Crop Improvement: seed Breeding Methods in Crop Plants
Classification of crop plants based on mode of pollination and mode of reproduction
Mode of pollination and reproduction
Examples of crop plants
Self Pollinated Crops
Rice, Wheat, Barley, Oats, Chickpea, Pea, Cowpea, Lentil, Green gram, Black gram, Soybean, Common bean, Moth bean, Linseed, Sesame, Khesari, Sunhemp, Chilli, Brinjal, Tomato, Okra, Peanut, Potato, etc.
Cross Pollinated Crops
Corn, Pearl millet, Rye, Alfalfa, Radish, Cabbage, Sunflower, Sugar beet, Castor, Red clover, White clover,Safflower, Spinach, Onion, Garlic, Turnip, Squash, Muskmelon, Watermelon, Cucumber, Pumpkin, Kenaf,Oil palm, Carrot, Coconut, Papaya, Sugarcane, Coffee, Cocoa, Tea, Apple, Pears, Peaches, Cherries,grapes, Almond Strawberries, Pine apple, Banana, Cashew, Irish, Cassava, Taro, Rubber, etc.
Often Cross Pollinated Crops
Sorghum, Cotton, Triticale, Pigeon pea, Tobacco.
BREEDING METHODS IN CROP PLANTS
In mass selection, seeds are collected from (usually a few dozen to a few hundred) desirable appearing individuals in a population, and the next generation is sown from the stock of mixed seed. This procedure, sometimes referred to as phenotypic selection, is based on how each individual looks. Mass selection has been used widely to improve old “land” varieties, varieties that have been passed down from one generation of farmers to the next over long periods.
An alternative approach that has no doubt been practiced for thousands of years is simply to eliminate undesirable types by destroying them in the field. The results are similar whether superior plants are saved or inferior plants are eliminated: seeds of the better plants become the planting stock for the next season.
A modern refinement of mass selection is to harvest the best plants separately and to grow and compare their progenies. The poorer progenies are destroyed and the seeds of the remainder are harvested. It should be noted that selection is now based not solely on the appearance of the parent plants but also on the appearance and performance of their progeny. Progeny selection is usually more effective than phenotypic selection when dealing with quantitative characters of low heritability. It should be noted, however, that progeny testing requires an extra generation; hence gain per cycle of selection must be double that of simple phenotypic selection to achieve the same rate of gain per unit time.
Mass selection, with or without progeny test, is perhaps the simplest and least expensive of plant-breeding procedures. It finds wide use in the breeding of certain forage species, which are not important enough economically to justify more detailed attention.
numerous superior appearing plants are selected from a genetically variable population;
progenies of the individual plant selections are grown and evaluated by simple observation, frequently over a period of several years;
when selection can no longer be made on the basis of observation alone, extensive trials are undertaken, involving careful measurements to determine whether the remaining selections are superior in yielding ability and other aspects of performance.
Any progeny superior to an existing variety is then released as a new “pure-line” variety. Much of the success of this method during the early 1900s depended on the existence of genetically variable land varieties that were waiting to be exploited. They provided a rich source of superior pure-line varieties, some of which are still represented among commercial varieties. In recent years the pure-line method as outlined above has decreased in importance in the breeding of major cultivated species; however, the method is still widely used with the less important species that have not yet been heavily selected.
A variation of the pure-line selection method that dates back centuries is the selection of single-chance variants, mutations or “sports” in the original variety. A very large number of varieties that differ from the original strain in characteristics such as colour, lack of thorns or barbs, dwarfness, and disease resistance have originated in this fashion.
During the 20th century planned hybridization between carefully selected parents has become dominant in the breeding of self-pollinated species. The object of hybridization is to combine desirable genes found in two or more different varieties and to produce pure-breeding progeny superior in many respects to the parental types.
