A. K. Joshi

Stay green trait: variation, inheritance and its association with spot blotch resistance in spring wheat (Triticum aestivum L.)

One thousand four hundred and seven spring wheat germplasm lines belonging to Indian and CIMMYT wheat programs were evaluated for stay green (SG) trait and resistance to spot blotch caused by Bipolarissorokiniana during three consecutive crop seasons, 1999–2000, 2000–2001 and 2001–2002. Disease severity was recorded at six different growth stages beginning from tillering to late milk stage. SG trait was measured by following two approaches: difference for 0–9 scoring of green coloration (chlorophyll) of flag leaf and spike at the late dough stage (GS 87) and a new approach of leaf area under greenness (LAUG). Germplasm lines showed a wide range (7–89) for LAUG and were grouped into four viz., SG, moderately stay green, moderately non-stay green and non-stay green (NSG). However, very few (2.2%) lines showed high expression of SG trait, i.e., LAUG >60. LAUG appeared to be a better measure of SG trait than a 0–9 scale. Mean spot blotch ratings of SG genotypes were significantly lower than those of NSG genotypes at all growth stages. Two spot blotch resistant genotypes (Chirya 3 and Chirya 7) having strong expressions of SG trait were crossed with NSG, spot blotch susceptible cv. Sonalika. Individually threshed F2 plants were used to advance the generations. SG trait and spot blotch severity were recorded in the parents and F1, F3, F4, F5, F6 and F6–7 generations under disease-protected and inoculated conditions. SG trait in the F1 generation was intermediate and showed absence of dominance. Evaluation of progenies (202–207) in the segregating generations revealed that SG trait was under the control of around four additive genes. Lines homozygous for SG trait in F4, F5, F6 and F6–7 generations showed significantly lower mean area under disease progress curve (AUDPC) for spot blotch than those with NSG expression. A positive correlation (0.73) between SG trait and AUDPC further indicated a positive influence of SG on severity of spot blotch. The study established that variation for SG trait exists in spring wheat; around four additive genes control its inheritance in the crosses studied and there is positive association between SG trait and resistance to spot blotch.

Wheat genetic resources enhancement by the International Maize and Wheat Improvement Center (CIMMYT)

The International Maize and Wheat Improvement Center (CIMMYT) acts as a catalyst and leader in a global maize and wheat innovation network that serves the poor in the developing world. Drawing on strong science and effective partnerships, CIMMYT researchers create, share, and use knowledge and technology to increase food security, improve the productivity and profitability of farming systems and sustain natural resources. This people-centered mission does not ignore the fact that CIMMYT’s unique niche is as a genetic resources enhancement center for the developing world, as shown by this review article focusing on wheat. CIMMYT’s value proposition resides therefore in its use of crop genetic diversity: conserving it, studying it, adding value to it, and sharing it in enhanced form with clients worldwide. The main undertakings include: long-term safe conservation of world heritage of both crop resources for future generations, in line with formal agreements under the 2004 International Treaty on Plant Genetic Resources for Food and Agriculture, understanding the rich genetic diversity of two of the most important staples worldwide, exploiting the untapped value of crop genetic resources through discovery of specific, strategically-important traits required for current and future generations of target beneficiaries, and development of strategic germplasm through innovative genetic enhancement. Finally, the Center needs to ensure that its main products reach end-users and improve their livelihoods. In this regard, CIMMYT is the main international, public source of wheat seed-embedded technology to reduce vulnerability and alleviate poverty, helping farmers move from subsistence to income-generating production systems. Beyond a focus on higher grain yields and value-added germplasm, CIMMYT plays an “integrative” role in crop and natural resource management research, promoting the efficient use of water and other inputs, lower production costs, better management of biotic stresses, and enhanced system diversity and resilience.

