen grain where they interact withproteins on the stigmatic surface to determine whetherthe pollen grain will germinate and initiate tubegrowth. Examples of plants with this type of incom-patibility are found in the Brassicaceae, Asteraceae,and Convolvulaceae families. Because multiple S alle-les are involved in this recognition system, pollen/stigma interactions can be complex (Fig. 5–7).

Cross-pollination is mostly carried out by themovement of pollen by wind or insects. Also, pollina-tion is sometimes by bats, birds, and water (48). Insectpollination is the rule for plants with white or brightlycolored, fragrant, and otherwise conspicuous flowersthat attract insects. The honeybee is one of the mostimportant pollinating insects, although wild bees, but-terflies, moths, and flies also obtain pollen and nectarfrom the flower (Fig. 5–8). Generally, pollen is heavy,sticky, and adheres to the body of the insect. Someimportant seed crops that require cross-pollinationare alfalfa (Trifolium), birdsfoot trefoil (Lotus), redclover (Trifolium pratense), white clover (Trifoliumrepens), onion (Allium), watermelon (Citrullis) andsunflower (Helianthus) (Fig. 5–8c). In addition,many flower and vegetable crops are insect pollinatedas are many fruit plants, ornamental plants, anddeciduous and broad-leaved evergreens used in thelandscape.

Figure 5–7Incompatibility mechanisms prevent self-pollination in somespecies. Top (cabbage): Sporophytic incompatibility. Eachpollen contains genes of both S1 and S2 alleles, and the pollentube will only grow down a style with a different genotype.Bottom (clover): Gametophytic incompatibility. Each pollengrain has a single S allele. A pollen tube will not grow down astyle where that allele is represented. Redrawn with permission from

Stoskopf, et al. Plant Breeding Theory and Practice. Westview Press: Boulder, CO.

(a) (b) (c)

Figure 5–8Important insect pollinators include (a) bees and (b) butterflies. (c) Bee hives are included in production fields to help pollination.



Wind pollination is the rule for many plants thathave inconspicuous flowers, or those with monoecious,dioecious, or dichogamous flowers. Examples aregrasses, corn, olive, and catkin-bearing trees such as thewalnut (Juglans), oak (Quercus), alder (Alnus), cottonwood(Populus), and conifers (Fig. 5–9). The pollen producedfrom such plants is generally light and dry and, in somecases, carried long distances in wind currents.

Most trees and shrub species are both heterozy-gous and cross-pollinated such that considerable poten-tial for genetic variability exists among the seedlingprogeny. Selection of seed source plants must take intoaccount not only the characteristics of the plant itselfbut also the potential for cross-pollination with otherspecies in the surrounding population. For example, thepresence of off-type individuals in seedlings propagated

from imported seed of Eucalyptus from Australia andpear (Pyrus) species from China and Japan (30) could betraced to hybridization with other species nearby.

ApomixisApomixis occurs when an embryo is asexually producedfrom a single cell of the sporophyte and does not developfrom fertilization of two gametes (28). This new “vegeta-tive” embryo may arise by mechanisms that weredescribed in Chapter 4. In each case, the effect is that seedproduction becomes asexual and seed reproduction resultsin a clone. In some species, both apomictic and sexualseeds are produced, sometimes within the same ovule(facultative); bluegrass(Poa pratensis) falls intothis category. Otherspecies are essentially100 percent apomictic(obligate); for example,Bahia grass (Paspalumnotatum) and buffelgrass(Pennisetum ciliare).

Breeding of apomictic cultivars requires that agenetic source for apomictic reproduction be foundwithin that species. This trait is not identifiable byvisual inspection of the parent plant but by its geneticperformance (i.e., unexpected uniformity of its prog-eny from among normally variable populations).Apomixis has been most important in the breeding ofgrasses, forage crops, and sorghum. Introduced culti-vars have included ‘King Ranch’ bluestem, ‘Argentine’Bahia grass (Paspalum), and ‘Tucson’ side oats grama,‘Bonnyblue’ and ‘Adelphi’ Kentucky bluegrass (Poa)(24), and buffelgrass (Pennisetum) (28). Relatively fewgenes apparently control apomixes, and breeding sys-tems have been described to incorporate this trait intocultivars and particular species.

Apomictic reproduction in woody plant speciesand cultivars is found in many Citrus (9), mango(Mangifera), and some apple (Malus) species (52).Although apomixis produces genetically uniformseedlings, it is not necessarily useful for growing spe-cific fruit cultivars because of undesirable juvenile ten-dencies, such as thorniness, excess vigor, and delayedfruiting. On the other hand, these characteristics makeapomictic seedlings useful as rootstocks, characteristicsexploited extensively in Citrus.

In apomixes, the seedling population is immedi-ately stabilized as a “true-breeding” line withoutseedling variation. Such plants exhibit the apomicticcycle and express typical juvenile traits of the seedlingpopulation. Apomixis is particularly appropriate for

(a) (b)

(c)

Female flower

Male flower

Figure 5–9Conifers are usually wind pollinated. Male strobili (a) releasepollen that is deposited on the female cone (b). Trueflowering plants (Angiosperms) developed along with insectpollinators. Wind pollination is a derived character that isusually associated with reduced flower parts (i.e., no petals)and unisexual flowers (c) as illustrated for chestnut (Castanea).

facultative apomicticA plant in which bothsexual and asexualembryos are producedby the same plant.

obligate apomicticA plant in which all theembryos are apomictic.



plants whose value lies in their vegetative characteris-tics—as occurs in forages and grasses—rather than inplants whose value depends on fruiting characteristics.

