Plant Breeding is art and science of improving the heredity of plants for benefit of mankind. In Evolutionary concept, plant breeding is merely a continuation of the natural evolution of the crop species, changing its course of direction in the benefit of greater use to mankind.

This also be defined as science of selection -Plant breeding is essentially an election made by man of the best plants within a variable population as a potential cultivar. In other words plant breeding is a ‘selection’ made possible by the existence of variability. Selections become the earliest form of plant breeding.

History of plant breeding

Plant breeding started with sedimentary agriculture and domestication of the first agriculture plants, the cereals were chosen by the early man. They learned to look for superior plants to harvest. The domestication was hastened by early practice of harvesting mutant plants with special traits. This forms the ancient type of plant breeding. Before Mendal’s discovery there was some plant breeding include selection and hybridization experiments. But the plant breeding was only hastened after discovery of Mendal’s law on pea, thus lead a new science “Genetics”. Modern plant breeding is applied genetics but its scientific basis is broader and uses conceptual and technical tools, molecular biology, cytology, systemetics, physiology, pathology, entomology, chemistry, and statistics (biometrics) and has also developed own technology.

Plant Breeding efforts may be divided into different historical landmarks-

1. Prehistoric plant breeding- domestication of crops

This includes domestication of crops by ancient people. Domestication is also continued so far.

2. Pre-mendal Plant Breeding

Economic botany and great Columbian exchange-

There were some experiments before Mendal on hybridization and selection. Some seed companies were also established based on success on selection. Another part discovery of North America by Columbus in 1492 triggered unprecedented transfer of plant resources, first from old world to the new world, or the New World to the old. This increased variability in total genetic resources.

3. Mendelian genetics and the green revolution

Mendel's experiment stimulated research by many plant scientists dedicated in improving crop production (plant breeders) through plant breeding. The most famous contribution of Mendelian genetics was hybridization. There was remarkable improvement in three economically important crops that made the food deficit world into a food surplus world. This is called the green revolution. The first, development of hybrid maize, the second development of high yielding and input responsive “semi-dwarf wheat” (CIMMYT breeder N.E. Borloug received Nobel prize for peace in 1970), the third is high yielding “short sutured rice” cultivars. Similarly the remarkable improvements were done in other crops like sorghum and alfalfa.

4. Molecular genetics and bio-revolution

Totipotency shown by plants gave rise to tissue culture techniques such as somatic hybridization, doubled haploid production, clonal propagation, and in-vitro selection. Intensive research in molecular genetics has led to development of recombinant DNA technology (popularly called Genetic Engineering). Advancement in Biotechnological techniques has opened many possibilities for breeding crops. Thus mendelian genetics allowed plant breeders to perform some genetic transformation in few crops, molecular genetics provides key not only the manipulation of the internal structure but also their “crafting” according to plan.

Physiological and Molecular Wheat Breeding

Plant breeding has traditionally applied a trial-and-error approach in which large numbers of crosses are made from many sources of parental germplasm. Progenies are evaluated for characters of direct economic interest (e.g., grain yield and grain quality) in target environments. Good performing parental germplasm, crosses, and progenies are selected for further use or testing. In many programs “breakthroughs” in improvement are made simply by finding superior sources of parental germplasm among the numerous sources tested. This conceptually simple approach has been highly successful in many crop species and numerous breeding programs. The approach has often succeeded in the absence of in-depth knowledge about the physiological basis for superior performance. In some crops such knowledge has been obtained by doing retrospective analyses of prior genetic gains. Breeders have not applied this knowledge to a significant extent as a guide to further improvements, but instead have taken any avenue of improvement that happens to arise from direct selection for yield and economic performance. However with increased population, there is need to increase yield further and breeding require more scientific approaches to handle the problem.