Breeding crops for reduced-tillage management in the intensive, rice-wheat systems of South Asia

The importance of reduced tillage in sustainable agriculture is well recognized. Reduced-tillage practices (which may or may not involve retention of crop residues) and their effects differ from those of conventional tillage in several ways: soil physical properties; shifts in host–weed competition; soil moisture availability (especially when sowing deeply or under stubble); and the emergence of pathogen populations that survive on crop residues. There may be a need for genotypes suited to special forms of mechanization (e.g. direct seeding into residues) and to agronomic conditions such as allelopathy, as well as specific issues relating to problem soils. This article examines issues and breeding targets for researchers who seek to improve crops for reduced-tillage systems. Most of the examples used pertain to wheat, but we also refer to other crops. Our primary claim is that new breeding initiatives are needed to introgress favourable traits into wheat and other crops in areas where reduced or zero-tillage is being adopted. Key traits include faster emergence, faster decomposition, and the ability to germinate when deep seeded (so that crops compete with weeds and use available moisture more efficiently). Enhancement of resistance to new pathogens and insect pests surviving on crop residues must also be given attention. In addition to focusing on new traits, breeders need to assess germplasm and breeding populations under reduced tillage. Farmer participatory approaches can also enhance the effectiveness of cultivar development and selection in environments where farmers’ links with technology providers are weak. Finally, modern breeding tools may also play a substantial role in future efforts to develop adapted crop genotypes for reduced tillage.

The relative importance of drought and other water-related constraints for major food crops in South Asian farming systems

Variation in water availability is a major source of risk for agricultural productivity and food security in South Asia. Three hundred and thirty expert informants were surveyed during 2008-09 to determine the relative importance of drought and water-related constraints compared with other constraints limiting the production of four major food crops (wheat, rice, sorghum, chickpea) in five broad-based South Asian farming systems. Respondents considered drought an important constraint to crop yield in those farming systems that are predominantly rainfed, but associated it with low yield losses (well below 10% of all reported losses) for crops in farming systems with well-developed irrigation. In these systems, other water-related constraints (including difficult access to sufficient irrigation water, the high cost of irrigation, poor water management, waterlogging and flooding of low-lying fields) were more important. While confirming the importance of drought and water constraints for major food crops and farming systems in South Asia, this study also indicated they may contribute to no more than 20–30% of current yield gaps. Other types of constraint, particularly soil infertility and the poor management of fertilizer and weeds for the cereals, and pests and diseases for chickpea, contributed most yield losses in the systems. Respondents proposed a wide range of interventions to address these constraints. Continued investments in crop-based genetic solutions to alleviate drought may be justified for food crops grown in those South Asian farming systems that are predominantly rainfed. However, to provide the substantial production, sustainability and food security benefits that the region will need in coming decades, the study proposed that these be complemented by other water interventions, and by improvements to soil fertility for the cereals and plant protection with chickpea.

Combining ability analysis for grain filling duration and yield traits in spring wheat (Triticum aestivum L. em. Thell.)

A diallel analysis of wheat (Triticum aestivum L. em. Thell) parents (n = 11) and their F1 (n = 55) and F2 (n = 55) offspring was carried out for the following four traits: grain filling duration (GFD), GFD for growing degree days (GDD), 1000 seed weight and seed yield per plant. Analysis of variance for general combining ability (GCA) and specific combining ability (SCA) displayed significant F1 and F2 general and specific combining ability effects for the four traits studied. For all the traits the GCA effects were relatively more important than the SCA effects, indicating that additive genetic effects were predominant. Crosses displaying high SCA effects for grain filling duration, seed weight and yield were observed to be derived from parents having various types of GCA effects (high x high, high x low, low x low and medium x low). The single seed descent method can be applied to exploit additive gene effects whereas dominance gene effects could be valuable in hybrid wheat breeding programs. Among the parents, genotypes from the International Maize and Wheat Improvement Center (Centro Internacional de Mejoramiento de Maiz y Trigo, CIMMYT) as well as South Asia were found to be superior general combiners for grain filling duration. Likewise, crosses involving diverse parents from CIMMYT and South Asia showed significant SCA effects for grain filling duration and other traits.