CATEGORIES OF SEED-PROPAGATED CULTIVARS AND SPECIESHerbaceous Annual, Biennial, and Perennial PlantsLandraces Historically, farmers throughout theworld have maintained seed-propagated plants by sav-ing selected portions of the crop to be used to producethe next cycle. These populations, called landraces,

evolved along withhuman societies andare still found in someparts of the world (56).These populations are

variable but identifiable and have local names. Thispractice results in genetic populations adapted to alocalized environment. Their inherent variability pro-vides a buffer against environmental catastrophe andpreserves a great deal of genetic diversity (Fig. 5–10).

Changes in cropping patterns have occurred dur-ing the 20th Century, particularly since about 1960.Many of the older populations around the world arebeing replaced by modern cultivars, which tend to beuniform and high yielding, particularly when grown inconjunction with high irrigation and fertility inputs.

Sometimes, new cultivars lack adaptation to local envi-ronments. Although the trend has been to increase theworld supply of essential food crops, concerns havebeen raised that a parallel loss of genetic diversity andgermplasm has occurred. Exploration and conservationefforts have expanded to maintain these important rawmaterials for future use (19).

Cultivars A cultivar is a uniform and stable plantpopulation that possesses recognizably distinct char-acteristics. Stated another way, a cultivar is a plantpopulation that shows a minimum of variation, thatcan be propagated true-to-type for at least one charac-teristic, and is unique compared to the wild species orother cultivars. The term variety is often used inter-changeably with cultivar especially when describingflower and vegetable populations. Care should betaken not to confusevariety with the con-cept of a true botani-cal variety (varietas orvar.) that describes atype of naturally occur-ring population.

Categories of seed-propagated cultivars includeopen-pollinated, lines, hybrids, synthetic, F2, andclonal cultivars.

Open-pollinated cultivars can be maintained incross-pollinated species that produce a relatively homo-geneous population for specific traits important forproduction of that crop. Open-pollinated seed is oftencheaper to produce compared to hybrid seed because

landrace Primitivevarieties developed andmaintained before themodern era of genetics.

Figure 5–10A landrace of soybeans (Glycine) inAfrica showing the diversity inherent inseeds saved over many generations.

botanical varietyA population of plantsoriginating in naturethat are within onespecies but arephenotypically distinct.



they do not require hand pollination to maintainthe cultivar. However, because open-pollinated culti-vars are a genetically heterogenic population, they canbe more variable than hybrids (41).

Historically, many open-pollinated vegetableand flower varieties were maintained by families intheir “kitchen gardens.” Many of these varieties havesince been maintained by generations of gardenersand local farmers and are being offered as heirloomvarieties. The preservation and distribution of infor-mation concerning these varieties has been an objec-tive of certain groups including Seed Savers Exchange,Inc., in Decorah, Iowa (4, 54, 59). There are alsonumerous commercial flower and vegetable crops pro-duced as open-pollinated cultivars including Begonia,marigold (Tagetes), cucumber (Cucumis), and squash(Cucurbita).

Lines result inseedling populationswhose genotype is main-tained relatively intactduring consecutive gen-erations. These may be

maintained as self- or cross-pollinated lines. An impor-tant type of seed population in this category is theinbred line, which are mainly used as parents for laterproduction of F1 hybrids (55).

Hybrid Cultivars include groups of individualsreconstituted each generation from specific parents. F1hybrids are the first generation of a planned cross. Forseed production, they result from the cross betweenseedling populations of two or more inbred lines.When crossed with another inbred line, the result is apopulation of uniform, but heterozygous, plants. Oftenthese populations exhibit greater vigor than the parentsdue to hybrid vigor (heterosis), depending on the com-bining ability of the parents. Hybridization is a meansof “fixing” the genotype of the population similar tothat described for self-pollinated lines. Hybrid lines

were first produced incorn (Zea mays) (55)but have since beenapplied to many agro-nomic, vegetable, andflower crops (1, 26).

Hybrids may be produced between two inbredlines (single-cross), two single-crosses (double-cross),an inbred line and an open pollinated cultivar (top-cross), or between a single-cross and an inbred line(three-way cross) (55). Seeds saved from the hybridpopulation normally are not used for propagation

line A population ofseedling plants whosegenotype is maintainedto a specific standard inconsecutive generations.

hybrid line A seedlingpopulation that isproduced by cross-pollinating two or moreparental lines.

because in the next generation, variability in size, vigor,and other characteristics may appear.

Synthetic cultivars are derived from the firstgeneration of the open cross-fertilization of severallines or clones. Forexample, ‘Ranger’ alfalfaseed is made from inter-cropping five seed-propagated lines thatresults in genetically dis-tinct but phenotypicallysimilar seedlings in theseeded crop. Othercrops in this categoryinclude pearl millet (Pennisetum glaucus), bromegrass(Bromus) and orchard grass (Dactylis).

F2 cultivars are derived for open-pollination ofan F1 hybrid. Some flower crops, (Petunia, pansy(Viola), and Cyclamen) and vegetables (tomato andmelon) can be maintained as F2 populations.