While wheat breeding programmes worldwide have achieved significant genetic gains in yield potential without the aid of physiological selection tools (Rajaram and van Ginkel, 1996), breeders, as well as physiologists, generally agree that future successes will be realized through a greater integration of disciplinary research (Jackson et al., 1996). There are two principal reasons for this. Until the year 2020 at least, demand for wheat is expected to grow by approximately 1.6 percent/year worldwide and by 2 percent/year in developing countries (Rosegrant et al., 1995). This implies a need to almost double the world average wheat yields in that period, and albeit steady, recent rates of yield growth, as well as improvement in genetic yield potential (Sayre et al., 1997), are too low to keep pace with future demand. Thus, there is an urgent need to develop new and more efficient wheat breeding methodologies to complement existing breeding techniques, as well as to identify new traits, which will drive faster yield gains.

Secondly, several recent studies suggest that physiological selection traits have the potential to improve genetic yield gains in wheat. At the International Maize and Wheat Improvement Center (CIMMYT), research has demonstrated associations of a number of physiological traits, including leaf conductance and photosynthetic rate, with performance of a historic series of cultivars in a high-yielding environment (Fischer et al., 1998). In addition, work emphasizing genetic improvement under marginal environments has illustrated that physiological traits, including canopy temperature depression, when measured in hot selection environments in Mexico, were strongly associated with performance in yield trials at a number of warmer wheat-growing regions worldwide (Reynolds et al., 1994a). In addition, physiological selection traits for drought tolerance have been incorporated into a number of Australian wheat breeding programmes, including higher transpiration efficiency, greater early vigour and reduced tillering (Richards et al., 1996). Physiological selection techniques are now being evaluated for their role as complementary tools in wheat breeding at CIMMYT (Reynolds et al., 1998a).

Physiological criteria are commonly though not explicitly used in breeding programs. A good example is selection for reduced height, which improves lodging resistance, partitioning of total biomass to grain yield, and responsiveness to management. Selection for reduced height and improved adaptation to environment has had a profound impact on modern plant breeding, and the improvement in yield potential of spring wheat since the Green Revolution has been shown to be associated with a number of other physiological factors (Reynolds et al.,1999). Nonetheless, most breeding programs do not put much emphasis on selecting physiological traits per se (Rajaram and van Ginkel, 1996). Exceptions would include: 1) the staygreen character, which has been selected for in relation to improved disease resistance and is associated with high chlorophyll content and photosynthetic rate in Veery wheats, for example Seri-82 (Fischer et al., 1998), and 2) more erect leaf angle, a common trait in many high yielding bread and durum wheat plant types that was introgressed into the CIMMYT germplasm pool in the early 1970s (Fischer, 1996).

In future physiological application impacts may arise through: • Focusing physiological work on an appropriate range of germplasm (which will depend on the specific breeding objectives); • Working with larger populations to enable extrapolation of findings to breeding methods; • Identifying traits for use as indirect selection criteria, in addition to those already used in core breeding programs; • Identifying traits for use as selection criteria in introgression programs; • Conducting selection trials in more representative environments, and • Developing tools that could be quickly and easily applied to large numbers of segregating lines.

Genetic basis of physiological traits

During the past two decades, molecular tools have aided tremendously in the identification, mapping, and isolation of genes in a wide range of crop species. The vast knowledge generated through the application of molecular markers has enabled scientists to analyze the plant genome and have better insight as to how genes and pathways controlling important biochemical and physiological parameters are regulated. Three areas of biotechnology have had significant impact: the application of molecular markers, tissue culture, and incorporation of genes via plant transformation. Molecular markers have enabled the identification of genes or genomic regions associated with the expression of qualitative and quantitative traits and made manipulating genomic regions feasible through marker assisted selection. Molecular marker applications have also helped us understand the physiological parameters controlling plant responses to biotic and abiotic stress or, more generally, those involved in plant development.