Genes, however, are always in the company of other genes in a collection called a genotype. The plant breeder’s problem is largely one of efficiently managing the enormous numbers of genotypes that occur in the generations following hybridization. As an example of the power of hybridization in creating variability, a cross between hypothetical wheat varieties differing by only 21 genes is capable of producing more than 10,000,000,000 different genotypes in the second generation. At spacing normally used by farmers, more than 50,000,000 acres would be required to grow a population large enough to permit every genotype to occur in its expected frequency. While the great majority of these second generation genotypes are hybrid (heterozygous) for one or more traits, it is statistically possible that 2,097,152 different pure-breeding (homozygous) genotypes can occur, each potentially a new pure-line variety. These numbers illustrate the importance of efficient techniques in managing hybrid populations, for which purpose the pedigree procedure is most widely used.
starts with the crossing of two genotypes, each of which has one or more desirable characters lacked by the other. If the two original parents do not provide all of the desired characters, a third parent can be included by crossing it to one of the hybrid progeny of the first generation (F1). In the pedigree method superior types are selected in successive generations, and a record is maintained of parent–progeny relationships.
The F2 generation (progeny of the crossing of two F1 individuals) affords the first opportunity for selection in pedigree programs. In this generation the emphasis is on the elimination of individuals carrying undesirable major genes. In the succeeding generations the hybrid condition gives way to pure breeding as a result of natural self-pollination, and families derived from different F2 plants begin to display their unique character. Usually one or two superior plants are selected within each superior family in these generations. By the F5 generation the pure-breeding condition (homozygosity) is extensive, and emphasis shifts almost entirely to selection between families. The pedigree record is useful in making these eliminations. At this stage each selected family is usually harvested in mass to obtain the larger amounts of seed needed to evaluate families for quantitative characters. This evaluation is usually carried out in plots grown under conditions that simulate commercial planting practice as closely as possible. When the number of families has been reduced to manageable proportions by visual selection, usually by the F7 or F8 generation, precise evaluation for performance and quality begins. The final evaluation of promising strains involves (1) observation, usually in a number of years and locations, to detect weaknesses that may not have appeared previously; (2) precise yield testing; and (3) quality testing. Many plant breeders test for five years at five representative locations before releasing a new variety for commercial production.
The bulk-population method
of breeding differs from the pedigree method primarily in the handling of generations following hybridization. The F2 generation is sown at normal commercial planting rates in a large plot. At maturity the crop is harvested in mass, and the seeds are used to establish the next generation in a similar plot. No record of ancestry is kept. During the period of bulk propagation natural selection tends to eliminate plants having poor survival value. Two types of artificial selection also are often applied: (1) destruction of plants that carry undesirable major genes and (2) mass techniques such as harvesting when only part of the seeds are mature to select for early maturing plants or the use of screens to select for increased seed size. Single plant selections are then made and evaluated in the same way as in the pedigree method of breeding. The chief advantage of the bulk population method is that it allows the breeder to handle very large numbers of individuals inexpensively.
Often an outstanding variety can be improved by transferring to it some specific desirable character that it lacks. This can be accomplished by first crossing a plant of the superior variety to a plant of the donor variety, which carries the trait in question, and then mating the progeny back to a plant having the genotype of the superior parent. This process is called backcrossing. After five or six backcrosses the progeny will be hybrid for the character being transferred but like the superior parent for all other genes. Selfing the last backcross generation, coupled with selection, will give some progeny pure breeding for the genes being transferred. The advantages of the backcross method are its rapidity, the small number of plants required, and the predictability of the outcome. A serious disadvantage is that the procedure diminishes the occurrence of chance combinations of genes, which sometimes leads to striking improvements in performance.
The development of hybrid varieties differs from hybridization. The F1 hybrid of crosses between different genotypes is often much more vigorous than its parents. This hybrid vigour, or heterosis, can be manifested in many ways, including increased rate of growth, greater uniformity, earlier flowering, and increased yield, the last being of greatest importance in agriculture.
CROSS POLLINATED CROPS
The most important methods of breeding cross-pollinated species are
development of hybrid varieties;
development of synthetic varieties.
Since cross-pollinated species are naturally hybrid (heterozygous) for many traits and lose vigour as they become purebred (homozygous), a goal of each of these breeding methods is to preserve or restore heterozygosity.