Clonal seed cultivars are maintained throughapomictic seed production (25, 51). Apomixis occurswhen an embryo is asexually produced and does notdevelop from fertilization of two gametes (28). Theresult is a clonal copy of the parent plant. Apomixisis discussed in detail in Chapter 4. The degree ofclonal seed production depends on whether the specieshas a facultative or obligate form of apomixis. Inspecies with facultative apomixis, both apomicticand sexual seeds are produced, sometimes within thesame seed. Bluegrass (Poa pratensis) falls into thiscategory. Other species show essentially 100 percentobligate apomictic seed production. Examples includeBahia grass (Paspalum notatum) and buffelgrass(Pennisetum ciliare).

Woody Perennial PlantsWild Populations In nature, most species can berecognized as a more or less phenotypically (and geno-typically) uniform seedling population that hasevolved over time through consecutive generations tobe adapted to the environment at a particular site. Ifthe species covers a wide area, local variation in envi-ronment can result in different populations becomingadapted to different areas even though the plants mayappear phenotypically similar. Plants within thespecies that show morphological differencescompared to the species, but that are reproduced byseed, may be designated as botanical varieties orvarietas or var. The term form indicates a particular

synthetic lineAcultivar seedlingpopulation that isproduced by combininga number of separatelydeveloping lines toproduce a heterozygousbut homozygousCultivar.



phenotypic difference,as a blue or white color.Subgroups of a particu-lar species that aremorphologically simi-lar but specificallyadapted to a particularenvironmental nicheare known as ecotypes.Variations that occur

continuously between locations are known as clines(35, 43).

Provenance The climatic and geographical localitywhere seed is produced is referred to as its seed originor provenance (2, 3, 38, 54). Variation can occuramong plants associated with latitude, longitude, andelevation. Differences may be shown by morphology,

physiology, adaptationto climate and to soil,and in resistance todiseases and insects.Natural plant popula-tions growing withina given geographical

area over a long period of time evolve so that theybecome adapted to the environmental conditions atthat site.

Consequently, seeds of a given species collectedin one locality may produce plants that are completelyinappropriate to another locality. For example, seedscollected from trees in warm climates or at low alti-tudes are likely to produce seedlings that will not stopgrowing sufficiently early in the fall to escape freezingwhen grown in colder regions. The reverse situation—collecting seed from colder areas for growth inwarmer regions—might be more satisfactory, but italso could result in a net reduction in growth resultingfrom the inability of the trees to fully utilize the grow-ing season because of differences in the response tophotoperiod (61).

Distinct ecotypes have been identified by meansof seedling progeny tests in various native forest treespecies, including Douglas-fir (Pseudotsuga menziesii),ponderosa pine (Pinus ponderosa), lodgepole pine(Pinus murrayana), eastern white pine (Pinus strobus),slash pine (Pinus caribaea), loblolly pine (Pinus sp.),shortleaf pine (Pinus echinata), and white spruce (Piceaalba) (66). Other examples include the Baltic race ofScotch pine (Pinus sylvestris), the Hartz Mountainsource of Norway spruce (Picea abies), the Sudeten(Germany) strain of European larch (Larix), the

Burmese race of teak, Douglas-fir (Pseudotxuga men-ziesii) from the Palmer area in Oregon, ponderosa pine(Pinus ponderosa) from the Lolo Mountains inMontana, and white spruce (Picea alba) from thePembroke, Ontario (Canada) area (16, 20, 31).Douglas-fir (Pseudotsuga menziesii) has at least threerecognized races—viridis, caesia, and glauca—with var-ious geographical strains within them that show differ-ent adaptations. For instance, progeny tests showedthat a viridis strain from the United States West Coastwas not winter-hardy in New York but was well suitedto Western Europe. Those from Montana andWyoming were very slow growing. Trees of the glauca(blue) strain from the Rocky Mountain region werewinter-hardy but varied in growth rate and appearance.Strains collected farther inland were winter-hardy andvigorous; similar differences occurred in Scotch pine(Pinus sylvestris), mugho pine (P. mugho), Norwayspruce (Picea abies), and others.

Improved Seed Sources Nursery propagation bywoody plant seed can be upgraded significantly by theselection and development of improved seed sources.This practice applies to the production of rootstocksfor fruit and nut trees (49, 58), shade trees (20, 43),and trees in the landscape. Likewise, foresters havebeen engaged in recent years in “domesticating” andupgrading forest tree production over that of “local”seed (38, 40, 60).

Elite Trees. Foresters refer to single seed-source treeswith a superior genotype, as demonstrated by a progenytest, as elite trees. Nursery progeny tests can identifyand characterize specificseed sources (e.g., forlandscape or Christmastree uses).

Clonal Seed Sources.Superior (elite) seed-source trees can bemaintained as clones inseed orchards to pre-serve the genotype ofthe parent. Seeds fromthis clonal source are then used to produce seedlingtrees in the nursery. This procedure is used to pro-duce rootstock seed for fruit and nut cultivars. Forinstance, ‘Nemaguard’ is a peach hybrid whose nema-tode-resistant seedling progeny are used for almond,peach, plum, and apricot trees in central California(49). Named cultivars of ornamental trees have been

ecotype A geneticallydistinct group of plantswithin a species that isadapted to a specificecological location.

cline Continuousgenetic variation fromone area to another inecological adaptation.

provenance A forestryterm used to indicatethe climatic andgeographical localitywhere the seedoriginated.

elite tree Anindividual tree withoutstanding phenotypiccharacteristics to beused as a seed source.

clonal seed sourceCultivar maintained as aclone selected forproducing outstandingseedlings.



identified as producing uniform, superior seedlingprogeny (16).