Traits of breeders' interest

Yield components

The most important step in improving genetic yield potential of wheat in favourable environments was the introduction of Rht alleles. The effect of the gene is to increase partitioning of assimilates to yield at the expense of non-grain biomass. It is interesting that progress in yield since the development of semidwarf lines has also been associated with increased partitioning (Austin et al., 1989; Sayre et al., 1997) and not, for example, improved radiation use efficiency (Slafer et al., 1996). Morphological traits associated with increased yield potential in CIMMYT wheat between 1962 and 1988 include grain number and harvest index (HI) (Sayre et al., 1997). While grain number can be used as a guide to visual selection, HI is less readily evaluated with the eye, and neither trait is reliably expressed in small plots or at low density in early generations. In addition, there is a theoretical limit to HI, estimated at 60 percent (Austin et al., 1980a), which would imply that unless biomass is raised, yields can increase by 20 percent at the most, using HI as a selection criterion.

Steady genetic gains in yield potential can be expected from recombining elite germ-plasm (Rasmusson, 1996) and refinement of selection methodologies. However, significant jumps in yield potential will almost certainly require introgression of genetically diverse sources (Kronstad, 1996) to permit evaluation of new yield determining genes in different backgrounds. At CIMMYT, one approach used is to cross parents with high expression for specific morphological traits, including large spikes, large grain size and large semi-erect leaves, based on the conceptual idea of improving both source (photosynthetic capacity) and sinks (grain number) simultaneously.

Physiological traits

A recent study conducted in a high-yielding environment in Mexico revealed that leaf photosynthetic rate, leaf conductance and canopy temperature depression (CTD) were all associated with yield progress in a set of eight spring bread wheat lines, representing progress in yield potential between 1962 and 1988 (Fischer et al., 1998). One important implication of this work is that such traits can be measured reasonably simply in the field, suggesting a potential methodology for screening physiologically superior lines. The idea is supported by studies in the same environment, where homozygous sisters from crosses between high- and low-CTD parents showed good association between yield and CTD (Reynolds et al., 1998b).

The physiological basis of the association of CTD and yield is unknown. However, since CTD is a direct function of evapotran-spiration rate, which itself is determined by a number of physiological and metabolic processes including stomatal conductance, photosynthetic rate, vascular capacity, etc., there are a number of alternate hypotheses. For example, high CTD may be indicative of a high demand for photo-assimilation caused by many, rapidly filling kernels (i.e. sink strength) in physiologically well-adapted lines. Alternate hypotheses are: (i) high CTD reflects an intrinsically higher metabolic capacity; (ii) high CTD is indicative of a good vascular system capable of meeting evaporative demand; and (iii) high CTD reflects a less conservative response to reduced soil water potential between irrigations. A precise understanding of the physiological basis of the association of CTD with yield will improve the likelihood of genetically improving yield potential. Canopy temperature depression, like yield, is a genetically complex trait, so selection for CTD directly is likely to be a slower approach to raising yields than selecting for the genes specifically related to current yield thresholds. Nonetheless, CTD offers the potential to discard genetically inferior lines during plant selection, adding efficiency to the breeding process. This possibility will be expanded on in section "Using canopy temperature depression to increase selection efficiency".

Additional physiological traits that may have implications on yield potential are translocation from the stems to the grain of soluble carbohydrates (stem reserves) and the ability to maintain green leaf area duration (stay-green) throughout grainfilling (Jenner and Rathjen, 1975). Both traits would be more important where a crop was assimilate limited, and physiological studies have indicated that higher yielding lines depend less on stem reserves than lower yielding ones (Stoy, 1965; Austin et al., 1980b). It has also been suggested that the two traits may be mutually exclusive, since loss of chlorophyll and stem reserve mobilization seem to be consequences of plant senescence (Blum, 1998).

Another area that has yet to be explored with respect to raising yield potential is the optimization of phasic development. The relative length of the cardinal phenological stages is a function of the interaction of environmental cues with genes determining earliness per se and sensitivity to photoperiod (Ppd) and vernalization (Vrn). The reproductive stage of development is pivotal in determining yield potential, and genetic variability for its duration relative to other phenological stages is known (Slafer and Rawson, 1994). The possibilities of manipulating this trait to improve yields will be discussed later.

