Major Cultivated Crop Varities OF Pakistan
Wheat Origin Wheat is the world’s leading cereal grain and most important food crop. This is evolved from wild grasses. The genetic origin of wheat is a classic example how closely related species combine in nature to form a polyploid series. The place of origin was the area known in early historical times as the fertile crescent – a rich soil region in the upper reaches of the Tigris– Euphrates basin also known as Asia minor center of origin or Near East. As reported by Vavilov, 1926, the primary centre of origin for bread wheat was the Central Asia while T. durum, T. turgidum and T. dicoccum were originated in the Abyssinian centre of origin. Substantial genetic variability among the wild relatives of wheat is found in Iran, Israel, and bordering countries.
Crop Systematics and Species Relationship Wheat belongs to the grass family Graminae and to the tribe Triticeae. The tribe forms a distinct natural group characterized by a compound spike, laterally compressed spikelets with two glumes, single starch grains and fairly large chromosomes in multiples of seven. The genera Triticum, Aegilops, Secale, Agropyron and Haynaldia are distinct and form a natural subtribe the Triticinae, within the tribe Triticeae. The genus Triticum has a large number of species including cultivated types (Table 1). All the species of wheats are grouped in three natural groups einkorn, emmer and dinkel wheat that form a polyploid series with chromosome numbers n=7, 14 and 21 respectively. The first domesticated forms of wheat are considered to have evolved through selectors of cultivable types from wild diploid species T. boeticum subsp. aegilopoides to produce T. monococuum (einkorn wheat) and the wild tetraploid T. dicoccoides to produce T. dicoccum (emmer wheat) simultaneously. The hexaploid wheats were the last to evolve and are the most modern. Genetic Evolution The genome analysis, the determination of evolutionary relationships on the basis of chromosome pairing in hybrids to understand the evolutionary and species relationship in Triticum has been extensively studied by Kihara and his colleagues. These studies indicated that allopolyploidy was involved and that wheat evolution followed a system of diploid divergence and polyploid convergence. Evidences indicate that the tetraploid wheats (AABB genome) evolved from an allopolyploid combining T. monococcum (AA) and an unknown, which was supposed to be the progenitor of BB genome. Though it was believed to be Ae .spelloides, the ‘B’ genome donor, recent studies revealed that it could not be progenitors of hexaploid wheat. Further, natural hybridization of a tetraploid with wild grass (Ae. squarrosa L. DD genome renamed as T. tauschii) gave rise to hexaploid wheats like T. aestivum, T. compactum etc. Origin of Hexaploid Wheat Triticum monococcum x Unknown (Ae. spelltoides) 2n = 2x = 14 (AA) 2n = 2x = 14 (BB) F1 AB Chromosome doubled Ae. squarrosa x Tetraploid wheat (= T. tauschii) (T. turgidum) 2n = 2x = 14 (DD) 2n = 4x = 28 (AABB) F1 ABD Chromosome doubling Hexaploid wheat 2n = 6x = 42 (AA BB DD)
Table 3: Ploidy levels and Genomes of different species of the genus Triticum Ploidy level Scientific name Common name Genome Diploid T. urartu Wild einkorn AA T. boeticum Bioss spp. aegilopoides ssp. thoudar Wild einkorn AA AA T. monococcum L. Cultivated einkorn AA T. sinskajae A. Filat and Kurk Cultivated einkorn AA T. speltoides (Tausch) Grene. x Richter Cultivated einkorn BB Tetraploid T. dicoccoides (Korn) Schweinf Wild emmer AABB T. dicoccum (Schrank.) Schulb Cultivated einkorn AABB T. palaecolchicum Men. - AABB T. carthlicum Nevski Persian wheat AABB T. turgidum L. Rivet or cone wheat AABB T. polonicum L. Polish wheat AABB T. durum Dest. Durum or Macaroni wheat AABB T. turanicum Jakabz Khorasan wheat AABB T. araraticum Jakabz Wild emmer AABB T. timopheevi Zhuk - AABB Hexaploid T. spelta L. Spelt or dinkel AABBDD T. vavilovi (Tum) Jakabz - AABBDD T. macha Dek and Men. Spelt AABBDD T. sphaerococcum Perc. Indian dwarf AABBD T. compactum Host. Club wheat AABBDD T. aestivum Bread or common wheat AABBDD T. zhukovskyi Men. AABBGG Miller, 1987 Floral Biology The inflorescence of wheat is a spike bearing two opposite rows of lateral spikelets and a single terminal spikelet on the primary axis. The unit of spike is called spikelet. Two to five florets are born in each spikelet, subtended by a pair of glumes. Each floret contains three anthers and a
pistil bearing two styles each with feathery stigma and two ovate lodicules which are modified perianth structure. Florets at anthesis are forced open by swelling of the lodicules. Flowering starts several days after the wheat spike emerges from the boot. Florets on the main culm flower first and those on the tillers flowering later. Flowering begins in the early morning and continues throughout the day. Two to three days are required for a spike to finish blooming. A wheat grain is caryopsis, a small dry, indehiscent, one seeded fruit with a thin pericarp consisting of a germ or embryo and an endosperm. Wheat Plant Wheat Spike Objectives of Breeding 1. Grain yield: The main objective is to create new genotypes improved in features that contribute to greater yield potential and improved product quality. Yield potential in wheat refers to the ability of the plant to manufacture, translocate and store food materials in the wheat grain. Emphasis is now being given to the breeding of high yielding wheat cultivars. The strategy for turning the green revolution which started during 1960’s with the introduction of Mexican dwarf wheat varieties such as Sonora 64 and Lerma Rojo into an ever-green revolution -includes • Collection, evaluation and utilization of germplasm from diverse sources. • Choice of appropriate parents using biometrical approaches. • Exploitation of segregating generations from chosen crosses. • Biparental mating for releasing locked up genetic variability. 