Selected Families.Genetic improvementof forest-tree species hasbrought about familyselection by growingprogeny trees either

from controlled crossing or selection from single open-pollinated superior (elite) trees. Seed orchards then maybe established either from seedlings of these trees orestablished by grafting the parent trees. A minimumnumber of individual genotypes are selected—usuallyaround twenty-five—to avoid the dangers of inbreed-ing and limits to the genetic range. Progeny trees areplanted in test sites and evaluated for various forestrycharacteristics. Over time, superior sources are identi-fied and preserved as parents to produce the next gener-ation of new families of improved seed genotypes.Inferior seed sources are identified and eliminated.

Hybrid Seed Sources. F1 hybrids of two species usuallyproduce uniform populations of plants in the same man-ner as hybrid seeds of corn and other inbred lines (seep. 157). For example, hybridization has been valuable inproducing vigorous almond × peach hybrids for almondand peach rootstocks (33, 34), Paradox hybrids (Juglanshindsii × J. regia) for walnuts and fast-growing poplarsfrom crosses with North American and European poplars(like Populus trichocarpa. × P. deltoides). Forest treehybrids, such as Pinus rigida × P. taeda in Korea and Larixdecidua × L. leptolepsis in Europe are not necessarily uni-form, however, but have been a focus of improved forests.Because of expense and uncertainty of production, seedsof F1 hybrids of the forest trees have been used to produceF2 seedling populations. The more vigorous hybrid plantsdominate and the weaker trees are crowded out.

CONTROL OF GENETICVARIABILITY DURING SEEDPRODUCTIONHerbaceous Annual, Biennial, and Perennial PlantsIsolation Isolation is used to prevent mechanicalmixing of the seed during harvest and to preventcontamination by unwanted cross-pollination with adifferent but related cultivar. Isolation is achieved pri-marily through distance, but it can also be attained byenclosing plants or groups of plants in cages, enclos-ing individual flowers, or removing male flower parts(i.e., de-tasseling corn) and then manually applyingpollen of a known source by hand or various otherdevices (Fig. 5–11). On a large scale, this goal can beachieved by using male-sterile parents (36). In anumber of crop species [e.g., tobacco (Nicotiana)and onion (Allium)], specific genes have been identi-fied that prevent normal formation of the male(pollen) reproductive structures (Fig. 5–12). Thismeans that no viablepollen is produced. Themost common form ofpollen sterility is cyto-plasmic male sterility,which is a complex inter-action between nuclearand mitochondrial plantgenes (12). Such traits canbe bred into parental linesof specific cultivars forthe production of hybridseed. Using molecularbiology to induce male sterility has also become apotential strategy to limit gene flow from transgenicplants into the environment (11).

selected familiesConsecutive groups ofprogeny trees relatedby origin and showingsuperior characteristics.

pollen sterilityGenetic phenomenonin which the pollen isnonviable.

hybrid seed Seedproduced by thecrossing of twodissimilar parents,usually producedwhen made betweenspecies.

(a) (b)

Figure 5–11Isolation is used to preventunwanted cross-pollinationduring seed production. (a) An onion hybrid cross beingisolated with an individualsac. (b) A small cage whereflies will be introduced topollinate onions for hybridproduction. Female plants inthe cage are made sterile toprevent self-pollination.



Figure 5–12Male sterility in tobacco (Nicotiana). Note how the flower onthe left lacks developed anthers compared to the perfectflower on the right with elongated and fertile anthers (arrow).

Self-pollinated cultivars of herbaceous plantspecies need only to be separated to prevent mechanicalmixing of seed of different cultivars during harvest. Theminimum distance usually specified between plots is 3 m (10 ft), but may be up to 50 to 65 m (150 to 200 ft)depending on the degree of cross-pollination capacity inthe crop. For example, bell pepper is a self-pollinatingcrop but, given the opportunity, will cross-pollinate to ahigh degree from bee pollinators. Careful cleaning of theharvesting equipment is required when a change is madefrom one cultivar to another. Sacks and other containersused to hold the seed must be cleaned carefully toremove any seed that has remained from previous lots.

More isolation is needed to separate cultivarscross-pollinated by wind or insects. The minimum dis-tance depends on a number of factors:

• the degree of natural cross-pollination• the relative number of pollen-shedding plants• the number of insects present• the direction of prevailing winds

The minimum distance recommended for insect-pollinated herbaceous plant species is 0.4 km (1⁄4 mi) to1.6 km (1 mi). The distance for wind-pollinated plantsis 0.2 km (1⁄8 mi) to 3.2 km (2 mi), depending onspecies.

Effective cross-pollination usually can take placebetween cultivars of the same species; it may also occurbetween cultivars of a different species but in the samegenus; rarely will it occur between cultivars belonging toanother genus. Since the horticultural classification maynot indicate taxonomic relationships, seed producersshould be familiar with the botanical relationshipsamong the cultivars they grow. It is also important to iso-late GMO (genetically modified organisms) crops fromnon-GMO seed crops of the same cultivar or species.

In seed production areas, such as regions ofOregon, Washington, and California, seed companiescooperate to locate seed production fields at appropri-ate isolation distances from each other. The fields arelocated on “pinning” maps (each colored pin indicatinga field and crop type), and the maps are located inCounty Extension offices within the production areas.Recently, these have also become available as virtualmaps on the Internet, as is the case in California.