Canopy-based traits

The erectophile leaf canopy has been proposed as a trait that could increase crop yield potential by improving light use efficiency in high-radiation environments. While some studies support the hypothesis, for instance in barley (Angus et al., 1972), others are less clear cut. For example, work at CIMMYT with near isogenic lines of spring wheat showed the erect leaf trait to be associated with higher grain number and increased rate of transpiration based on measurements of CTD, carbon isotope discrimination and relative water content of flag leaves, but it was not associated with yield itself (Araus et al., 1993). Based on this hypothesis, a large number of accessions from germplasm collections were screened for erect leaves at CIMMYT in the early 1970s. The trait was introgressed into the wheat germplasm base, and it is present in some of CIMMYT’s best yielding durum and bread wheat lines (Fischer, 1996).

The idea that higher yield potential could be achieved by designing a plant type that is well adapted to the commercial practice of sowing high-density monocultures was introduced 30 years ago by Donald (1968). He used the word ‘communal’ to describe the ideotype. In a more recent study, yield progress in CIM-MYT lines seemed to be associated with the communal trait, defined as the relative lack of yield response of higher yielding lines to a reduction in interplant competition; in contrast to lower yielding lines that responded considerably to removal of neighbouring plants after flag leaf emergence (Reynolds et al., 1994b). Such observations have important implications to plant breeding methodologies where individual plant selection, or even mass selection, is used on segregating generations and bulks. Competition among genotypes is likely to reduce the gene frequency of the communal trait, especially if visual selection favours more competitive plant types. Several studies have shown that selection for yield potential in early generations can be enhanced by reducing interplant competition between genotypes in bread wheat (Lungu et al., 1987), durum wheat (Mitchell et al., 1982), oat (Robertson and Frey, 1987) and rye (Kyriakou and Fasoulas, 1985). Studies have not yet shown a physiological basis for the communal trait but they seem to suggest that it can be selected for empirically. While wider spacing between plants in early generations would increase breeding costs, avoiding selection bias based on plant type in early generations may be a useful compromise. The relative success of single seed descent methodology in European winter wheat breeding would appear to back these conclusions.

Stressed environments

Two of the most important stresses of wheat are heat and drought. Wheat yields can be severely reduced in moisture-stressed environments (Morris et al., 1991), which affect at least 15 million ha of spring wheat alone in the developing world. Over 7 million ha of spring wheat are grown under continual heat stress, namely environments with mean daily temperatures of greater than 17.5°C in the coolest month (Fischer and Byerlee, 1991). In addition, terminal heat stress can be a problem in up to 40 percent of the irrigated wheat-growing areas in the developing world.

Nonetheless, wheat has been traditionally cultivated in many stressed environments, and it is not surprising that the crop is relatively stress tolerant. Wheat’s drought hardiness is apparent from the linear relationships observed between grain yield and water application when measured under moisture stress in field experiments (for example, Sayre et al., 1995), or the simple observation of a plant under severe stress, which will complete its life cycle yielding perhaps only a single viable kernel. At high temperatures, the rate of plant development is increased (Midmore et al., 1984), thus reducing the potential for biomass accumulation. Nonetheless, extensive testing of 16 spring wheat cultivars throughout the heat-stressed regions of the developing world by CIMMYT and the national programme collaborators have shown that warmer environments reduce intrinsic growth rates, as well as the length of the growth cycle, and that there is significant genetic variation in heat tolerance of modern semidwarf wheats (Reynolds et al., 1998a).

Drought-adaptive traits

As one might expect, root characteristics, such as depth and abundance, are known to be associated with performance under drought in many studies with wheat (Hurd, 1968; see also Blum, 1988). Nonetheless, decreased investment in roots in the top 30 cm of the soil has been shown to be stress adaptive, when stress occurs before flowering, and is apparently associated with a strategy that conserves stored soil moisture (Richards, 1991). It is interesting to note that no study has shown a clear effect of dwarfing genes on drought adaptation or rooting patterns, despite the fact that specific height categories may be advantageous over others under certain water-stressed environments (Richards, 1992).