Mass selection in cross-pollinated species takes the same form as in self-pollinated species; i.e., a large number of superior appearing plants are selected and harvested in bulk and the seed used to produce the next generation. Mass selection has proved to be very effective in improving qualitative characters, and, applied over many generations, it is also capable of improving quantitative characters, including yield, despite the low heritability of such characters. Mass selection has long been a major method of breeding cross-pollinated species, especially in the economically less important species.
The outstanding example of the exploitation of hybrid vigour through the use of F1 hybrid varieties has been with corn (maize). The production of a hybrid corn variety involves three steps: (1) the selection of superior plants; (2) Selfing for several generations to produce a series of inbred lines, which although different from each other are each pure-breeding and highly uniform; and (3) crossing selected inbred lines. During the inbreeding process the vigour of the lines decreases drastically, usually to less than half that of field-pollinated varieties. Vigour is restored, however, when any two unrelated inbred lines are crossed, and in some cases the F1 hybrids between inbred lines are much superior to open-pollinated varieties. An important consequence of the homozygosity of the inbred lines is that the hybrid between any two inbreds will always be the same. Once the inbreds that give the best hybrids have been identified, any desired amount of hybrid seed can be produced.
Pollination in corn (maize) is by wind, which blows pollen from the tassels to the styles (silks) that protrude from the tops of the ears. Thus controlled cross-pollination on a field scale can be accomplished economically by interplanting two or three rows of the seed parent inbred with one row of the pollinator inbred and detasselling the former before it sheds pollen. In practice most hybrid corn is produced from “double crosses,” in which four inbred lines are first crossed in pairs (A × B and C × D) and then the two F1 hybrids are crossed again (A × B) × (C × D). The double-cross procedure has the advantage that the commercial F1 seed is produced on the highly productive single cross A × B rather than on a poor-yielding inbred, thus reducing seed costs. In recent years cytoplasmic male sterility, described earlier, has been used to eliminate detasselling of the seed parent, thus providing further economies in producing hybrid seed.
Much of the hybrid vigour exhibited by F1 hybrid varieties is lost in the next generation. Consequently, seed from hybrid varieties is not used for planting stock but the farmer purchases new seed each year from seed companies.
Perhaps no other development in the biological sciences has had greater impact on increasing the quantity of food supplies available to the world’s population than has the development of hybrid corn (maize). Hybrid varieties in other crops, made possible through the use of male sterility, have also been dramatically successful and it seems likely that use of hybrid varieties will continue to expand in the future.
A synthetic variety is developed by intercrossing a number of genotypes of known superior combining ability—i.e., genotypes that are known to give superior hybrid performance when crossed in all combinations. (By contrast, a variety developed by mass selection is made up of genotypes bulked together without having undergone preliminary testing to determine their performance in hybrid combination.) Synthetic varieties are known for their hybrid vigour and for their ability to produce usable seed for succeeding seasons. Because of these advantages, synthetic varieties have become increasingly favoured in the growing of many species, such as the forage crops, in which expense prohibits the development or use of hybrid varieties.
Physical mutagens include various types of radiation, viz X-rays, gamma rays, alpha particles, beta particles, fast and thermal (slow) neutrons and ultra violet rays. A brief description of these mutagens is presented below:
Type of Radiation
X – rays
S.I., penetrating and non-particulate
S.I., very penetrating and Non-particulate
D.I., particulate, less penetrating and positively charged.
Fast and Thermal Neutrons
D.I., particulate, neutral particles, highly penetrating.
Ultra Violet Rays
Non-ionizing, low penetrating
X-rays were first discovered by Roentgen in 1895. The wavelengths of X-rays vary from 10-11 to 10-7. They are sparsely ionizing and highly penetrating. They are generated in X-rays machines. X-rays can break chromosomes and produce all types of mutations in nucleotides, viz. addition, deletion, inversion, transposition, transitions and transversions. X-rays were first used by Muller in 1927 for induction of mutations in Drosophila. In plants, Stadler in 1928 first used X-rays for induction of mutations in barley.
Gamma rays have shorter wave length than X-rays and are more penetrating than gamma rays. They are generated from radioactive decay of some elements like 14C, 60Co, radium etc. Of these, cobalt 60 is commonly used for the production of Gamma rays. Gamma rays cause chromosomal and gene mutations like X-rays.
