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Crop Improvement :: Emasculation and Pollination Techniques

EMASCULATION AND POLLINATION TECHNIQUES IN VEGETABLES

Table 1. Mode of reproduction and diploid chromosome number in major vegetable crops

Crop Botanical Name 2n No Mode of reproduction
Potato Solanum tuberosum 48 VP
Sweet potato Ipomea batatas 90 VP
Tapioca Manihot esculenta 36 (72) VP
Yam Dioscorea alata 40 VP
Arvi (colocasia) Colocasia esculenta 28 VP
Egg plant Solanum melongena 24 VP
Pepper C. annuum 24 SP
Tomato Lycopersicon esculentum 24 SP
Peas Pisum sativum 14 SP
Broad bean Vicia faba 12 (24) SP
Cluster bean Cyamopsis tetragonoloba 14 SP
Cowpea Vigna unguiculata 22 SP
French-bean Phaseolus vulgaris 22 SP
Indian bean Dolichos lablab 22 SP
Soybean Glycine max 40 SP
Sword bean Canavalia gladiata 22 (44) SP
Okra Abelmoschus esculentus 72 OCP
Garlic Allium sativum 16 VP
Onion Allium cepa 16 CP
Cauliflower Brassica oleracea var. botrytis 18 SP/CP
Cabbage B. oleracea var. capitata 18 CP
Knol-khol B. oleracea var. gongylodes 18 CP
Brussels sprouts B. oleracea var. gemmifera 18 CP
Ashgourd Benincasa hispida 24 CP
Bittergourd Momordica charantia 22 CP
Bottlegourd Lagenaria siceraria 22 CP
Cucumber Cucumis sativus 14 CP
Muskmelon Cucumis melo 24 CP
Pumpkin Cucurbia species 40 CP
Ridgegourd Luffa acutangula 26 CP
Snakegourd Trichosanthes anguina 22 CP
Watermelon Citrullus lanatus 22 CP
Amaranths Amaranthus tricolour 32 CP
Beet leaf (palak) Beta vulgaris 18 CP
Spinach Spinacea oleracea 12 CP
Lettuce Lactuca sativa 18 CP
Beetroot Beta vulgaris 18 CP
Carrot Daucus carota 18 CP
Radish Raphanus sativus 18 CP
Turnip Brassica rapa 20 CP
Asparagus Asparagus officinalis 20 CP
VP-Vegetative  propagation CP=Cross-pollinatedSP=self-pollinated OCP – often cross - pollinated

Table. 2 Centres of diversity of the major vegetable crop species

            Mega gene centre Primary centres of origin and domestication Secondary centres
Chinese – Japanese Eggplant, waxgourd, Chinese cabbage, kangkong, welsh onion Watermelon, amaranath
Indo-Chinese Waxgourd, spongegourd, ridgegourd, bittergourd, swordbean, winged bean, Taro, cumcumber, bottlegourd, chayote, yam Cucumber, bottlegourd, chayote, shallot, yam bean, yardlong bean, Chinese cabbage, amaranath, kangkong
Hindustani Eggplant, waxgourd, cucumber, ridgegourd, spongegourd, bittergourd, hyacyinth bean, kangkong, okra, drumstick Watermelon, melon, bottlegourd, amaranath, roselle
Central Asia Onion, garlic, mustard, spinach, carrot Eggplant, watermelon, melon, cauliflower
Near East Onion, garlic, leek, mustard beet Okra
Mediterranean White cabbage, cauliflower, broccoli, bean, radish Sweet pepper, garlic, okra
African Eggplant, watermelon, melon, bottlegourd, locust bean, cowpea, okra, roselle Onion, shallot, limabean, mustard, amaranth, Corchorus
European-Siberian Lettuce Onion, common bean, white cabbage, cauliflower, spinach, carrot
South American Tomato, hot peeper, pumpkins and squashes, lima bean, common bean, cassava, xanthosoma, chayote Common bean
Central American and Mexican Region Tomato, hot pepper, pumpkins and squashes, yam bean, common bean, amaranth, sweet potato -
North America - Tomato, eggplant, pepper, melon, watermelon, pumpkin, and squashes, onion, lettuce, common bean, lima bean, okra

TOMATO

Emasculation is usually done one day prior to anthesis/flower opening. At this stage, the sepals have started to separate and the anthers and corolla are beginning to change from light to dark yellow. The stigma is fully receptive at this stage allowing for pollination even immediately after emasculation.