Roguing The removal of off-type plants, plants ofother cultivars, and weeds in the seed production fieldis known as roguing (37).During the development ofa seed-propagated cultivar,positive selection is practicedto retain a small portion ofdesirable plants and to maxi-mize the frequency of desir-able alleles in the population. During seed production,roguing following visual inspection exerts selection byeliminating the relatively small population that is not “trueto type,” thus keeping the cultivar “genetically pure.”

Off-type characteristics (i.e., those that do notconform to the cultivar description) may arise becauserecessive genes may be present in a heterozygous condi-tion even in homozygous cultivars. Recessive genes aris-ing by mutation would not be immediately observed in the plant in which they occur. Instead, the plantbecomes heterozygous for that gene, and, in a later gen-eration, the gene segregates and the character appearsin the offspring. Some cultivars have mutable genesthat continuously produce specific off-type individuals(45). Off-type individual plants should be rogued outof the seed production fields before pollination occurs.Systematic inspection of the seed-producing fields bytrained personnel is required.

Other sources of off-type plants include contami-nation by unwanted pollen due to inadequate isolationor volunteer plants arising from accidentally plantedseed or from seed produced by earlier crops. Seed pro-duction fields of a particular cultivar should not havegrown a potentially contaminating cultivar for a num-ber of preceding years.

Weeds are plant species that have been associatedwith agriculture as a consequence of their ability toexploit disturbed land areas when cultivation occurs(29). Some weed species have evolved seed types thatclosely resemble crop seeds and are difficult to screenout during seed production.

Seedling Progeny Tests Planting representative seedsin a test plot or garden may be desirable to test for

roguing The act ofremoving off-typeplants, weeds, andplants of othercultivars in seedproduction fields.



trueness-to-type. This procedure is used in the devel-opment of a cultivar to test its adaptability to variousenvironments. The same method may be necessary totest whether changes have occurred in the frequency ofparticular genes or new gene combinations may havedeveloped during seed increase generations. Thesechanges can result from selection pressure exerted bymanagement practices or environmental interaction.For example, intensive roguing may result in a genetic

drift due to changes inthe frequency of partic-ular genes or gene com-binations (24). Shiftsmay also occur due toenvironmental expo-sure in a growing area

which is different from the initial selection area.Seedlings of particular genotypes may survive betterthan others and contribute more to the next generation.If sufficiently extensive, genetic drift could producepopulations of progeny plants that differ somewhatfrom those of the same cultivar grown by other produc-ers. Or the cultivar may have changed from the originalbreeder’s seed.

Problems can result if seed crops of particularperennial cultivars are grown in one environment (suchas a mild winter area) to produce seed to be used in adifferent and more severe environment (such as an arearequiring cold-hardiness). This situation has occurred,for example, with alfalfa (24) where rules for produc-tion of forage crop seed in a mild winter area can spec-ify only one seedling generation of increase.

Woody Perennial PlantsUse of Local Seed “Local seed” means seed from anatural area subjected to a restricted range of climaticand soil influences. As applied to forest tree seed, thisusually means that the collection site should bewithin 160 km (100 mi) of the planting site andwithin 305 m (1,000 ft) of its elevation. In theabsence of these requirements, seed could be usedfrom a region having as nearly as possible the sameclimatic characteristics. The reason for the historicalemphasis on local seed is the phenomenon previouslydefined as seed origin or provenance (27). The use oflocal seed for herbaceous and woody plants is partic-ularly important in the effort to restore any nativeecosystem where the use of exotic species would beinappropriate (43).

Pure Stands. Pure stand refers to a group of pheno-typically similar seedling plants of the same kind. This

concept could applyboth to plants growingin a natural environmentor in a planting such as awood lot. These popula-tions are useful in seedcollection because cross-pollination would likely occurfrom among this group of plants and one can judgeboth the female and the male parents. Although theindividuals are likely to be heterozygous, they shouldproduce good seeds and vigorous seedlings. The popu-lation should be homogeneous and reproduce theparental characteristics.

Phenotypic Tree Selection Versus Genotypic TreeSelection. Phenotypic selection refers to evaluationof a seed source throughvisual inspection of thesource plant(s). The basisof this procedure is thatmany important traits inforestry—such as stemform, branching habit,growth rate, resistance to diseases and insects, presenceof surface defects, and other qualities—are inheritedquantitatively. Geneticists refer to this relationship ashigh phenotypic correlation between parents and off-spring. In practical terms, this means that the parentalperformance can be a good indicator of the perform-ance of the offspring (57).

When individual trees in native stands show asuperior phenotype, foresters call them “plus” or elitetrees and sometimes leave them for natural reseeding oras seed sources. Such dominant seed trees may con-tribute the bulk of natural reseeding in a given area.

Genotypic selection refers to evaluation of a seedsource based on theperformance of theirseedling progeny test(39). Seeds may be pro-duced by open pollina-tion (OP) where onlyone parent is known. Or the test may be made from acontrolled cross, where both parents are known and thecontributions of each can be evaluated. A progeny testestablishes the breeding value of a particular seedsource (5) because genetic potential is based on actualperformance of the progeny.

A representative sample of seeds is collected,planted under test conditions, and the progenyobserved over a period of years. A high correlationbetween the average phenotypic traits of the parent(s)

genetic drift Changein the frequency ofspecific genes as aresult of environmentalor other types ofselection.

pure stand Aninterbreeding group ofphenotypically similarplants of the same kindgrowing in a given site.

phenotypic selectionSelection of a seedsource based on thephenotypicappearance of thesource tree.

genotypic selectionSelection of a seedsource based on thephenotypic appearanceof the seedling progeny.



