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Hybrid Seed Production in Tomato R.T. Opeña, J.T. Chen, T. Kalb and P. Hanson Introduction Hybrid tomato varieties have many advantages compared to open-pollinated varieties. Hybrids usually produce higher yields. They generally mature earlier and more uniformly. Many hybrids have better fruit quality and disease resistance. With all of these advantages, many farmers prefer to sow hybrid seeds in spite of the higher seed costs. The demand for hybrid tomato seeds can open a new market for growers interested in seed production. Hybrid tomato seed production is not easy. First, it requires much labor (see page 5). Fortunately, this is not a problem in developing countries where affordable labor is available. Second, it requires the mastery of special skills and a close attention to detail. This publication will teach these skills. Crop Management Climate Ideal growing conditions are needed to produce high quality seed. Tomatoes grow best in the dry season under day temperatures of 21-25°C and night temperatures of 15-20°C. Vines will struggle to set fruit if temperatures exceed 30°C. Humidity is an important factor. Humidity levels higher than 60% at the time of fruit maturity will increase disease problems and reduce seed yields. Seed production during the rainy season generally leads to low yields and poor seed quality. Field Requirements Avoid planting your tomato seed crop in a field previously planted with tomato, pepper, eggplant or other solanaceous crops. This will prevent the build-up of diseases and insects. Growing tomato after paddy rice reduces the incidence of diseases and nematodes. Optimum soil pH for tomato is 6.0 to 7.0; disor- ders such as blossom end rot are common if soil pH is lower than 5.5. Figure 1. Producing hybrid tomato seed requires special skills. In this photo, a seed producer is carefully preparing a flower for hybridization. International Cooperators’ AVRDC Guide October 2001 AVRDC pub # 01-527

Hybrid Seed Production in Tomato

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Page 1: Hybrid Seed Production in Tomato

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Hybrid Seed Production in TomatoR.T. Opeña, J.T. Chen, T. Kalb and P. Hanson

IntroductionHybrid tomato varieties have many advantagescompared to open-pollinated varieties. Hybridsusually produce higher yields. They generallymature earlier and more uniformly. Many hybridshave better fruit quality and disease resistance.With all of these advantages, many farmersprefer to sow hybrid seeds in spite of the higherseed costs.

The demand for hybrid tomato seeds can open anew market for growers interested in seedproduction.

Hybrid tomato seed production is not easy. First,it requires much labor (see page 5). Fortunately,this is not a problem in developing countrieswhere affordable labor is available. Second, itrequires the mastery of special skills and a closeattention to detail. This publication will teachthese skills.

Crop Management

ClimateIdeal growing conditions are needed to producehigh quality seed. Tomatoes grow best in the dryseason under day temperatures of 21-25°C andnight temperatures of 15-20°C. Vines willstruggle to set fruit if temperatures exceed 30°C.

Humidity is an important factor. Humidity levelshigher than 60% at the time of fruit maturity will

increase disease problems and reduce seedyields. Seed production during the rainy seasongenerally leads to low yields and poor seedquality.

Field RequirementsAvoid planting your tomato seed crop in a fieldpreviously planted with tomato, pepper, eggplantor other solanaceous crops. This will prevent thebuild-up of diseases and insects. Growingtomato after paddy rice reduces the incidence ofdiseases and nematodes.

Optimum soil pH for tomato is 6.0 to 7.0; disor-ders such as blossom end rot are common if soilpH is lower than 5.5.

Figure 1. Producing hybrid tomato seed requiresspecial skills. In this photo, a seed producer iscarefully preparing a flower for hybridization.

InternationalCooperators’

AVRDC Guide October 2001AVRDC pub # 01-527

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Cultural PracticesHealthy plants produce healthy seeds. Pestsshould be controlled. Nutrient and water man-agement should be optimal to achieve good fruitand seed yield. For more information on growingtomato, see AVRDC International Cooperators’Guide, Suggested Cultural Practices for Tomato.

Selecting ParentsHybrid seed production involves the crossing of afemale line to a male line. Either line can be thefemale or male parent, but normally the best seedyielder is selected as the female parent. Bothparents should be pure, preferably being self-pollinated for more than six generations (this iscalled inbreeding). The inbred parents areselected for their desirable traits (e.g., high yields,disease resistance, fruit quality, earliness, etc.).