The chemical mutagens can be divided into four groups, viz.
RICE (Oryza sativa) (2 n = 24) (Family – Poaceae)
The inflorescence is a terminal panicle borne on the peduncle or last internodes’ with single flowered spikelets. The main axis is glabrous to ciliate and one to several sub branches at each node called secondary branches which bear spikelets. Usually spikelets appear singly. Each spikelet consists of two glumes (empty bracts), one lemma, one palea enclosed by lemma, monocarpellary ovary with a single style and a bifid stigma and six stamens with bilobed anthers. There are two lodicules at the base of the ovary which help in opening of the flower.
In rice the panicle completes emergence from flag leaf in 2 to 4 days. Anthesis commences shortly after emergence of panicle. Spikelets at the tip bloom first and proceed downwards. Anthesis time 8-10 am. Each spikelet remains open 30 minutes and then closes. The anther dehiscence takes place immediately after the opening of the spikelets. Receptivity of stigma is maximum during first three days after opening of the spikelets and then gradually decreases and is lost within 7 days. Viability of pollen grains is only for five minutes.
Bag the panicle and put the labeled tag to the desired lines when panicle starts emerging from the flag leag.
Emasculation and Crossing techniques
Emasculation is necessarily followed by controlled pollination. Emasculation is done during early morning between 6 and 8 AM in spikelets, due to open on the same day. Emasculation should be over well ahead of the time of anthesis. Crossing techniques in rice differ based on the method of emasculation. Since maximum number of spikelets open on the 3rd or 4th day of anthesis, panicles of that stage are selected for emasculation. The following methods are widely used for hybridization in rice.
Hand Emasculation (Removal of anthers)
For hand emasculation, the spikelets which are expected to open the next morning are selected and are made to open by slightly separating the lemma and pales and the anthers are removed with the help of forceps.
In the previous day evening, top 1/3rd and bottom 1/3rd portions in the panicle of the desired female parent are clipped off by using scissors leaving the middle spikelets. With the help of scissors again, top 1/3 portion in each spikelet is clipped-off in a slanting position. The six anthers present in each spikelet are removed with the help of the needle (Emasculation). Care must be taken during emasculation for not to damage the gynoecium. Then to prevent contamination from the foreign pollen, the emasculated spikelets are covered with a butter paper big. In the next day morning (usually at 9.00AM), the bloomed panicle from the desired male parent is taken. The top portion of the butter paper bag which was originally inserted in the emasculated female parent is now cut to expose the panicle. The male parent panicle is inserted in an inverted position into the butter paper bag and sturned in both ways in order to disperse the pollen. After ensuring the abundant disbursement of pollen, the opened butter paper bag is closed using a pin. Colored thread may be tied at the base of the panicle to identify the crossed ones. After ensuring pollination, the bag may be removed.\
Hot water method
A method of hot water emasculation is used to about the same extent as the clipping method. Panicles in 3rd (or) 4th day of blooming are chosen as female parents. An hour or so before blooming (i.e. normally at 7. A.M.), the panicle is selected and under developed and opened spikelets are removed. Now, the tiller is bent over (carefully to avoid breaking) and the selected panicle is immersed in hot water contained in a thermos bottle at 40-440C for a period of 5 to 10 minutes. This treatment causes the florets to open in a normal manner and avoids injury. Then, emasculation is done by removing the six stamens by fine forceps or needles and then dusting should be done.
Dr. Ramiah method
Panicles on the 3rd or 4th day of its blooming are selected; top and lower splikelets are removed leaving only the middle. It is covered with a wet cloth and air is blown from mouth. This facilitates opening of spikelets. After 2-3 minutes, wet cloth is removed and spikelets are found to be open. Then, the six anthers are removed.
Vacuum emasculation method
This works on the principle of suction pressure. The spikelets are clipped off prior to operation. The minute pipette is to be shown at the point of clipping and pollen is sucked in. Six panicles can be emasculated at a time. By hand emasculation, 100 flowers can be emasculated by a person. With the vaccum emasculator, six persons can operate and emasculate 3000 to 3600 florets/hour.