Anthers are removed as a group with or without the surrounding corolla, by inserting forceps between the sepals to grip the base of the anthers and / or petals which are then removed by a firm but steady pull. If anthers seem reluctant to part company from flower receptacle as a group, it is advisable to remove a single one first by careful manipulation of the forceps. Following this, the remaining four may be gripped firmly without any fear of damaging the style. Pollen is best applied in experimental crosses by slitting the inside of the anthers of mature flowers of the male parent with the forceps in such a way that a small amount of pollen is collected at the tip of the forceps. This can then be lightly applied to the stigmatic surface and should be visible as a white covering. Forceps should be sterilized by dipping in alcohol or methylated spirit after each pollination. Pollen may be collected in large amounts by inverting the mature flower and tapping pollen into the thumbnail (Watts, 1980). Protection of pollinated flowers by wrapping with cotton or small pollination bags is essential.

BRINJAL

Flowers generally emerge 40-45 days after transplanting. Anthesis occurs at about 6-8 a. m. in August – September and usually between 9.30 to 11.15 am during winter (December-January). Stigma receptivity is highest during anthesis i.e. flower opening. Anthers usually dehisce 15 to 20 minutes after the flowers have opened. The receptivity of the stigma can be observed from its plump and shiny period of effective receptivity ranges from a day prior to flower opening until about four days after opening. Pollen usually remains viable for a day during summer and 2-3 days in winter under field conditions.

For emasculation, a healthy long or medium styled, well developed bud from the central portion of the plant is selected. The bud is opened gently with the help of fine pointed forceps one or two days before the opening of the bud and all the five anthers are carefully removed. For pollination, freshly dehiscing anthers are picked up and are slit vertically with fine needle to get sufficient pollen at the tip of the needle. Pollen are applied on the stigma of the emasculated flower bud. It is labelled and covered with small pollination bag.

CHILLI

Flowers are emasculated in bud stage. Pollen is transferred to the stigma either from mature undehisced anthers by scooping it out through the lateral sutures with the needle or by touching a freshly dehisced anther to the stigma with the forceps. Hands and tools (a pair of sharp-pointed forceps, a needle, a pair of scissors) are washed with 95% ethyl alcohol. A roll of cheese cloth, some light weight cotton string and balls of different colours of thread are also needed. Pollinated flowers are identified by loosely typing coloured thread around the delicate pedicels, preferably enclosing a leaf petiole for protection. Different colours of string can be used for different crosses on the same plant, and white for the selfs. Pollinated flowers are protected from bees by a double layer of cheese cloth, loosely wrapped around the branch, enclosing leaves and flowers, and securely fastened. Appropriately marked plastic labels describing the cross, the date, are attached to a bamboo stake marking the chosen plant. Pollinated flowers are periodically checked and the cheese cloth removed in 4-6 days. Fruits normally mature in about 45 days.

BHENDI

 For emasculation, buds likely to pen the next day are given a slight ring cut at the base of the flower bud with the help of a blade. Petals along with calyx sheath are removed and staminal column and stigma are exposed. The underhisced anthers are removed with a pair of forceps. The emasculated buds are bagged. Next morning, the bags are removed and the emasculated buds are pollinated with the male flowers bagged a day earlier. Pollinated flowers are bagged and labelled. After a few days the bags may be removed.

GARDEN PEA

For emasculation the flower bud chosen should have developed to the stage just before anther dehiscence, indicated by extension of petals beyond sepals. Flowers can be emasculated at any time. The first step in emasculation is to tear away with the forceps the tip of the sepal from in front of the keel. The forefinger is positioned behind the flower and thumb in front and a light pressure is applied. This spreads the standard and wings to expose the keel. The exposed keel is slit-open by tips of forceps. Pressure can be applied by the thumb and finger on keel for increased exposure of the pistil and stamens. The 10 stamens are pulled out.

Pollen can be obtained throughout the day, preferably from a freshly opened flower. For pollen collection, it is more convenient to pick the male flowers, remove the standard and wings, pull back the keel so that the style protrudes and use the pollen covered stylar brush as an applicator to transfer the pollen to the stigma of the emasculated bud. Older flowers and other flower buds not used in crossing are removed the peduncle to increase the pod set after crossing.