Ratio of Male to Female PlantsIt is important to have plenty of pollen available formaking hybrid crosses. Since tomato vines bloomprofusely, a ratio of one male for every four femaleplants is recommended.

Sowing DatesSeeds of male plants are sown three weeksearlier to ensure that pollen is available from thebeginning of hybridization.

IsolationEach tomato flower has both male and femaleparts (see Figure 3 on next page). Nearly alllines, including modern varieties, are self-pollinating. In other words, the pollen from eachflower's stamens pollinate only the style of thesame flower. Isolation of male and female parentallines is not necessary. Male plants can be plantedas close as two meters from female plants.

There are a few exceptions. Currant tomatoes (L.pimpinellifolium) and potato-leaf types of L.esculentum generally have styles that protrudeoutside their blossom. These lines are moreattractive to insects and thus, more likely to becross-pollinated. Netting or caging to excludepollinating insects is required if any of these typesare used as female lines. Also, do not collectseeds of double fruits since double flowers aremore prone to insect pollination.

Plant Location and SpacingsMale lines are planted in a different location tofacilitate operations and avoid shading fromcompeting plants. Select a sunny spot to pro-mote maximum production of flowers and pollen.

Male and female lines are planted in double-rowraised beds, with centers of beds spaced 150 cmapart. For female lines, plants are spaced 50 cmapart within the row (Figure 2). Male plants arespaced 40 cm apart to maximize flower productionper hectare.

StakingThe female parent is staked. Staking facilitatesthe handling of plants during emasculation andpollination. Staking also keeps the ripening fruitsabove the ground and prevents rotting. Plants aretrellised along with plants from the adjoining bed sothat work operations are done on the raised bedrather than in the furrow.

Among male lines, only indeterminate types needto be staked. If male lines are staked, trellisingcan be done within beds (as shown for male linesin Figure 2) or across adjoining beds (as shownfor female lines in Figure 2).

Removing Off-TypesThe male and female lines must be 100% pure.Know the plant habit, leaf type, and immature fruitcharacters (e.g., shape, size, and shouldercoloring) of each parent. Regularly inspect theplants.

Remove any off-type (usually inferior) or virus-infected plants before hybridization proceduresbegin. Symptoms of viruses include yellowmottling of leaves; severe curling, cupping orother distortion of foliage; and stunting of plants.

Figure 2. Spacing and staking options for femaleand male lines.

Female Male

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Emasculation

OverviewSelf-pollination cannot be allowed in hybrid seedproduction. The female flower must be pollinatedby the pollen from the male line.

To prevent self-pollination, remove the stamensfrom the flower buds of the female line beforethey shed their pollen (Figure 3). This process iscalled emasculation.

Figure 3. Tomato flower before and afteremasculation. The anther and surroundingcap are removed to prevent self-pollination.The petals and sepals are cut to identify theflower for future pollination.

ProcedureEmasculation begins about 55-65 days aftersowing. Flower buds from the second clusterwhich will open in two to three days are chosenfor emasculation (Figure 4). The petals will beslightly out of the flower bud but not opened, andthe corolla color is slightly yellow or even paler.Flowers from the first cluster are removed.

Sterilize the forceps, scissors and hands bydipping them in 95% alcohol before emasculationis started. If gloves are used, these should alsobe dipped in 95% alcohol to prevent pollencontamination.

Use sharp-pointed forceps to force open theselected buds. Then, split open the anther cone(Figure 5). Carefully pull the anther cone out ofthe bud, leaving the calyx, corolla and pistil(Figure 6).

To help identify the hybrid fruits from selfed fruits atthe time of harvest, cut the corolla and calyx (all ortwo sepals) (Figure 7).

Figures 4-7. Emasculation of tomato: selection of buds, removal of anther cone, and cutting of petals.

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Pollen CollectionCollect flowers from the male parent to extractpollen (Figure 8). The best time for pollencollection is during the early morning before thepollen has been shed. Avoid pollen collection onrainy days.

Remove the anther cones from the flowers andput them in suitable containers, such as glassine,cellophane, or paper bags (Figure 9).