Figure 5–13Pedigree system for seed production. See text for discussion.

and the average pheno-typic response of theoffspring is referred toas high additive heri-tability and justifiesusing the “best” treesfor seed sources of thenext generation (38). Alow statistical correla-tion between parentand progeny character-istics is referred to aslow additive heritabil-ity in that the desiredtraits of the progeny

cannot be predicted from inspection of the parents.Progeny testing is useful to verify the suitability of

individual seed sources for future seed collecting. Theprocedure is an important component to the improve-ment of woody plants whether in forestry or horticulture.

SEED PRODUCTION SYSTEMSHerbaceous Annual, Biennial, and Perennial PlantsCommercial Plantings Traditional seed selection ofherbaceous plants utilized a portion of the seed from oneyear’s crop to plant a crop for the next year. This systemwould be satisfactory for self-pollinated cultivars thatare easy to maintain genetically. For cross-pollinatedcultivars, knowledge of the production requirements ofindividual crops is needed and specific conditions arepracticed depending upon the plant (3). Note, how-ever, that inadequacies of this method led to its replace-ment by the pedigree system.

Pedigreed Stock System (4, 42) Commercial seed pro-duction of most self-pollinated and cross-pollinated linesis carried out in three steps (Fig. 5–13). The purpose of apedigreed stock system is to maintain genetic purity

through consecutiveseed generations fol-lowing appropriatestandards of isolation,inspection, and rogu-ing (with high costs) atthe initial release withdecreased standards(and lower costs) in thedistribution of com-

mercial seed. The overall program includes three phases.Phase 1 includes the development phase, which ends

with the production of a small quantity of seeds(breeder’s seed) that is maintained by the originatinginstitution as the primary reference for the cultivar. Phase2 is a maintenance phase in which a quantity of seedcalled foundation seed (for certified seed classes; see Box5.1, page 154) or stock seed (in commercial enterprises)is maintained under high standards of isolation, inspec-tion, and roguing. Phase 3 is the distribution phase, whichmay include two steps: a second-generation increase blockand a third-generation block to produce commercial seedfor distribution to the public. A foundation planting orig-inates only from breeder’s seed or another foundationplanting. An increase block originates only from a foun-dation seed or another increase planting. A seed produc-tion planting originates from foundation seed orincrease block seed. This entire production process iscarried out either by large commercial firms or groupsof independent growers joined within a CropImprovement Association to produce certified seed.

Seed Certification (4, 13, 17, 42) Seed certification isa legalized program that applies the previously men-tioned principles to theproduction of specificseed-propagated plant cul-tivars to ensure the main-tenance of seed purity.The system was estab-lished in the United Statesand Canada during theearly 1920s to regulate the

high additiveheritability Highcorrelation betweenphenotypic traits ofoffspring with thephenotypic traits of theparents.

low additiveheritability Lowcorrelation betweenphenotypic traits ofoffspring with thephenotypic traits of theparents.

pedigreed stocksystem A controlledseed-production systemof consecutivegenerations withstandards to maintaingenetic purity leading to commercialdistribution.

seed certificationA system of seedproduction utilizingpedigreed stock prin-ciples, which providesfor legally enforceablestandards of quality and genetic purity.



BOX 5.1 GETTING MORE IN DEPTH ON THE SUBJECT

CLASSES OF CERTIFIED SEEDS

Breeder’s seed: that which originates with the sponsoringplant breeder or institution and provides the initial sourceof all the certified classes. Foundation seed: progeny ofbreeder’s seed that is handled to maintain the higheststandard of genetic identity and purity. It is the source ofall other certified seed classes but can also be used to pro-duce additional foundation plants. (Select seed is a com-parable seed class used in Canada.) Foundation seed islabeled with a white tag or a certified seed tag with theword “foundation.” Registered seed: progeny of founda-tion seed (or sometimes of breeder’s seed or otherregistered seed) produced under specified standards

approved and certified by the certifying agency anddesigned to maintain satisfactory genetic identity andpurity. Bags of registered seed are labeled with a purpletag or with a blue tag marked with the word “registered.”Certified seed: progeny of registered seed (or sometimesof breeder’s, foundation, or other certified seed) that isproduced in the largest volume and sold to crop produc-ers. It is produced under specified standards designed tomaintain a satisfactory level of genetic identity and purityand is approved and certified by the certifying agency.Bags of certified seed have a blue tag distributed by theseed-certifying agency.

commercial production of new cultivars of agriculturalcrops then being introduced in large numbers by stateand federal plant breeders. The principles (as describedfor the pedigreed stock system) and accompanying regu-lations of seed maintenance were established through thecooperative efforts of public research, extension, regula-tory agencies, and seed-certifying agencies known asCrop Improvement Associations, whose membershipincluded commercial producers. These organizationswere designated by law through the Federal Seed Act(1939) to conduct research, establish production stan-dards, and certify seeds that are produced under thesestandards. Individual state organizations are coordinatedthrough the Association of Official Seed CertifyingAgencies (AOSCA) (4) in the United States and Canada.Similar programs exist at the international level wherecertification is regulated through the Organization forEconomic Cooperation and Development (OECD).

The principal objective of seed certification is toprovide standards to preserve the genetic qualities of acultivar. Other requirements of seed quality also maybe enforced as well as the eligibility of individual culti-vars. The seed-certifying agency may determine pro-duction standards for isolation, maximum percentageof off-type plants, and quality of harvested seed; makeregular inspections of the production fields to see thatthe standards are being maintained; and monitor seedprocessing.