FRENCH BEAN

For emasculation an unopened flower bud is selected. The standard is detached from below with a pair of forceps and bent backward. The keel is pulled in pieces with the forceps. Care should be taken to turn in the same direction as the spiral winding, otherwise the style may break. After pulling off keel, the stamens are removed. For the supply of pollen, freshly opened flowers are collected. The thickly pollinated stigmas emerge as soon as the wings are pressed downward. This stigma is rubbed against the stigma of the emasculated bud. Sometimes, the thickly pollinated stigma of the open flower to be used as male is hooked into the stigma of the flower bud to be pollinated (Buishand, 1956). This method has been found successful by the author also.

Pollination without emasculation has also been suggested. In this method the standard is detached and unfolded. By pressing the left-hand wing downward, the unpollinated stigma emerges. Pollen is rubbed into this stigma. The emasculation and pollinations are done simultaneously in the forenoons.

COWPEA

Cowpea flowers are large and showy. Flowers open only once between 7 and 9 a.m. On cloudy days the flowers may open even in the afternoon. Though the flowers open late in the morning, the dehiscence of the anthers is much earlier. It may vary farm 10 pm to 0.45 a.m. The dehiscence is influenced by environmental factors like presence of moonlight, a clear sky and a dry warm atmosphere. During dark nights the dehiscence tends to be delayed. Due to dehiscence taking place before the opening of flowers, the cowpea is strictly self-pollinated in nature.

Since the dehiscence of anthers is much in advance of the blooming, the emasculation needs to be carried out in mature flower buds in the preceding evening. The flower buds likely to bloom the next day (recognized by large size, the yellowish colour of the back of the standard petal) is selected for emasculation. The bud is held between the thumb and fore-finger with the keel side uppermost. A needle is run along the ridge where the two edges of the standard unite. One side of the standard is brought down and secured in position with thumb. Same thing is done with one of the wings. After this the exposed keel is slit on the exposed side, about 1/16 inch from the stigma. A section of keel is also brought down and secured in position under the end of thumb. Now 10 stamens are seen. They are removed with pointed forceps. Afterwards, the disturbed parts of standard, wing and keel are brought in original position as far as possible. To prevent drying out of the emasculated bud, a leaflet may be folded and pinned around the bud. A tissue paper can be used to cover and protect the bud.

Pollination is done next morning from a freshly opened flower. The standard and wings of male flower are removed. By slight depression of the keel, stigma covered with pollen grains protrudes out. This itself can be used as a brush for pollination. Cowpea flowers are highly sensitive and drop off easily with slight mechanical disturbance or injury. Therefore, much labour and time is devoted to get enough crossed seed.

CAULIFLOWER

Pollination Technique

Selfing

The self-compatible varieties of cauliflower can be selfed by simply bagging the flower-stalk. Selfing is also done by caging some plants with flies in cages or by isolation planting of lines having decreased level of self-incompatibility. With self-incompatible plants, bud pollination gives better results. In this system, the pollination is carried out in buds before 2-4 days of opening, with emasculation or without emasculation.

Crossing

The flowers may be emasculated by removing 6 stamens using a pair of forceps. In self-compatible cauliflowers (European types), the stamens are removed before the opening of the buds as the flowers are already fertile in the bud stage, crossing can be done at the same time. In self-incompatible types, emasculation may be omitted. When pollination cages are available, crosses between self-incompatible types can be made by insects such as honey bees, bumble bees and flies.

CABBAGE

Flowers are slightly protogynous and cabbage is naturally cross-pollinated due to sporophytic self-incompatibility. Pollination is brought about by bees and flies. Bud pollination is effective to achieve selfing. For cross-pollination flower buds expected to open within 1-2 days are emasculated and are pollinated immediately with desired pollen using a brush/flower stamens.

BOTTLEGOURD

There are three stamens, two as compound and one as single. Since possible, during night also, it could be done.

BITTERGOURD

Anthesis and dehiscence occur early in the morning. Therefore selfing and crossing should be attempted in forenoon preferably in early hours.

RIDGE GOURD and SPONGE GOURD

The flowers of ridge gourd like those of bottle gourd start anthesis (opening) in the evening and remain open throughout the night and are ready for selfing and pollination in the early morning/forenoon. The flowers of sponge gourd open in early morning hours and are suitable for selfing/crossing almost throughout the day.

 
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