A non-invasive crop ideotype to reduce invasive potential

Summary In plant breeding programs, qualitative and quantitative traits confer market value and, thus, constitute the basis for developing breeding criteria during crop domestication. Some traits such as high male/female fertility are advantageous in the wild and could enable the evolution of cultivated crops into invasive weeds. Other traits, e.g. sterility, are not expected to confer invasiveness. To date there has been very limited involvement in invasion risk assessment by plant breeders. Thus, in this paper we propose that trait-based selection of potential crop species be coupled with species design in the creation of a “non-invasive crop ideotype” as an avenue to reduce invasiveness during domestication. The non-invasive crop ideotype embodies the ideal characteristics for a crop to excel in cultivated environment(s) but minimizes the likelihood it will establish and spread in non-cultivated environments, constituting the underlying foundation for all breeding objectives, choice(s) of breeding methodologies, and propagation techniques for non-invasive crop release. Using ornamental (floriculture) horticultural crops as an example, we identify 10 traits to be used individually or in combination to reduce invasiveness while retaining commercial value: reduced genetic variation in propagules, slowed growth rates, non-flowering, elimination of asexual propagules, lack of pollinator rewards, non-shattering seed, non-fleshy fruits, lack of seed germination, sterility, and programmed death (apotopsis). A non-invasive crop ideotype would constitute the underlying foundation for all breeding objectives, the choice(s) of breeding methodologies, and propagation technique(s). The ideotype should be flexible and should adjust to species- and crop-specific traits to account for the intended use. For example, development of sterile cultivars may have negligible effects in reducing invasiveness if the crops spread vegetatively. A non-invasive crop ideotype may increase the direct participation of plant breeders, who are the professionals directly involved in the collection, development and release of new crops, in reducing the invasive potential of ornamental crops. Future research is required to determine the feasibility of incorporating each trait into various crops, use of classical or molecular techniques for creation of non-invasive crops, trait stability (lack of genotype × environment interaction over years and locations), consumer acceptance, and long-term viability.     

Hybridization as a stimulus for the evolution of invasiveness in plants?

Summary Invasive species are of great interest to evolutionary biologists and ecologists because they represent historical examples of dramatic evolutionary and ecological change. Likewise, they are increasingly important economically and environmentally as pests. Obtaining generalizations about the tiny fraction of immigrant taxa that become successful invaders has been frustrated by two enigmatic phenomena. Many of those species that become successful only do so (i) after an unusually long lag time after initial arrival, and/or (ii) after multiple introductions. We propose an evolutionary mechanism that may account for these observations. Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness. We present and review a remarkable number of cases in which hybridization preceded the emergence of successful invasive populations. Progeny with a history of hybridization may enjoy one or more potential genetic benefits relative to their progenitors. The observed lag times and multiple introductions that seem a prerequisite for certain species to evolve invasiveness may be a correlate of the time necessary for previously isolated populations to come into contact and for hybridization to occur. Our examples demonstrate that invasiveness can evolve. Our model does not represent the only evolutionary pathway to invasiveness, but is clearly an underappreciated mechanism worthy of more consideration in explaining the evolution of invasiveness in plants. This paper was presented at the National Academy of Sciences colloquium “Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins,” held January 27–29, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA. Reprinted from PNAS 97(13): 7043–7050 (2000).     

Light competition for invasive species control: A model of cover crop–weed competition and implications for Phalaris arundinacea control in sedge meadow wetlands