The international OECD scheme includes simi-lar classes but uses different terms. These include basic(equivalent to either foundation or registered seed),certified first-generation (blue tag) seed, and second-generation (red tag) seed.

Hybrid Seed Production (1) Hybrid cultivars (Fig. 5–14)are the F1 progeny of two or more parental lines. Parent

plants are maintained either as inbred lines (corn, onion)or as vegetatively propagated clones (asparagus). Thesame standards of isolation as for nonhybrid seed pro-duction may be required. To mass-produce hybrids,some system must be used to prevent self-pollinationand to enforce cross-pollination. Hand pollination issometimes practiced to produce seed in crops or situa-tions in which the production of seed per flower is veryhigh and/or the high value of the seed justifies theexpense (Fig. 5–15, page 156). Hand pollination is usedto produce some hybrid flower seeds and in breedingnew cultivars (Fig. 5–16, page 157).

Perennial SourcesCommercial Sources Seeds for fruit and nut cropshistorically have been collected more or less successfullyfrom commercial orchards particularly where the spe-cific cultivar or origin is known. Fruit tree seeds such asapple (Malus), pear (Pyrus), and peach (Prunus) havebeen collected from canneries and dry yards where spe-cific commercial cultivars, such as ‘Lovell’ peach, areused. Pure stands of local seedling landscape trees mightbe used. In several forest-tree species, seed has been col-lected from phenotypically above-average trees in com-mercial plantations. In New Zealand, seed from suchtrees is designated “CS” (“climbing selects”) and ratedhigher in value than the seed from the surroundingtrees, but below that of seed orchards or from well-tested families.

Seed-Collection ZonesA seed-collection zonefor forest trees is an areawith defined boundariesand altitudinal limits inwhich soil and climate

seed-collection zoneNaturally occurring zone(forest plants) designatedby elevation, latitude, andlongitude that identifies aspecific seed source.



Figure 5–14Hybrid seed corn production. Four inbred lines are produced to be used as parents for cross-pollination utilizing either detasseling(removal of male flower) or a pollen-sterile parent. The resulting F1 plants are used as parents of the next (F2) generation which arethen sold for commercial crop production. The individual progeny plants are highly heterozygous, but the population is highlyhomogeneous, showing high vigor and production. Redrawn from USDA Yearbook of Agriculture 1937, Washington, D.C.: U.S. Government Printing

Office.

are sufficiently uniform to indicate a high probabilityof reproducing a single ecotype. Seed-collectionzones, designating particular climatic and geographi-cal areas, have been established in most of the forest-tree–growing areas in the world (61). California, forexample, is divided into six physiographic and cli-matic regions, 32 subregions, and 85 seed-collectionzones (Fig. 5–17, page 158). Similar zones are estab-

lished in Washington and Oregon and in the centralregion of the United States.

Seed-Production Areas (43)A seed-production areacontains a group of plantsthat can be identified and setaside specifically as a seed

seed-productionarea An area ofsource plantsspecifically utilizedfor seed collection.



(a) (b)

(c) (d)

(e) (f )

Figure 5–15(a and b) Hybrid pollination in petunia. Hand pollination involves removal of the male anthers (emasculation) before theflower opens; (c and d) followed by transfer of the pollen to the receptive stigma. Pollen is collected and stored dry in small vials at cold temperature. Pollen is transferred to the stigma using a small transfer stick or brush. (e) A successfulpollination/fertilization is evident by continued development (red arrow). (f) Seed yield per capsule is high and the seed isvery valuable justifying the use of hand labor.

species that would interfere with the operations.Competing trees can be eliminated to provide adequatespace for tree development and seed production. Inforestry, an isolation zone at least 120 m (400 ft) wide

source. Seed plants within the area are selected for theirdesirable characteristics. The value of the area might beimproved by removing off-type plants, those that donot meet desired standards, and other trees or shrub



(a)

(c)

(b)

Figure 5–16F1 hybrid cultivars of many vegetable and flowering annualsare created following large-scale hand pollination in a con-trolled environment. (a) Removing male parts in snapdragonprior to hand pollination. (b) Hand pollination. (c) Pepper fruitprior to harvest for seed extraction.

from which off-typeplants are removedshould be establishedaround the area.

Seed orchards Seedorchards are estab-lished to produce treeseeds of a particular

origin or source. For example, fruit tree nurseriesmaintain seed orchards to produce seeds of specificrootstock cultivars under conditions that will preventcross-pollination and the spread of pollen-borneviruses. A clonal cultivar such as ‘Nemaguard’ peachis budded to a rootstock, planted in isolation to avoidchance cross-pollination by virus-infected commer-cial cultivars, and grown specifically for rootstockseed production as part of the nursery operations.

seed orchardA planting used in forestryor in fruit tree nurseries tomaintain seed sources asseedling populations ofselected seed families orof a clone (fruit and nuttrees) or collections ofclones (forestry).


TREE CERTIFICATION CLASSES

Certification of forest-tree seeds is available in somestates and European countries similar to that for crop seed(17, 18, 61). Recommended minimum standards are givenby the Association of Official Seed Certifying Agencies (4).Forest-tree seed have different standards than agriculturalseeds.