The technique was developed by CRRI, Cuttack. The panicle to be emasculated is inserted into hallow piece of bamboo closed at one end and plugged with cotton wool and split cork at the other end. The flowers thus enclosed will open within 5-10 minutes. The anthers are removed.
Brown paper method
The panicles are enclosed in a Brown paper cover before a couple of hours of blooming. Heat develops inside due to which the anthers extrude, but do not dehisce. This happens in 15-30 minute then the anthers are easily clipped off. Stigmatic surface is then dusted with pollen grains collected from the chosen male parent. The crossed panicle is then properly tagged and protected with paper cover which is retained in a position for 7 – 10 days.
In this method hot water is kept in the flask and it is poured outside. After pouring out the water inside of the flask will be warm and humid. The panicle to be emasculated will be inserted into the flask and kept for some time. Due to high temperature and humidity the spikelets will get opened and the anthers are exposed which can be removed with the help of forceps.
From the male parent, the anthers which are about to shed the pollen are collected and the pollens are transferred on to the stigma of emasculated spikelets with the help of a camel brush. Bagging and labeling is done is done immediately after pollination.
WHEAT (Triticum aestivum) (2n = 14, 28, 42) (Family – Poaceae)
The inflorescence is a spike of spikelets (15-20 or more in numbers). Spikelets are borne on a zig zag rachis in two alternate rows with a terminal spikelet. The spikelets are sessile and each spikelet contains 3-7 florets. Only the lateral florets are fertile and the central ones may be sterile. At the base of each spikelet, there are two oppositely placed empty glumes. Each floret comprises of a lemma ending in an awn, a palea, 2 lodicules, androecium and gynoecium consists of monocarpellary superior ovary with 2 feathery stigmas. Lodicules are 2 in number, triangular in shape and are present at the base of the ovary.
Main culm flowers first and the tillers bloom later in the order of their formation. Flowering starts at approximately 2/3 from the base and proceeds in the both the directions. Blooming remains throughout the day and it takes 3-5 days for completion. Flower opening is usually during warmer part of the day i.e., between 9 am to 2pm and peak period is between 10 Am to 1 pm. Anther dehiscence takes place simultaneously and hence the crop is highly self- pollinated (< 1 % cross- pollination).
The inflorescence is covered with a butter paper cover prior to anthesis, and kept undisturbed till the flower opening completed.
On emergence of the ear upper 1/3rd of the spikelet is cut and lower spikelets are also removed. Of the remaining spikelets alternate ones on both sides of the axis are removed. The top spikelet is held with forceps and pulled downwards and upwards to remove the upper florets of the spikelets. The glumes are separated and anthers left exposed are removed carefully and covered with butter paper cover.
On the next day ear head selected from the pollen parent are used for crossing. The upper half of the glumes of the few medium spikelets are cut of and the ripened bright yellow anthers are rubbed on the styles of the emasculated florets and then covered.
MAIZE (Zea mays L.); Family: Poaceae (2n=20)
The maize plant bears its flowers in the form of spikelets. The male inflorescence is produced terminally as a tassel on the main stalk while the female spikelets are borne on the ear which is produced from a modified branch from a lateral bud. Staminate or male spikelets are borne in pairs on the tassel, one of them is pedicelled with a short stalk, and other sessile without a stalk, both are identical in size, shape and structure. Each spikelet is covered by two glumes and has two fertile flowers. Each flower is protected by a lemma and palea and has 3 anthers, a pair of lodicules, and rudimentary pistil. The spikelets are arranged on a much branched panicle. The central spike is thick, more pronounced and has distinctly higher condensation of spikelets than on the primary and secondary branches whose number would vary considerably in different varieties. The pistillate or female spikelets are borne on a thickened axis called cob which is fully covered by a number of modified sheaths or husk leaves. The ear is attached to the main stalk through a shank. Which has a number of condensed internodes, each bearing a husk leaf. The paired pistillate spikelets are usually sessile. Each spikelet has two florets but usually only one of them, frequently the upper one, is fertile. This explains the usual observation of paired rows in most varieties. In the pistillate spikelets, the lodicules are absent or poorly developed, and has a well developed ovule. The slender styles of the spikelets elongate and emerge at top of the husk leaves as silks. The basal spikelets of the ear produce longer styles which are bifid at the tip, hairy and are receptive along most of its length.