Traits associated with drought tolerance that are easily evaluated with the eye include rapid early ground cover by leaves, leaf glaucousness, leaf pubescence (Richards, 1996a) and erect leaf posture (Innes and Blackwell, 1983). All are associated with conserving available moisture by reducing radiation load to the leaves, or at the soil surface in the case of early ground cover. More difficult to measure, but with apparent value under drought, are abscisic acid (ABA) accumulation (Innes et al., 1984) and spike photosynthesis, which can provide over 70 percent of the assimilates for grainfilling under drought (Evans et al., 1972). Despite the difficulties of measuring spike photosynthesis, it is the awns with their very high wateruse efficiency relative to leaves or glumes that are the major contributors to spike photosynthesis under stress, and these are selected for readily with the eye.

Early escape from progressively intensifying moisture stress, through the manipulation of plant phenology, is a commonly exploited genetic strategy to ensure relatively stable yields under terminal drought conditions (for example, Richards, 1991). In order to exploit a longer growth cycle, adaptive strategies must be employed that enable physiological rather than temporal escape from moisture stress. Probably the best documented is the maintenance of leaf turgor through osmotic adjustment (OA). The benefit of OA was demonstrated by Morgan and Condon (1986) using the progeny of high by low OA crosses. In random F4 - derived sibs grown under drought, OA was shown to be associated on the one hand with yields of field plots and on the other, their increased water use, which in turn was directly related to root function through improved water extraction between 25 cm and 150 cm in the soil profile.

While OA is measured using a laboratory protocol, some of its beneficial effects can be assessed using relatively easy-to-measure traits, such as leaf rolling, which is scored visually, canopy temperature using an infrared (IR) thermometer, or stomatal conductance. In addition, there are techniques such as spectral reflectance (Araus, 1996), which can be used to estimate a range of physiological characteristics, including plant water status and leaf area index. The technique is based on the principal that certain crop characteristics are associated with the absorption of very specific wavelengths of electromagnetic radiation (e.g. water absorbs energy at 970 nm). Solar radiation reflected by the crop is measured and calibrated against light reflected from a white surface. Different coefficients can be calculated from specific bands of the crop’s absorption spectrum, giving a semi-quantitative estimate (or index) of a number of crop characteristics.

Other techniques are available that can integrate physiological processes over the whole or part of the crop cycle. For example, water-use efficiency (WUE) can be estimated using carbon isotope discrimination. The methodology is based on higher affinity of the carbon-fixing enzyme (Rubisco) for the more common 12C isotope over the less common 13C. As the internal [CO2] falls in the leaf, the 12C:13C ratio falls permitting less discrimination in favour of 12C. Lower internal [CO2] is normally associated with reduced stomatal conductance, which would increase WUE, assuming CO2 fixation is not primarily limited by other factors (e.g. thermal deactivation of photosynthesis or other metabolic processes). A lower discrimination value would be associated with higher WUE. While the trait appears to be fairly heritable, its precise association with yield under drought is yet to be fully characterized (Richards, 1996a). A probable and cheaper alternative to carbon isotope discrimination is ash analysis (Araus, 1996), based on the principal that relative ash accumulation in leaf tissue is related to evapotranspiration rate and inversely related to WUE. Relative ash content is measured after complete combustion of tissue.

One drought-adaptive trait that relates specifically to improved partitioning, though not to reproductive growth, is translocation of soluble stem carbohydrates to the grain. While time consuming to measure directly, the trait can be measured indirectly by artificially inducing some of the physiological problems attendant to drought stress through chemical desiccation of green tissue (Blum et al., 1983). Remobilization of stem reserves is associated with increased levels of ABA, which presumably is involved in the triggering of enzymes prerequisite to remobilization.

Heat-adaptive traits

Studies in controlled environments have shown genetic variability in photosynthetic rate among wheat cultivars when exposed to high temperatures (Wardlaw et al., 1980; Blum, 1986). Such differences in photosynthesis under heat stress have been shown to be associated with a loss of chlorophyll and a change in the a:b chlorophyll ratio due to premature leaf senescence (Al-Khatib and Paulsen, 1984; Harding et al., 1990). Studies at CIMMYT demonstrated genetic variability for photosynthetic rate under heat-stressed field conditions (Delgado et al., 1994). In addition, both CTD and flag leaf stomatal conductance, as well as photosynthetic rate, were all highly correlated with field performance at a number of international locations (Reynolds et al., 1994a).