Dry the anther cones by placing them 30 cmbelow a 100-watt lamp for 24 hours (Figure 10).The lamp creates a drying temperature of about30°C. Pollen can also be sun-dried, but avoiddrying at midday when temperature is very high.

Put the dried anther cones in a plastic pan or cup.Cover the cup with a fine mesh screen (200-300mesh) and then seal it with a similar tight-fittingcup, serving as a lid (Figure 11).

Figures 8-13. Pollen is collected, dried, and prepared for making hybrid crosses.

Shake the cup about 10-20 times so that thepollen is collected in the "lid" cup (Figure 12).Transfer the pollen into a small convenient-to-handle container for pollination (Figure 13).

Fresh pollen is best for good fruit-set. It can bekept for one day at moderate room temperature.

When weather conditions are not suitable forpollination, dried or dehydrated pollen can bestored in a sealed container (capsule or vial) andkept in the freezer for about a month. Withoutfreezing, the pollen can be kept in an ordinaryrefrigerator for two to three days without anysignificant loss in viability.

The pollen should be taken from the freezer orrefrigerator and kept closed until the containerwarms to room temperature. This will prevent thepollen from getting wet due to condensation.

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PollinationEmasculated flowers are generally pollinated twodays later. Try to avoid pollination on rainy days.The corolla of the emasculated flower turns brightyellow, signalling that the stigma is ready forpollination (Figure 14).

Dip the stigma into the pool of pollen in the pollencontainer (Figure 15) or pollinate by touching thestigma with the tip of the index finger dipped inthe pollen pool (Figure 16).

Pollination is usually done three times weeklyover a three to five week period. Successfulpollinations are easily seen within one week bythe enlargement of the fruit.

Labor Demands forHybrid Seed Production

Emasculation and pollination are very laborintensive! About 0.1 ha of indeterminatefemale plants requires two to three per-sons for five to six weeks. The same areaof determinate female plants requires fourto six people for three weeks.

After breeding operations are completed, anynon-crossed flowers on the female plants areremoved to lessen the chance of contaminationfrom selfed seeds before harvest.

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Figures 14-16. Pollination of emasculated flowers.

Fruit ProductionThe number of hybrid fruits produced per plantdepends on the fruit size of the maternal parent.As a rule of thumb, maintain the following: 30fruits for large-fruited parent; 40 fruits for me-dium-fruited parent; and 50 or more fruits forsmall-fruited parent.

Hybrid fruits are easily recognized by their cutsepals (Figure 17).

Remove the naturally-pollinated (non-hybrid) fruits,if any, from the female plants. This removal willprevent the accidental mixture of non-hybrid withhybrid fruits. Furthermore, non-hybrid fruits willsteal nutrition away from the ripening hybrid fruits.

Figure 17. Ripening seed crop. Note that a fewsepals on each fruit have been cut to indicate thefruit contains hybrid seed. Non-hybrid fruits havebeen removed.

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HarvestingTomato fruits ripen about 50-60 days afterpollination, but may take longer if temperaturesare cool.

Keep the fruits on the vine until they are fullymature, preferably to the pink or red ripe stage.This enables the seed to develop normally andfully. If fruits are harvested at an earlier stage,place them in a covered, cool dry place for threeor four days until they become red ripe.

Be sure to check for theclipped sepal before harvest-ing fruit.

Collect fruits in nonmetalliccontainers, such as nylonnet bags, plastic buckets, orcrates. Metal containersmay react with acids in thetomato juice and affect seedviability. Hence, they shouldnot be used.

Figure 18.Harvesting

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Seed Extraction

Option 1: Manual ExtractionHarvest the ripe fruits and keep them in nylonbags (Figure 19). Crush the fruits by tramplingwith feet (Figure 20).

Put the bags of crushed fruits into big plasticcontainers and ferment to separate the gel massembedding the seeds. To hasten the fermenta-tion process, put weights over the bags or keepthe fruits submerged in the liquid fruit mass(Figure 21).

The time of fermentation depends upon theambient room temperature. If temperature isabove 25°C, one day of fermentation may besufficient. If cooler, two days of fermentation maybe needed. Fermentation for more than threedays may spoil the seeds' quality.