Summary Since resource competition plays a critical role in many plant invasions, controlling invasive vegetation may require managing the supply of limiting resources. For example, lowering light availability with a cover crop might prevent invasions during community establishment in light-limited restored ecosystems. However, most cover crops evaluated for invasive species control either do not adequately suppress invasives or equally suppress desired species. To improve our ability to predict cover crop effectiveness, we use a theoretical model of plant competition to identify potential mechanisms by which cover crops might favor desired species over invasives. In addition, we consider the model's implications for controlling an invasive forage, Phalaris arundinacea, in restored sedge meadows. The model suggests that cover crops will improve the outcome of competition between desired and invasive species only when (1) desired species have lower minimum light requirements than invasive species and (2) invasive species dominance results from rapid establishment and resource preemption. Cover crops in the model favor desired species over faster-growing invasives because faster-growing invasives are positioned higher in the canopy. Invasive species higher in the canopy shade desired species more than desired species shade invasives. Consequently, by reducing invasive species biomass, cover crops give desired species a competitive advantage. The simple requirements for cover crop success in the model suggest that cover crops may be effective for invasive species control in light-limited restored ecosystems. The available information on P. arundinacea responses to shade suggests, however, that cover crops are unlikely to favor sedge meadow species over P. arundinacea.     

How to maintain improved cultivars

Improved cultivars loose their identity and healthiness unless maintained properly. Contaminating and degrading forces, such as outcrossing, volunteer plants, mixing, natural selection, mutation and seed-borne diseases, are at the root of this. Maintenance selection can prevent this deterioration. How it is carried out depends on the reproduction system of the crop. Crops are therefore classified into four categories; typical cross-pollinating crops, self-pollinating crops with a substantial amount of outcrossing, typical self-pollinating crops with little outcrossing, and the vegetatively reproduced crops. Generally some of the “breeder seed” is used to plant a small plot with spaced plants. A fair number of healthy plants of the cultivar type is selected and the seed is harvested per plant. The progenies of the selected plants are grown in small plots. Non-uniform or deviating plots and plots with a seed-borne disease are removed. The seed of the progenies that are healthy, uniform and similar (and so of the cultivar type) are harvested per progeny to be tested next season on larger plots. The same selection is applied and only the seed of the progenies that are healthy, uniform and similar are harvested together to produce the “breeder seed”. The details of this maintenance selection vary with the reproduction system, the multiplication rate of the crop and the possibilities available to the breeder. Seven crops, potato, common bean, barley, wheat, faba bean, quinoa and maize are discussed here as they represent the different reproduction systems and multiplication rates, while being important Andean food crops.     

Doubled haploid production in nutraceutical species: a review

There is a growing interest in herbs, spices, nutraceuticals, and medicinal plants worldwide. These species are used for food, flavourings, cosmetics, and for medicinal purposes. However, there is very little activity underway to improve the genetics and breeding of these crops. Much of the industry relies on “wild” plants (i.e. not genetically improved/enhanced); therefore, the potential for variability in crop performance and active ingredients is high, presenting significant challenges for the industry. Uniform varieties would be beneficial for use in agronomic and clinical trials. Doubled haploid technology can accelerate the breeding of new varieties as has been demonstrated in other species. However, very little work has been done on developing doubled haploid technology for herbs, spices, and nutraceutical species. This review describes some of the research conducted in this field.     

作物种植模式对土壤微生物和农田有害生物的影响

土壤是联系地上和地下生态系统的纽带,土壤微生物在土壤养分循环和作物吸收中起关键作用,被称为土壤质量的指标。土壤微生物通过分解土壤有机物,促进养分循环利用,并调控植物生长发育,随着现代农业种植结构的变化,特别是种植制度的改变,地上植物种类和生长状况常常影响土壤中微生物的群落结构和多样性,调控着作物生长发育,农田有害生物的发生,而影响农作物生产。综述了现代农业中几种主要种植模式,如轮作、连作和覆盖等对土壤微生物群落结构及与农作物有害生物的影响,强调科学合理选择种植模式的重要性,最后,讨论了这些种植模式有待深入研究的一些关键问题。     