2. Stability and adaptability: Breeding for stability implies that the variety developed is affected to minimal loss from vagaries of climate, stress or destructive pest. Breeding for adaptability implies that that variety is adaptable over a wide range of environments with consistency in performance. For a given agro-climate, it is also necessary to develop varieties / hybrids that have high yield with specific adaptation. 3. Breeding for disease and pest resistance: The development of cultivars of wheat with resistance to destructive disease pathogens like rust, karnal bunt, leaf blotch, smuts, powdery mildews is also a major objective for increased yield in wheat breeding programme. Similarly
breeding for resistance to insect pests such as hessian fly, particularly resistance to various biotypes is also important. 4. Breeding for quality: Wheat is cultivated primarily for its grain, which are mainly processed in to flour utilized for numerous end products. The quality of end product is of utmost consideration to the wheat consumers. Broadly the wheat grain quality criteria include features like physical appearance, processing qualities, nutritional values and biological properties each of these is composed of several components influenced by genetic make up of the variety. (i) Breeding for physical quality: The objective is to develop a variety with well accepted physical characteristics like colour, vitreousness, texture / hardness, appearance, grain weight, test weight. (ii) Breeding for chemical composition: Wheat grain is one of the important source of human nutrition and is a rich source of protein, starch and minerals. Some of the objectives that determine the chemical composition of wheat grain that has implications on higher quality include: a. Starch composition – modification of functionality of starch and amylose and amylopectin content as per desirable end product such as noodles, pasta, thickness, binding agents, bread etc. If the objective is to produce starch with no amylase, then breeding for waxy type wheat would be necessary. b. Protein content - Wheat grain has a special significance of breeding for high protein and low protein for bread and biscuit purposes respectively and also for different end products. c. Protein quality: The ratio of gluten / gliadin fractions dictates the quality of end produce. The control of expression of these two under genetic control is targeted for specific end product quality. (iii). Nutritional quality: Objective is to improve the amino acid balance for better nutritional quality. Wheat grains deficient in lysine and there is a negative correlation between protein and lysine content. Efforts to improve lysine as well as high protein content are needed to improve nutritional quality of wheat. (iv). Breeding for market quality: Includes physical characteristics, flour recovery milling quality, dough quality as well as gluten content useful for specific product. Methods of Breeding 1. Yield improvement: The wheat improvement programme in India is one of the biggest national programmes in the world. Being self pollinated crop, the basic methods of wheat improvement include pure line, pedigree, bulk and back cross method. The first phase for development of improved wheat genotypes was the adoption of pure line selection from indigenous landraces and then introduction of improved exotic types. Later hybridization programme involving intercrossing of systematically selected genotypes in a system of single, double or complex multiple crossing schemes followed by various forms of pedigree selection. Most systematic varietal improvement work was started by Sir Albert Howard and his wife Mrs. G.L.C. Howard at IARI in 1906. They made a comprehensive collection of local sorts / landraces
grown in various parts of the country and selection among them led to the development of several varieties such as Pusa 4, Pusa 6 and Pusa 12. In durum wheat, the variety Ekdania 69 was developed through pure line selection. (i). Hybridization: In the first stage, local selections and their derivatives were intercrossed to breed superior strains. Several improved varieties were developed such as NP 42, NP 80-5, NP 125, NP 165, NP 718, NP 842, NP 846, PbC 518, PbC 591, PbC 281 etc. Development of semi-dwarf varieties : The semi-dwarf wheat varieties developed by Norman E. Borlaug and his colleagues using Norin 10 dwarfing genes Rht-B1 and Rht-D1 attracted the attention of Indian wheat breeders. Introduction of semi dwarf spring wheat, Sonora 63, Sonora 64 and Lerma Rojo 64A and Mayo 64 in 1963 became the base material for enhanced productivity. These varieties become popular with the farmers because of their higher yield and the source of quantum jumps in wheat production. In1964, these varieties significantly out yielded the Indian check varieties NP824 and C306 by 15-30% margin. Selection from 613 advance lines led to the development of Kalyan Sona, Sonalika, Chhoti Lerma and Safeda Lerma which triggered “wheat revolution in India. The next jump in yield improvement by the release of varieties