In maize protandry is common and in most varieties the tassels become visible 1-3 days before the emergence of the silks. Under unfavorable environments like moisture stress, the emergence of silks may be substantially delayed, whereas the emergence of tassel remains unaffected. The pollen shedding starts from the spikelets located on the central spike, 2-3 cm from the tip of the tassel, and proceeds downwards in central spike and in all primary and secondary branches of the tassel. Lateral branches start blooming a little later. Blooming period varies from 2-14 days in different varieties. Flower opening starts early in the morning and completes by afternoon. Upper florets in a spikelet open prior to lower floret.
A single anther may shed 2500 pollen grains and a medium- size tassel may give out 15-30 million or even more pollen grains. Pollen grains are light and fall by gravity or are blown away by wind and cross-pollination is common. The extent of cross-pollination may ranges from 95 percent upwards.
Depending on the variety, Temperature and relative humidity the pollen shedding may continue for 3-6 days under normal field conditions pollen grains stay viable for 4-16 hours after shedding. Emergence of silks or styles from the cob husk leaves take 2-5 days. Stigma remains receptive for 3-8 days and sometimes up to 14 days. When placed on the silk, the pollen grains germinate and the pollen tube grows into the style through the stigmatic hairs. Fertilization is completed in 24-36 hours. The silks after fertilization turn brown and dry. Following fertilization, kernel development is initiated, the development is visibly more pronounced after 10-15 days. The length of the ear and number of kernels per ear largely determined by both heritable and environmental factors.
Based on floral biology, the maize plant in which tassel blooms first before the silk becomes receptive (protandry) is grouped under naturally cross-pollinated crop (<5% self pollination) by wind borne pollen grains.
In maize, Selfing is done to develop inbred lines from open pollinated population. For Selfing, We must first bag the female inflorescence (cob) in which silks will emerge after 1-2 days using butter paper bags of 7.5cm x 16cm size. The tassel of the same plant is also covered with a Kraft paper bag (tassel bag) of 10cm x 30cm size for collection of pollen dust. On the next day morning, the pollen is collected in the tassel bag. The butter paper bag which is used for cover the cob (silks) of the same plant is slightly lifted & the silks are dusted with the pollen grains. The same tassel bag is used to cover the pollinated cob. The procedure of collecting & dusting the pollen is repeated in the following day. The cob is labeled.
Sometimes, instead of Selfing, sibbing is adopted in which, the tassel of all plants of a variety or line is covered with tassel bag and pollen grains collected from all plants next day morning, mixed and used to pollinate the silks (covered by silk bags) of all the plants of the same line and again bagged and labeled.
Since maize plant is highly cross-pollinated, for crossing purpose, we use controlled hand pollination as detailed below:
Cutting of silks 2-3 days after its emergence and covering with a butter paper bag.
The tassel of the male parent should be covered with tassel bag 18-24 hours before pollination (usually on the previous day evening).
Collection of pollen dust in the tassel bag on the next day morning by bending the tassel & shaking it vigorously.
Pollinating the silks of female parent by quickly removing silk bag (butter paper bag) & dusting the pollen or silks and covering with the same tassel bag & stapling it firmly on the stalk opposite to ear short.
The alternate methods of controlled pollinations are
The method of direct pollination also called as “bottle method” was suggested by Jenkins (1923). In the method, the silks are covered as in “hand pollination” methods. However, when the silks have fully emerged and the tassel is shedding pollen, the silks are cut back and covered with the tassel bag in which a tassel actively shedding pollen removed from the plant is placed. The base of the tassel is kept in a small bottle containing water and disinfectant. The tassel would provide pollen for the next 2-3 days when the silks would emerge. This procedure would provide for full seed set. Water bottle and tassel may be removed after 304 days. This procedure is now rarely used.