Assimilates are more likely to be yield limiting under stress than in temperate environments, especially as stress typically intensifies during grainfilling. Evidence for this comes from the observation that under stress, total above-ground biomass will typically show a stronger association with yield than partitioning, i.e. harvest index (for example, Reynolds et al., 1994a), while the situation is reversed under temperate conditions (for example, Sayre et al., 1997). For these reasons, stay-green is a trait that has been promoted for heat and drought tolerance. However, as mentioned earlier, evidence suggests that the trait may in fact be a disadvantage under heat stress due to it being associated with the tendency not to translocate stem reserves to the grain (Blum, 1998).

In a number of studies, conductometric measurement of solute leakage from cells was used as a methodology to estimate heat damage to the plasma membranes. Genetic variation in membrane thermostability has been inferred using conductometric measurements in various field-grown crops including spring wheat (Blum and Ebercon, 1981). Shanahan et al. (1990) obtained a significant increase in yield of spring wheat in hot locations by selection of membrane-thermostable lines, as determined by measurements on flag leaves at anthesis. Applying the membrane thermostability test on winter wheat seedlings, Saadalla et al. (1990) found a high correlation in membrane thermostability between seedlings and flag leaves at anthesis for genotypes under controlled environmental conditions. Measurements of membrane thermostability (MT) of 16 spring wheat cultivars were compared with performance at several heat-stressed locations. Variation in MT of both field, heat-acclimated flag leaves, as well as seedlings grown in controlled conditions, were associated with heat tolerance in warm wheat-growing regions (Reynolds et al., 1994a).

The physiological basis for the association of MT with heat tolerance is unknown, and in fact plasma membranes are known to be more heat tolerant than is photosynthesis for example (Berry and Bjorkman, 1980). While loss of membrane integrity is a possibility, the phenomenon of ion leakage from the cell could also be caused by thermal-induced inhibition of membrane bound enzymes, which are responsible for maintaining chemical gradients in the cell. Direct evidence for a biochemical limitation to heat tolerance in wheat comes from studies of the enzymes involved in grainfilling, specifically soluble starch synthase, which is deactivated at high temperatures (Keeling et al., 1994). If conversion of sucrose to starch is a limitation to yield under heat stress, this would explain the observation of increased levels of carbohydrates in vegetative tissue of wheat when grainfilling was limited by heat stress (Spiertz, 1978).

There are a number of other processes that are clearly affected by high temperatures, but that are not discussed in depth here since they do not lend themselves to simple screening. Respiration costs are higher with increasing temperature leading eventually to carbon starvation because assimilation cannot keep pace with respiratory losses (Levitt, 1980). However, this apparently wasteful process would seem unavoidable, at least in current germplasm, as evidenced by positive associations observed between dark respiration at high temperature and heat tolerance of sorghum lines (Gerik and Eastin, 1985) and in wheat (Reynolds et al., 1998a). Heat shock proteins are synthesized at very high rates under high-temperature stress and are thought to have a protective role under stress; nonetheless their role in determining genetic differences in heat tolerance is not established. Another trait that may have more promise as a screening trait is chlorophyll fluorescence; associations between heat tolerance and lower fluorescence signals have been reported in a number of crops including wheat (Moffat et al., 1990), though screening protocols are yet to be evaluated.

While a definitive picture of the physiological basis of reduced growth rates under heat stress is still lacking, many of the drought-adaptive traits discussed above are likely to be useful under heat stress. Examples would include leaf glaucousness to reduce the heat load, awn photosynthesis when high temperatures reduce assimilation rate of the leaves and early escape from heat stress. Heat stress is almost certainly a component of drought stress since one of the principal effects of drought is to reduce evaporative cooling from the plant surface. Nonetheless, not all traits conferring heat tolerance are also associated with genetic variability in drought tolerance, a good example being membrane thermostability (Blum, 1988). In addition, wheat germplasm that typically performs well under heat stress is not necessarily useful under drought (S. Rajaram, personal communication).