To wash the seeds, put them in an open plasticcontainer. Then fill the container with water andstir the seeds to allow the pieces of flesh and skinsticking on the seeds to float.

Figures 19-24. Tomatoes are harvested in bags. The fruit are crushed and then fermented. Seeds arewashed by careful removal of floating pieces of flesh and skin.

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Incline the container and gently remove thefloating refuse, making sure that the seedsremain at the bottom (Figures 22 and 23). Re-peat the washing several times, adding freshwater to the container every time until all the fleshand gel are completely removed, leaving cleanseeds at the bottom (Figure 24).

Option 2: Mechanical ExtractionMechanical seed extraction is used by large-scaleoperations. Put ripe fruits into a mechanical seedextractor for crushing and separation of the seedsand gel from the pulp (Figure 25).

Gather the seeds and gel mass in a suitablecontainer such as plastic tub or bucket (Figure26).

Instead of fermentation, treat the seed-gel masswith 0.7% hydrochloric acid (HCl) at a rate ofseven milliliters of HCl per kilogram of seed-gelmass (Figure 27). Stir the seed-gel mass whilethe acid is being added. Continue stirring for 40minutes until the gel is visibly softened or dis-solved. Do not use a higher concentration of acid

nor a longer treatment time, otherwise you willinjure the seeds.

When the seed is separated from the gel, pourthe acid-treated seeds into a clean fine-meshbag. Wash the bag with tap water thoroughly sothat no acid is left on the seeds’ surface (Figure28). While washing, step on the bag to squeezeout the remaining gel.

Place the seeds into a plastic container, filling it toone-third capacity. Then, fill up the container withtap water. Stir the seeds to enable the smallpieces of flesh and skin to float. Incline thecontainer and remove the floating debris (Figure29). Make sure the seeds remain at the bottomof the container.

Repeat the washing procedure several times untilall the debris is gone and the seeds are clean.

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Figures 25-29. A machine is used to extract seeds from fruits. Seeds are then treated with hydrochloricacid to loosen their gelatinous coating, and then cleaned using water.

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Figure 33. Seed storage room

Seed DryingPlaced the washed seeds in bags. Excess watercan be removed by hanging the seeds in theshade for a day. An even quicker way to removewater is to place the seeds in a spin dryer (Figure30).

After the excess water is removed, uniformlyspread the partially dried seeds in a flat plasticcontainer or aluminum pan. Loosen any clumpsof seeds (Figure 31). Enclose this container withits seeds into a net nylon bag.

Place the container into an air drier (Figure 32).Drying continues for three to four days, maintain-ing a temperature of 28-30°C. Higher tempera-tures at the time of drying may cause seeds togerminate.

Stir the seeds two to three times daily so thatseeds dry uniformly. Loosen any seeds thatclump together. These procedures will get theseeds to the desired 6-8% moisture content.

Seed Packaging and StoragePack and deliver the dried seeds according tospecifications of the seed company or contractagency.

If necessary, tomato seeds can be safely storedfor at least three to five years. Place seeds inmanila envelopes, cloth or mesh bags, plasticcontainers, or foil envelopes. The best containersare airtight, such as a sealed glass jar, metal can,or foil envelope.

Label each container carefully. Note the namesof the hybrid and parents, the date, and any otherinformation you feel is valuable.

Store seeds in a cool, dry place. Small quantitiescan be kept in an airtight container inside arefrigerator. For larger quantities, a special roomwith controlledhumidity andtemperatureshould be used.If possible, thetemperaturesshould notexceed 20°Cand relativehumidity (RH) inthe storage areashould notexceed 30%.

More InformationFor more information on hybrid tomato seedproduction, contact Dr. Peter Hanson at<[email protected]>.

A slide set on this topic is available. In addition,International Cooperators' Guides have beenwritten on numerous topics related to tomatocultivation and vegetable seed production. Thisincludes a guide that describes the production ofopen-pollinated tomato seed. All of this informa-tion is available at the AVRDC web site,<http://www.avrdc.org>.

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Figures 30-32. Seeds are spun-dried, spread and loosened in a flat container, then rebagged andplaced into an air drier.