Hemp as a raw material for industrial applications

Summary Developed countries need to promote alternatives to crops produced in excess (such as cereals), and cultivation with limited environmental impact. Hemp (Cannabis sativa L.) is a potentially profitable crop, fitting into sustainable farming systems. Interest in new fibre crops is in fact increasing, to find alternatives to a high-input crop such as cotton, or to relieve the pressure of the paper industry on remaining natural forests. Besides, innovative applications are provided by hemp in renewable raw materials (oils from seeds and essential oils or secondary metabolites from inflorescences for food, cosmetic or pharmaceutical industry; straw and hurds for the building industry and for energy production). The development of more specialized hemp plants, needs a better understanding and control of metabolism. Since the renewed interest for this crop is increasing all over the world, it is important to update the knowledge on this crop, to understand whether it is a really sustainable, alternative and economically rewarding crop, and what contribution plant breeding, agronomical practice, and processing techniques are making, or could further make, to improve the relevant traits. The symposium Hemp: perspectives for advanced utilization, held in Bologna (Italy) on March 2004, was an opportunity to update the research carried out in different fields.     

Aspects of participatory plant breeding for quinoa in␣marginal areas of Ecuador

Summary Field trials were carried out in Ecuador with two indigenous communities, Ninín Cachipata and La Esperanza, to determine farmers’ preferences for quinoa (Chenopodium quinoa Willd.) cultivars and to improve PPB processes. More women than men participated, reflecting that quinoa, a primarily subsistence crop, is mainly managed by women. Farmers’ field selection criteria for quinoa in the field were mostly based on yield, earliness and plant colour; however only breeders’ measurements of yield and panicle height significantly correlated to farmer selection scores. Older women gave higher scores than younger women or men, apparently due to a concept of no cultivar being without value. Working in same gender pairs improved evaluation richness. INIAP technicians were more discriminating in their evaluations than farmers. They also used additional selection criteria of disease resistance and uniformity. At seed selection, farmers from Ninín Cachipata, where food security is not assured, chose lines based on yield, while farmers from La Esperanza, where resources are less limiting, also considered seed size, colour, saponin content and marketability. Field characteristics were not taken into consideration at seed selection, signifying that farmers are less interested in those characteristics, or that it was difficult for them to correlate field data when presented in tabular form with seed characteristics. Future trials with small farmers should have fewer lines or replications to avoid farmer fatigue during evaluation. Farmers who grow primarily for subsistence in semi-arid environments have more interest in growing quinoa, and more to gain from having improved cultivars; therefore future participatory efforts should focus on them.     

Physiological characteristics of a functional stay-green rice “SNU-SG1” during grain-filling period

Functional stay-green has been regarded as a promising characteristic to be introduced for improving rice yield potential. A functional stay-green rice “SNU-SG1” that was identified from japonica rice collections was compared with two regular high-yielding rice cultivars (HYVs) for the temporal change of leaf chlorophyll, soluble protein, and root activity, and nitrogen accumulation and remobilization during the grain-filling period. SNU-SG1 had slower decreasing rate and maintained higher concentration of chlorophyll and soluble protein in upper four leaves during the grain-filling period than HYVs “Suweon490” and “Andabyeo”, revealing a typical stay-green characteristic. Even though SNU-SG1 remobilized almost the same proportion of N accumulated before heading as HYVs to grain, it maintained much higher leaf N concentration due to the significantly higher N accumulation that is ascribable to the higher root activity sustenance during grain-filling period. The functional stay-green trait of SNU-SG1 seems to stem not only from the genetic control preventing chlorophyll degradation but also from the higher capacity to absorb N from soil due to the sustained strong root activity during grain-filling period. SNU-SG1 exhibited higher crop growth rate during late grain-filling period than HYVs, resulting in higher grain-filling percentage and non-structural carbohydrate re-accumulation in the stem at the final stage of grain filling. It is concluded that SNU-SG1 has a promising trait “functional stay-green” contributable to rice yield potential improvement through the improved grain filling.     