The in situ or “Overall method” suggested by Hume (1941) involves covering either the entire plant or from tassel up to the base of the ear, with large cloth bag at the flowering time. The leaves between the ear and the tassel are removed to avoid any interference in the transfer of pollen from tassel to silk being expensive and time consuming this method has not been favoured by the maize breeders.
The monoecious nature of the maize plant has been used to provide simpler and inexpensive methods of controlled pollination for specific programmes. Detasselling (removal of tassels before pollen is shed) of breeding materials sown in an isolated plot would render them as purely pistillate or female plants. The pollen can then be provided from (a) outside, from another field and followed by hand pollination or (b) from pollinator rows specially sown in the same field. The latter approach has been extensively used in developing hybrids, with a common pollinator or half sib progenies. SORGHUM (Sorghum bicolor L.) Family: Poaceae (2n = 20)
The inflorescence varies from a compact head to an open panicle carrying two types of spikelets in hairs. The sessile spikelet (perfect) is relatively broad and large as compared to the pedicillate one (sterile or staminate). The sessile spikelet is fertile and sets grain which consists of two glumes, one hairy lemma, and a small pales. Androecium consists of three stamens, two lodicules. Gynoecium is monocarpellary, unilocular, superior ovary with two styles having feathery stigma.
Sorghum inflorescence blooms in the early morning from top to bottom of the panicle. The anthers and stigma push-out as the glumes open. The anthers dehisce as they are exerted from glumes or little later. The stigmas are receptive from m1-2 days before blooming to 8-16 days after blooming depending on the variety and climatic conditions. Stigma exposed before the anther dehiscence are subject to cross-pollination (<5 to 50%). Cross- pollination is greater in varieties with open panicle than in varieties with compact panicle.
Hand emasculation and hand pollination:
Artificial cross-pollination can be done by hand emasculation of the floret in the selected portion of the panicle of the seed parent in the previous evening and hand pollinating with the pollen collected from pollen parent on the next day morning, whose panicles are bagged in the evening hours of previous day. The emasculated panicle is to be bagged after emasculation & after pollination until seed set & should be labeled properly.
Emasculation by hot water treatment & hand pollination:
Amass method of emasculation has been devised wherein the sorghum earhead (ready to bloom) is immersed in hot water at 480 C for 10 minutes, which will kill the pollen grains but do not affect stigma. The emasculated heads are bagged & hand pollinated on the next day morning with pollen grains collected from male parent in a bag & the bag is replaced after pollination until seed set.
Use of male sterile line & hand pollination:
Flowers of male sterile plant (used as female or seed parent) need not be emasculated but must be bagged before natural cross- pollination occurs & then pollinated with the pollen of desired male parent when the stigmas are receptive. Both genetic and cytoplasmic male sterility are available in sorghum.
BAJRA (pennisetum typhhoides) Family: Poaceae 2n=14
The inflorescence of bajra is a cylindrical spike. It consists of a central rachis on which infinite number consisting of cluster of spikelets and a whorl of bristles are arranged spirally by means of rachillae. The number of spikelets in each fascicle is usually two (sometimes up to five). Each spikelet contains two sterile glumes and two florets. The lower floret is usually staminate and the upper one is perfect. The lower flower is reduced to a single lemma which more or less encloses three stamens (no lodicules) with pennicilliate anthers. The upper floret consists of a lemma, palea, three stamens and an ovary with a single style & bifid, non- plumose stigmas without lodicules.
Protogyne is present in this crop. And hence the crop is highly cross-pollinated. First the stigmas of perfect floret begin to appear on the inflorescence from top to bottom 2-3 days after emergence of the spike from the flag leaf & remain receptive for 1-2 days. Next, the anthers of perfect floret emerges from top to bottom. Lastly, the anthers of staminate floret emerges from top to bottom.
For Selfing, spikes are usually bagged by using long narrow Kraft paper bags before emergence of stigmas. As the spike elongates, it may be necessary to adjust the bag to cover the lower spikelets. Since the plant tillers profusely and there is variation in spike emergence in the tillers of some plant, anther special. Procedure to produce selfed seeds is to encloses within a same bag, two full spikes of different tillers of the same plant, one a few days older than other & ready to shed pollen as the stigmas are emerging from the another younger spike.