like WL711 and Arjun (HD 2009) in 1975 and these varieties were dominated in north western plain zone comprised of plains of J & K and Himachal Pradesh, Punjab, Haryana, Rajasthan, Delhi, Western U.P. and Uttarakhand till they were replaced by HD 2329 because of its greater adaptability, short stature and other agronomic advantages. During the period of yield plateuing (1975-85), IB/IR translocations have played a significant role in breaking the yield barriers resulting in spurt in wheat yields.The more recent genotypes like PBW 343, UP 2338 and WH 542 derived from crosses with Veery genotypes, but still carrying IB/IR translocation have higher yield potential. (ii). Backcross breeding: It is important in improvement of one or more highly heritable characters in an otherwise promising variety. It is most rapid and inexpensive method for achieving short term goals in wheat breeding. Incorporation of genes for rust resistance in Indian wheat varieties through backcross method led to the development of several near isogenic lines like HW 2004 (backcross line of C 306 carrying Lr24), HW 2044 (backcross line of PBW 226) for commercial cultivation. (iii). Mutation breeding: In mutation breeding, using mutagenic agents, several mutants were induced and the combination of traits from two or more mutants in the same genetic background to develop new varieties is carried out. Similarly mutant traits are recombined with those from other germplasm. Using γ-rays, amber grained mutants of Sonora 64 and Lerma Rojo were produced and released as Sharbati Sonora and Pusa Lerma respectively. 2. Systematic breeding for rust resistance: Most organized work on breeding for rust resistance was carried out at IARI under the leadership of Dr. B.P. Pal. The research carried out during the period from 1935 to 1948 by Prof. K.C. Mehta, Dr. B.B. Mundkur and their associates had generated very comprehensive knowledge on the annual cycle of rust occurrence in India and also the physiological specialization of races.
Efforts to incorporate high degree of resistance to individual rusts led to the development of brown rust resistant varieties NP783, NP784, yellow rust resistant varieties NP785 and NP786 and black rust resistant varieties NP789 and NP790. Later on, attempts to combine resistance to more than one rusts resulted in the development of varieties like NP 792, NP 797, NP 798 and NP 799 combined high degree of resistance to black rust and fair degree of resistance to brown rust. Finally combining resistance to all the three rusts resulted in the development of wheat variety NP 809. Similarly karnal bunt caused by Nevossia indica which was considered as minor disease became a major problem with severe epidemics. Using artificial inoculations methods, varieties resistant to karnal bunt have been identified. Durum variety PDW 215 and triticale TL210 showed immunity. Bread wheat varieties like HD29, HD30, HP1531, ISWRN191 and ISD227-5 were found promising for karnal bunt resistance. 3. Breeding for quality: In India, three types of wheat T. aestivum (Bread wheat). T. durum and T. dicoccum are being cultivated of which bread wheat accounts for a major share to prepare wide array of home foods and chapatti. Durum wheat is used for semolina preparation. Wheat is the only cereal which has gluten and this makes it unique in terms of processing possibilities into different products. Breeding programme for developing wheat cultivars targeted to specific food markets include understanding the genetic control of specific grain components as well as their relationship with processing qualities. Grain hardness is an important criterion for starch quality and wheat end use and is the major determinant of the level of starch damage during milling. Starch from hard grains fracture more during milling and absorb more water by the flour. Hard grains are used for baking breads and noodle production while soft wheat are used for biscuit flour. The major factor controlling grains hardness is a single locus Ha on chromosome 5D of wheat encoding a 15 kDa protein called Friabilin. Friabilin is found to consist of three polypeptides namely puroindoline a, puroindoline b and GSP-1 which confer softness to the grains. Mutations at this locus transform grain from soft to hard type. Several mutants at this locus have been characterized. Similarly grain protein content and quality are also most targeted traits in quality breeding and the highly influenced by the environmental conditions. The various components of wheat grain protein are Albumins, Globulins, Gliadins and Glutenins. Gliadins and glutenins together are known as gluten. Glutenins confer elasticity, while gliadins confer mainly viscous flow and extensibility to the gluten complex. The genes encoding glutenin and gliadins are characterized by group 1 and group 6 chromosomes of wheat. Several studies revealed that HMW (high molecular weight) glutenin subunits are important for various quality parameters for different end uses. The best breads are produced from dough that has a mix of strength, elasticity and plasticity properties largely determined by the balance between the gliadin and glutenin subunits. Genetic control of quality traits has been well characterized. Introgression of a piece of 6B dicoccoides chromosome, a source of high protein into low protein lines of durum wheat as well as glutenin subunits from hexaploid wheat has been carried out to improve the quality characteristics for bread making in durum wheat which lacks ‘D’ genome.