Tree certification classes are defined as source-identified: tree seed collected from natural stands wherethe geographic origin (source and elevation) is known andspecified or from seed orchards or plantations of knownprovenance, specified by seed-certifying agencies. These

seeds carry a yellow tag. Selected: tree seed collectedfrom trees that have been selected for promising pheno-typic characteristics but have not yet been adequatelyprogeny-tested. The source and elevation must be stated(44). These seeds are given a green label. Certified: twotypes of seed are recognized. Seeds are from trees grow-ing in a seed orchard whose genetic potential is based onphenotypic superiority. These are identified by a pink tag.When seedlings or seeds have been proven to be geneti-cally superior in a progeny test, they are classified astested and identified by a blue tag.



Physiographic and climaticregion boundaries

Physiographic and climaticsubregion boundaries

Zones

Figure 5–17Seed collection zones in California. The 85 zones areidentified by a three-digit number. The first gives 1 ofthe 6 major physiographic and climatic regions, thesecond gives 1 of the subregions, of which there are32, and the third gives the individual zone. 0 = Coast;1 = South coast; 3 = North coast; 5 = Mountains ofSierra Nevada and Cascade ranges; 7 = Northeastinterior; 9 = Valley and desert areas, further dividedinto the central valley (6), southern California (9), andthe desert areas (7). Redrawn from G. H. Schubert and R. S.

Adams. 1971. Reforestation Practices for Conifers in California.

Sacramento: Division of Forestry.

Fruit-tree rootstock clones that are self-pollinated areplanted in solid blocks. An isolation zone 120 m(400 ft) wide should be established around theorchard to reduce pollen contamination from othercultivar sources. The size of this zone can be reducedif a buffer area of the same kind of tree is presentaround the orchard. Hybrid seed production involvesplanting both the parental clones in adjoining rowsunder the same condition.

Three general types of seed orchards are used forforest trees (42, 61): (a) seedling trees produced fromselected parents through natural or controlled pollina-tion; (b) clonal seed orchards in which selectedclones are propagated by grafting, budding, or rootingcuttings; and (c) seedling-clonal seed orchards inwhich certain clones are grafted onto branches ofsome of the trees. The choice depends on theparticular strategy used in the seed improvementprogram.

A site should be selected for good seed produc-tion. Forest trees and most other native species should

include a range of genotypes in a suitable arrangementto ensure cross-pollination and to decrease the effects ofinbreeding. Seven to thirty unrelated genotypes havebeen recommended to avoid this problem in a purelyclonal orchard.

Nursery Row Selection Sometimes phenotypicallyunique individuals appear in nursery seed populationsplanted in the nursery row and can be identified visu-ally. This is referred to as nursery row selection.Identification requiresthat the character bedistinctive in vigor,appearance, or both(Fig. 5–11). For exam-ple, Paradox hybrid wal-nut seedlings (Juglansregia � ( J. hindsii) used as a rootstock in California areproduced by planting seeds of specific seed tree sourcesof black walnut (Juglans hindsii). Once germinationtakes place, leaf characteristics, bark color, and greater

nursery row selectionA system of selectionwhere specific progenytrees can be identified inthe nursery row due tophenotype.




PLANT VARIETY PROTECTION ACT

Breeders of a new seed-reproduced plant variety (cultivar)in the United States may retain exclusive propagationrights through the Plant Variety Protection Act, estab-lished in 1970 (7), and revised in 1994 (46). The breederapplies to the U.S. Department of Agriculture for a PlantVariety Protection Certificate. For one to be granted, thecultivar must be “novel”: it must differ from all knowncultivars by one or more morphological, physiological, orother characteristics. It must be uniform; any variationmust be describable, predictable, and acceptable. It

must be stable (i.e., essential characteristics must remainunchanged during seed propagation). A certificate isgood for 20 years. The applicant may designate that thecultivar be certified and that reproduction continue onlyfor a given number of seed generations from the breederor foundation stock. If designated that the cultivar becertified, it becomes unlawful under the Federal SeedAct to market seed by cultivar name unless it is certified.The passage of this law has greatly stimulated commer-cial cultivar development.

vigor of the hybrid seedlings allow identification of thedesired hybrid seedlings, whereas the black walnutseedlings are rogued out of the nursery row or separatedat a later date. Genotypic progeny tests based on previ-ous commercial experience indicate which walnut treesources produce the highest percentages of hybrids, pre-sumably from natural crossing with surrounding Persianwalnut (J. regia) orchards.

Sometimes the phenotype of the propagatedplant is sufficiently striking as to allow for selection inthe nursery. For example, blue seedlings of theColorado spruce (Picea pungens) tend to appear amongseedling populations having the usual green form.Differences in fall coloring among seedlings ofLiquidambar and Pistacia chinensis necessitate fallselection of individual trees for landscaping.

DISCUSSION ITEMS

Propagators of many plant species and cultivars may notbe involved directly in the selection and handling of theseeds used but depend on the skill and knowledge of thespecialized seed industry. Nothing is more important,however, than using seeds that are true-to-type and true-to-name. Consequently, knowledge of the basic princi-ples and practices that are required to produce geneticallypure seeds is important to propagators whether or notthey are directly involved in seed selection.

1. From the propagator’s standpoint, why do youthink crop plants such as wheat, rye, and barleyplayed such an important role in human history?

2. What are some major reasons why seed producerslike to produce hybrid seed lines?

3. What are the differences and similarities amongapomictic, inbred, and hybrid seed lines?

4. What is the function of seedling progeny tests inseed production?

5. Why is the seed origin (provenance) important tousers of tree crops?

6. Would it be better to collect seeds from a singlewoody plant or from multiple plants?

REFERENCES

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Principles and Practices of Seed Selection