Study of genetic diversity in safflower genotypes using agro-morphological traits and RAPD markers

This research was conducted to study the genetic diversity in safflower (Carthamus tinctorius L.) using agro-morphological traits and RAPD markers. Sixteen selected lines derived from landraces growing in various agro-climatic regions of Iran along with four exotic genotypes were evaluated in a randomized complete block design with three replications under field conditions. Days to emergence, days to initial flowering, days to flowering, days to maturity, plant height, branches per plant, capitula per plant, seeds per capitulum, 1,000-seed weight, seed yield per plant, seed yield, and reaction to powdery mildew (Leveillula taurica Arnaud) were evaluated in this study. Genetic diversity of the genotypes was assessed by RAPD markers. The results indicated significant differences among genotypes for the agro-morphological traits and clustering based on these traits classified the genotypes into five groups. Analysis of the RAPD markers revealed 15 polymorphic primers out of 50 used primers. Based on RAPD data, the highest genetic similarity was observed between the cultivars of “AC Sunset,” “AC Sterling” from Canada and the lowest relatedness observed between a local breeding line “E₂₄₂₈” and genotype “GE₆₂₉₂₃” from Germany. Cluster analysis based on RAPD markers and 54% coefficient of similarity divided the genotypes into five distinct groups. Comparing the clusters based on agro-morphological traits with those from molecular markers showed slight similarities. The finding of high genetic variation for agro-morphological traits and polymorphism at DNA level reveal that agronomic traits can be improved by selection programs.     

Commercialization decisions and the economics of introduction

Summary A commercial horticultural industry that establishes plant-breeding nurseries for an exotic species throughout a regional economy will expand until the marginal profit of the last nursery established is zero. However, a regional government concerned with social welfare will take into account not only the profits of the horticultural industry but also any expected costs of an accidental invasion. The latter costs will consist of the discounted expected social damages due to an increase in the rate of invasion over time and the increase in expected damage cost per hectare caused by an additional nursery. A government can employ an “introducers' pay” tax equal to the latter social costs to ensure that the plant breeding industry establishes the optimal number of nurseries. We illustrate this outcome with the example of purple loosestrife (Lythrum salicaria) in North America. In the absence of any tax, the horticultural industry will establish n = 3528 nursery operations, and the expected damages from invasion are US$ 28.2 million per year. With the tax, only n = 300 nurseries are established but the expected damages from invasion are reduced to US$ 1.3 million per year. Although profits for the horticultural industry are lower from the tax, the net gains in overall social welfare more than offset the losses. Although these results are illustrative only, they show that the problem of plant invasives initiated by commercial operations is amenable to standard economic analysis and solutions, such as implementation of an “introducers' pay tax”.     

The importance of population growth, seed dispersal and habitat suitability in determining plant invasiveness

Summary This paper examines the roles of plant demography, seed dispersal ecology and habitat suitability in influencing invasiveness of horticulturally important species. Section one investigates the relative invasiveness of two woody species, Crataegus monogyna and Prunus mahaleb, and concentrates on differences in demographic and dispersal traits. The second section delineates the invasion of two Asparagus spp. and concentrates on differences in seed dispersal ecology. Section three reports the use of a geographical information system analysis to determine whether habitat suitability, seed dispersal or land management is more important in determining threat of invasion by adventive Olea europaea. C. monogyma, P. mahaleb are closely related with similar habits and overlapping home ranges in Europe. Crataegus monogyna is very invasive in northern New South Wales, having spread rapidly and conspicuously throughout the region and elsewhere in southern Australia at rates of 80–120 m yr⁻¹. Prunus mahaleb is far less invasive, being restricted to a small population, which is expanding at 20 m yr⁻¹. Demographic analysis showed that potential growth rates of P. mahaleb (1.713–1.490) are greater than those for C. monogyna (1.138–1.103). Assessment of the seed dispersal ecology of both species revealed that C. monogyna had seeds dispersed by one bird and three mammals over many kilometers. P. mahaleb had seeds dispersed by six birds and four mammals over distances generally < 100 m. The role of humans in introducing both species and the characteristics of seed dispersal are more influential than demography in determining invasiveness. The more invasive Asparagus asparagoides has smaller fruit with a bicoloured display up to 1.5 m above ground. Less invasive A. declinatus has larger, translucent white fruit displayed at heights < 0.5 m above ground. The role of humans has also been important in shaping the course of invasion. Sensitivity analysis confirms that land management practices, which affect Olea europaea seedling establishment and survival, are most important at a landscape scale in determining invasiveness. The main factors determining the difference in invasiveness relate directly to the changing nature of human management of each species and the ecological interactions between the plants and the invaded environment. This research has implications for the prediction and management of biological invasions; emphasizing the importance of seed dispersal and human activities in determining the course of invasions: both of which cannot be readily predicted.     