The presence of translocations 1B/1R and 1 A/1R resulted in the poor quality due to the production of secalins from rye chromosome. The complex and genetically additive nature of inheritance of most quality traits has led to the development of a range of indirect tests. By applying these tests in early generations, the population mean is favourably shifted and increased frequency of homozygous lines with desired quality characteristics can be expected at the end of the breeding process. Selection and testing for quality begins in early generations. Crop management x quality interactions is of critical importance. At least one parent with desired quality must be selected in designing crossing strategies as end use requirements determine the potential new cultivars. In general, pedigree or modified pedigree method has been widely used. Exploration of genetic variation for quality traits present in wild relatives and alien species may require pre-breeding before they are widely used in the breeding programme. Novel biotechnology techniques have opened the possibilities of investigating the basic and biochemical aspects of individual protein subunits and of other molecules contributing to the end use quality of wheat. Biofortification is the process of breeding food crops that are rich in bioavailable micronutrients. CIMMYT (International Maize and Wheat Improvement Center) Mexico, is leading the Harvest Plus research effort in collaboration with national agricultural research and extension systems for biofortification of wheat for high iron and zinc content using conventional and molecular breeding approaches. New Plant Type for Quantum Jump in Yield To cope up the ever increasing demand of wheat which will be 109 million tons by the year 2020, the present level of productivity has to be increased to 4.4 tons / ha. The only approach for achieving quantum jump in productivity is to restructure the wheat plant architecture which can yield up to 8 tons / ha. the Indian Agricultural Research Institute, New Delhi developed new plant type (NPT) wheat’s, utilizing a local germplasm SFW and released wheat’s and genetic stocks, which have high 1000 grain weight; high grain number per spike; higher biomass; thick, broad, semi erect and dark green leaves; thick stem; plant height 85-100 cm and good root system. In these NPT genotypes the negative correlation between grain weight and grain number per spike has been broken. These genotypes are also having post-anthesis mobilization of stem reserve to sink. New Plant Type Wheat
Biotechnology in Wheat Improvement Biotechnology techniques and tools have immense potential in crop improvement programme, encompasses utilization of gametoclonal variation, somaclonal variation, genetic selection for biotic and abiotic stresses, gene transfer through embryo rescue, protoplast technology, somatic hybridization and recombinant DNA technology. The important aspects of wheat biotechnology are: 1. In vitro production of haploids: Haploids are of significance for studies on the induction of mutations and for the production of double haploids / homozygous plants. Most of the anther culture studies in wheat have been conducted with T. aestivum and callus, embryoids or haploid plants have been obtained with varied success. 2. Somaclonal variation: The variation observed in plants produced through tissue culture is known as somaclonal variation. Wheat has proved to be an excellent material for the induction of somaclonal variation. The variations have been observed for plant height, size and shape of leaves, length of awns, fertility of spikes and the size shape and colour of the seeds. 3. Molecular markers and wheat breeding: The development in molecular genetics in wheat have been relatively slow due to its ploidy level, the size and complexity of its genome, the very high percentage of repetitive sequences and the low level of polymorphism. However, due to the large number of disease and pest resistances controlled by major genes, the mapping of such genes has dominated the research activities in wheat molecular genetics. On the other hand, the hexaploid nature of wheat and its amenity to cytogenetic manipulation have offered unique tools for molecular genetics of wheat. The use of various aneuploid stocks, such as nullitetrasomic and ditelosomic lines to assign molecular markers to specific chromosome arms, chromosomal deletion stocks for physical mapping of markers and single chromosome substitution lines to map genes of known chromosomal location, are being taken up seriously. Despite the low level of variability available in wheat, extensive molecular maps have now been prepared and as many as 36 traits have been tagged using different molecular markers. The availability of a large number of molecular markers in wheat suggests their use in intra- specific analysis, comparative analysis as well as gene introgression studies. However, the application of molecular markers to wheat breeding involving marker assisted selection is still in its infancy despite the availability of a lar