When considering deployment of selection traits it may be useful to divide them, some-what arbitrarily, into two categories: (i) simple traits associated with a particular morpho-physiological attribute such as root depth or leaf waxiness; and (ii) integrative traits, the net effect of a number of simpler traits, an example being canopy temperature. Being a function of several simpler traits, integrative traits are potentially powerful selection criteria for evaluating breeding progeny, while the simpler traits might be considered when choosing possible parental characteristics. Clearly the heritability of traits, as well as the ease with which they can be measured would modify any such rule of thumb.

Molecular Wheat Breeding

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Molecular wheat breeding is application of biotechnological tools in wheat improvement such as gene transfor (genetic engineering) and marker aided selection. Such changes aims to alter the physiological pathways through change in genetic structures. There are many successful examples of such kind.

References on application

PHYSIOLOGICAL TRAITS FOR IMPROVING WHEAT YIELD UNDER A WIDE RANGE OF CONDITIONS pdf file

Physiological approaches to wheat breeding FAO site

Physiological traits for abiotic stress tolerance breeding

Physiological Traits to Improve the Yield of Rainfed Wheat: Can Molecular Genetics Help?CIMMYT site link

Evaluating Potential Genetic Gains in Wheat Associated with Stress-Adaptive Trait Expression in Elite Genetic Resources under Drought and Heat Stress crop science

Physiological traits for biotic stress tolerance breeding

M. J. Foulkes, N. D. Paveley, A. Worland, S. J. Welham, J. Thomas, J. W. Snape. Major Genetic Changes in Wheat with Potential to Affect Disease Tolerance. Phytopathology, July, Volume 96, Number 7, Pages 680-688 (doi: 10.1094/PHYTO-96-0680)click link

Rosyara, U.R., K. Pant, E. Duveiller and R.C. Sharma. 2007. Variation in chlorophyll content, anatomical traits and agronomic performance of wheat genotypes differing in spot blotch resistance under natural epiphytotic conditions. Australasian Plant Pathology 36 : 245–251.

Rosyara, U.R., R.C. Sharma, and E. Duveiller. 2006. Variation of canopy temperature depression and chlorophyll content in spring wheat genotypes and association with foliar blight resistance. J. Plant Breed. Gr. 1 : 45-52.

Rosyara, U.R., R.C. Sharma, S.M. Shrestha, and E. Duveiller. 2005. Canopy temperature depression and its association with helminthosporium leaf blight resistance in spring wheat. Journal of Institute of Agriculture and Animal Science 26: 25-28.

Rosyara, U.R., R.C. Sharma, S.M. Shrestha, and E. Duveiller. 2006. Yield and yield components response to defoliation of spring wheat genotypes with different level of resistance to Helminthosporium leaf blight. Journal of Institute of Agriculture and Animal Science 27. 42-48.

Rosyara, U. R. 2002. Physio-morphological traits associated with Helminthosporium leaf blight resistance in spring wheat. Masters’ Thesis. Tribhuvan University, Institute of Agriculture and Animal Science, Rampur, Chitwan, Nepal. supported by CIMMYT International. Available at CIMMYT library

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Useful Books

Hayward, M. D., N. O. Bosemark, and I. Romangosa. 1993. Plant Breeding: Principle and Prospects. Chapman and Hall, London.

Wood, D. R., K. M. Rawal, and M. N. Wood (eds). 1983. Crop Breeding. American Society of Agronomy, Crop Science Society of America, Madison, Wisconsin.

Allard, R. W. 1960. Principles of Plant Breeding. John Wily and Sons Inc. New York.

Simmonds, N. W. 1979. Principles of Crop Improvement. Longman Group Limited, London.

Singh, B. D. 2000. Plant Breeding. Sixth ED. Kalyani Publishers, New Delhi.