The use of general and specific combining abilities in a context of gene expression relevant to plant breeding

Many common traits are believed to be a composite reflection of multiple genetic and environmental factors. Recent advances suggest that subtle variations in the regulation of gene expression may contribute to quantitative traits. The nature of sequence variation affecting the regulation of gene expression either in cis (that is, affecting the expression of only one of the two alleles in a heterozygous diploid) or in trans (that is, affecting the expression of both alleles in a heterozygous diploid) is a key and usually unknown feature for the breeders. If the change in expression acts entirely in cis, then the structural gene can be treated as a candidate gene and a potential target for marker-assisted selection. Therefore, gene surveys for cis-regulatory variation are a first step in identifying potential targets for marker-assisted breeding. Here, we discuss in detail the “genome-wide analysis of allele-specific expression differences” (GASED) approach. The GASED approach was developed to screen for cis-regulatory variation on a genome-wide scale. In GASED, mRNA abundance is treated as if it were a quantitative phenotypic response variable, whose genetic between-F₁ hybrid variance is partitioned into additive and non-additive components. In plant breeding, this partitioning of the genetic variance is well known in the context of estimation of general and specific combining abilities for diallel crossing schemes. We demonstrate the GASED method using Arabidopsis thaliana data. The method can be used to screen for cis-regulatory variation in any crop species for which diallel crossing schemes are appropriate and genomic tools are available.     

Effects of individual selection on agronomical and morphological traits and essential oil of a “Greek basil” population

“Greek basil” (Ocimum basilicum L.) is a common local cultivar in Greece, also known as “fine-leaved” basil, because of its small-narrow leaves. It is used mainly as ornamental plant in pots and gardens. Its variation in terms of the time of florescence and the shape of the vegetative development (compact or loose ball, reverse cone, other shapes) led to the improvement of a “Greek basil” population, by selecting plants with late blooming, compact ball shape and big size, potentially suited for ornamental uses. After three cycles of individual selection in nil competition conditions, by applying the above selection criteria, the florescence of the selected population started at the beginning of August, instead of mid June of the initial population, and the percentage of plants with compact ball shape increased from 55 to 90%. Moreover, significant variation was observed in the essential oil yield and composition due to the applied selection. The percentage oil yield increased from 0.15 to 0.53%. The main oil constituents eugenol and linalool raised from 8.60 to 12.90% (49%) and from 36.80 to 49.75% (34.90%) respectively, while the methyl (E) cinnamate, though it was present (19.20%) in the initial population, it was not even detected in the improved population.     

Use of crop models to understand genotype by environment interactions for drought in real-world and simulated plant breeding trials

Crop simulation models of plant processes capture the biological interactions between the sensing of signals at an organ level (e.g. drought affecting roots), the response of the plant at a biochemical level (e.g. change in development rate) and the result at the organ (or crop) level (e.g. reduced growth). In dissecting the complex control of phenotypes like yield, simulation models have several roles. Models have been used to generate an index of the climatic environment (e.g. of drought stress) for breeding programme trials. In wheat and sorghum grown in northern Australia, this has shown that mid-season drought generates large genotype by environment interaction. By defining gene action to calculate the value of input trait parameters to crop models, simulated multi-environment trials estimate the yield of ‘synthetic’ sorghum cultivars grown in historical or artificial climates with current or potential management regimes. In this way, the biological interactions among traits constrain the crop yields to only those that are biologically possible in the given set of environments. This allows the construction of datasets that are more ‘realistic’ representations of gene by trait by environment interaction than is possible using only the statistical attributes (e.g. means, variances and correlations) of real-world trait datasets. This approach has an additional advantage in that ‘biological and experimental noise’ can be manipulated separately. These ‘testbeds’ for statistical techniques can be extended to the interpretation of a crossing and selection programme where the processes of chromosomal recombination are simulated using a quantitative genetics model and applied to the trait parameters. Statisticians are challenged to develop improved methods for the resulting simulated phenotype datasets, with the objective of revealing the (known) underlying genetic and environment structure that was input to the simulations. These improved methods can then be applied to existing plant breeding programmes.     