Emasculation in bajra is a laborious and difficult due to small size of flowers & late development of anthers in relation to the stigmas. Actually the crop also does not require emasculation for making crosses. The interval between appearance of the stigmas and anthers can be used advantageously for artificial cross-pollination. The interval is greater in the lower most region of the spike. Hence 4/5 of the upper portion of the spike is removed and the rest 1/5 portion is bagged before the stigmas appear on the spike to prevent cross-pollination by insects. The pollen grains are collected in bags from the male parent by bagging the spikes of male parent as soon as the anthers start emerging from spike on the previous evening. The receptive stigmas of female parent (before emergence of anthers) are pollinated on next day morning with the pollen of the male parent by dusting fresh pollen collected in the bag on them or by shaking a spike from the desired male parent which is shedding pollen over the exposed stigmas. It is necessary to ensure that no stigmas remain unpollinated. Crossing can also be done by using male sterile lines which serve as female parent only m& pollinating them by pollen grain of male parent either through hand pollination or by growing both male sterile plant and male parent in definite row proportion in isolation & allowing the female parent to receive pollen grain through pollinating agents like wind or insects.
FINGER MILLET (Eleusine coracana (L.) Gaertn.); Poaceae (2n=36)
Inflorescence of ragi is borne on a long peduncle, from the end of which four to five spikes radiate in a whorl called ‘the fingers’, with an odd one a little lower down the whorl called ‘the fingers’ with an odd one a little lower down the whorl called ‘the thumb’. The rachises of the spikes are flat, spikelets are4bsessile arranged in two rows alternately attached to one side of the rachis. Each spikelet contains 3 to 7 flowers with common glumes at the base. The flowers are perfect except for the terminal flower which may be either staminate or sterile. The flowers overlap each other forming a compact tapering spikelet. Each floret contains one lemma, one palea which enclose three stamens with short anthers & long lodicules & one ovary with branched stigmas.
The spikes may take 6 to 13 days to complete flowering. In a spikes or finger, the floret in the top most spikelet opens first with flowering proceeding downwards, but within a spikelet, the lowest flower opens first and the process continues upward. The flowers open between 1 and 5 am. Pollen viability is very short lasting only 10-15 minutes. Flowering takes place simultaneously in all fingers. The anthers requires about 45 minutes for dehiscence after emergence. The delay in opening permits emasculation after the anthers have emerged. The stigma is very short, self pollination is the rule. However a very small percent of cross pollination may occur.
In ragi we find following four types of panicle shapes.
Since a little amount of out-crossing occurs in ragi to ensure self- fertilization, butter paper bags are used to cover the panicle (ear heads). After blooming completes in all the spikes, these bags are removed
The florets that are going to open next day morning are selected on the evening of previous day. Other florets in the spikelets are cut-off. The three stamens of the selected florets are removed with the help of fine forceps by separating the lemma & pales. However, hand emasculation’s are difficult to make due to make due to small of florets & usually done with the magnifying lens. The flowers after emasculation are bagged to avoid natural crossing.
Test- tube method:
The delayed dehiscence of anthers after emergence may be utilized in
making emasculation. A wide test tube (or small flask) lined with moist filter paper is inverted over the spike and plugged with cotton. Due to high humidity developed inside the tube, the anthers emerge intact without shedding the pollen which may be cut-off & removed.
Immersing the panicles for 2-2.5 minutes in hot water at 520 C is effective in killing the pollen grains.
Pollination are made next day morning by collecting ripe anthers & breaking them over the stigmas or by dusting the pollen from a flowering spike of the male parent over the emasculated spikes.
Contact method of production of crossed seeds:
This is a special method of obtaining crossed because of minutes size of florets. If the male parent has a dominant marker gene & that of female parent has corresponding recessive gene, both the male & female plants are grown together side by side & the ear heads of both the plants are enclosed in the same bag until blooming is completed. The F1 plants with dominant character may be identified by saving seeds set on female parent having recessive character.