Genetic analysis of yield and quantitative traits in pigeonpea (<Emphasis Type="Italic">Cajanus cajan</Emphasis> L. Millsp.)

Basic information on genetics and inheritance of quantitative characters, which is necessary to develop future breeding programme, is not widely studied in pigeonpea. Hence, present study was conducted among 5 generations in four pigeonpea crosses to know significance of additive-dominance model, gene action involved in inheritance of quantitative characters, heritability and genetic advance. “Scaling” and “joint scaling test” was significant for most characters indicating that additive-dominance model alone is not enough to explain the inheritance of a character. Though additive variance was more, dominance variance also played important role for most of the traits. Positive and negative alleles were found to be distributed between parents. Additive gene effect (d) was significant for pods per plant and seeds per pod whereas dominance gene effect (h) was more predominant among pod yield and seed yield. Dominance × Dominance inter-allelic interactions (l) was more important than Additive × Additive type (i) for most of the traits studied which could be exploited by selecting individuals based on their performance in recurrent selection. Complementary gene action was observed among many traits with few exhibiting duplicate gene action. Heritability and genetic advance was high indicating the effectiveness of selection. Since dominance effects is also present along with additive effects selection could be practised in later generations to identify high yielding genotypes.     

Development of synthetic Brassica napus lines for the analysis of “fixed heterosis” in allopolyploid plants

Summary Allopolyploids are widely spread in the plant kingdom. Their success might be explained by positive interactions between homoeologous genes on their different genomes, similar to the positive interactions between different alleles of one gene causing heterosis in heterozygous diploid genotypes. In allopolyploids, such interactions can also occur in homozygous genotypes, and may therefore be called “fixed heterosis”. As to our knowledge, no experimental data are available to support this hypothesis. We propose an experimental approach to quantify “fixed heterosis” in resynthesised Brassica napus and the detection of loci contributing to “fixed heterosis” via comparative QTL mapping in B. napus and its parental species B. rapa and B. oleracea. In order to develop a genetically balanced material, interspecific crosses between 21 Brassica rapa and 16 Brassica oleracea doubled haploid or inbred lines were performed. In total 3485 vital embryos have been obtained from 9514 pollinated buds. The success of interspecific hybridisation was highly depending on the maternal genotype (B. rapa) and ranged from 0 to 1.18 embryos per pollinated bud. For the genetic characterisation of the B. rapa and B. oleracea lines, a dendrogram was constructed based on 273 RAPD markers. Thus a well-characterised material is now available, which is suitable to analyse the effects of “fixed heterosis” and the interactions between homoeologous genes in allopolyploid species.     

Participatory crop improvement and formal release of <Emphasis Type="Italic">Jethobudho</Emphasis> rice landrace in Nepal

Jethobudho is an aromatic rice landrace of the Pokhara valley in middle hills of Nepal. Although local consumers are willing to pay a high price for its purchase, the landrace has a problem with quality variation. Decentralized participatory population improvement for specific market-identified traits was conducted on “Jethobudho” populations collected from farmers’ fields in seven geographic regions of the valley in Nepal. The preferred post harvest quality traits, field tolerance to blast and lodging, and superior post harvest quality traits of Jethobudho were established by a consumer market survey. These traits were used for screening the materials. 338 sub-populations of Jethobudho were evaluated for yield, disease, lodging resistance, and post harvest quality traits. Significant variation was found for culm strength, neck blast tolerance, awn characteristics, panicle length, number of grains per panicle, test grain weight and post harvest quality traits, whereas no significant variation was found in grain yield, plant height, tiller number, maturity period and leaf blast. Based on these identified traits and micro-milling evaluations, 183 populations were screened in on-farm and on-station nurseries, and in succeeding years populations were further screened by plant breeders and expert farmers in research trials, resulting in the selection of 46 populations for post harvest quality traits. Six accessions with similar agronomic traits, field tolerance to blast and lodging, and superior post harvest quality traits, were bulked and evaluated on-farm using participatory variety selection (PVS). The enhanced Jethobudho accessions were also evaluated for aroma using simple sequence repeat (SSR) and found to have unique aromatic genetic constitution. Community based seed production groups were formed, linked to the Nepal District Self Seed Sufficiency Programme (DISSPRO), and trained to produce basic seeds (truthfully labeled) of Jethobudho. The National Seed Board of Nepal released the enhanced landrace in the name of “Pokhareli Jethobudho” in 2006, as the first bulk variety of traditional high quality aromatic rice improved through participatory plant breeding to be formally released in Nepal for general cultivation under the national seed certification scheme. Landrace improvement is shown as an important option for supporting programmes for in situ conservation of landraces on-farm.     

Genetics of the resistance to powdery mildew disease in mungbean (Vigna radiata (L.) Wilczek)

Powdery mildew disease caused by the fungus Erysiphe polygoni D.C. is an important disease of mungbean. Loss can be more serious if the disease attacks at seedling stage. In this paper, we report genetics of the resistance to powdery mildew disease in mungbean using a recombinant inbred line (RIL) population derived from a cross between the susceptible parent “KPS1” and the resistant parent “VC6468-11-1A”. Five hundred and ninety-two RILs were developed by random descending from 200 F₂ plants. The population was evaluated against the fungus in field and greenhouse conditions. The data were analyzed following a nested design for selfpollinating plants to determine genetic heritability of powdery mildew resistance. The severity of the infestation was measured by using disease index (DI) and area under disease progress curve (AUDPC). Both values showed continuous distribution in the RILs. Under field conditions, the narrow-sense heritability of DI and AUDPC were 0.67 and 0.48, respectively. While under greenhouse conditions, the values were 0.68 and 0.62, respectively. The results suggested that the resistance is quantitatively inherited with high heritability and predominantly additive gene action. To develop powdery mildew-resistant mungbean varieties, the plant breeder can select for resistant lines by using standard selection procedures for self-pollinating crops, viz. pedigree selection, bulk selection, early generation testing, and single-seed descent.     

High yield potential,shuttle breeding,genetic diversity,and a new international wheat improvement strategy

The main elements of the international wheat improvement program of the Centro Internacional de Mejoramiento de Maíz y Trigo (CIMMYT), also known as the International Maize and Wheat Improvement Center, have been shuttle breeding at two contrasting locations in Mexico, wide adaptation, durable rust and Septoria resistances, international multisite testing, and the appropriate use of genetic variation to enhance yield gains of subsequently produced lines. Such an approach yielded successes known collectively as the Green Revolution. However, at the beginning of the 21st century, this “cultivar assembly line” approach needs fine tuning to address crop needs under increasingly adopted resource conserving practices, as well as those related to nutritional requirements of the end-users. International wheat improvement will therefore focus on the targeting of traits in respective mega-environments, and the use of participatory methods, especially in marginal environments. The main features of this wheat improvement strategy include the introduction of new and novel sources of genetic variation through wild species, landraces, and, potentially, the use of transgenes for intractable traits. This variation will be combined using international shuttle breeding, and increased breeding efficiency will be achieved through marker-aided methods, more targeted use of crop physiology, plant genetics, biostatistics, and bioinformatics. Likewise, CIMMYT will increase its focus on the needs of end-users by emphasizing regional efforts in participatory research and client-oriented plant breeding.