Sources of inoculum and reappearance of spot blotch of wheat in rice–wheat cropping

A study was undertaken to examine the main source of inoculum of Bipolaris sorokiniana responsible for its reappearance in rice–wheat cropping regions of eastern India. Soil samples were collected at monthly intervals during April–October in the years 2000 and 2001 from fields having rice–wheat cropping. Bipolaris sorokiniana conidia were isolated and their viability was found to decline sharply with the onset of flooding in the month of August. In contrast to 82% in April, viability was 4% and <1% in August and September, respectively. Viable conidia were multiplied in the laboratory and inoculated on to susceptible cv. Sonalika under controlled conditions for test of pathogenicity. Appearance of symptoms typical to spot blotch were recorded after 7 days. Twenty-two different species (weeds and grasses) normally found to be associated with rice–wheat fields were tested for the presence of B. sorokiniana to evaluate their possible role as alternative hosts. Only three species, i.e. Setaria glauca, Echinochloa colonum and Pennisetum typhoids, were found to naturally harbour B. sorokiniana. Isolates from these hosts were tested for pathogenicity and also for their possible spread to wheat. When reisolated from these hosts, the pathogen did not infect wheat. Seeds of 25 different wheat genotypes were tested for B. sorokiniana infection. All genotypes were infected and the incidence of infection varied from 26% to 86%. Five isolates of wheat and one isolate from each of the three species (S. glauca, E. colonum and P.typhoids) were subjected to RAPD analysis. Two broad clusters were formed, suggesting that the wheat isolates were different from the isolates originating from other hosts. The results indicate that seeds are the most important source of inoculum for the reappearance of spot blotch of wheat in rice-wheat cropping systems in eastern India.

Variation for Staygreen Trait and its Association with Canopy Temperature Depression and Yield Traits Under Terminal Heat Stress in Wheat

Nine hundred and sixty three wheat advanced lines from various sources, including Indian and CIMMYT germplasm, were screened for the presence or absence of staygreen (SG) trait during two cycles, 2003–05. Staygreen was evaluated based on visual scoring (0–9 scale) and a new parameter, Leaf Area under Decline (LAUD). Approximately 5.5% of the lines were found to show staygreen character, 10.6% were moderately staygreen, and the remaining showed some or no expression of this trait. From this germplasm, one hundred lines were selected with the objective to find the effect of staygreen on yield and yield traits. These lines were sown under three different sowing dates (timely, late and very late) for three consecutive years to find the association between staygreen trait and heat tolerance. Canopy Temperature Depression (CTD), used as selection criteria for heat tolerance, was recorded at 12h, 14h and 16h at 7days interval, on bright sunny days. Correlation study showed that LAUD and CTD were strongly correlated (r=0.90) . LAUD was also found to be significantly associated with yield traits like grain filling duration (GFD) (r=0.83) , grain yield (r=0.89) , biomass (r=0.84) , but its association with test weight was non-significant. CTD also displayed significant correlation with yield traits like GFD (r=0.78) , grain yield (r=0.84) and biomass (r=0.81) . There was significant association between grain yield, biomass, and GFD under all the sowing dates but the association between CTD, LAUD and test weight was significant only under very late sown conditions. Genotype x Year interaction was found to be non-significant for LAUD and CTD. Genotype x Year x Sowing date was non-significant for LAUD but significant for CTD. T-test done to compare SG and non-SG genotypes was found to be significant for all the traits for both the years and under all sowing dates, except for test weight, under timely sown condition. This investigation revealed that substantial variation exists for SG trait in wheat and there is significant difference between SG and non-SG genotypes for CTD. Hence, SG trait along with CTD can be used as effective selection criteria for tolerance to heat stress

Identification of Molecular Marker and Aggressiveness for Different Groups of Bipolaris sorokiniana Isolates Causing Spot Blotch Disease in Wheat (Triticum aestivum L.)

One hundred fifty-five isolates of Bipolaris sorokiniana of wheat were studied for their morphopathological characterization. These isolates were grouped in five categories—black, brown/dull black, gray cottony growth, dull white/greenish black, and white—on the basis of their growth pattern. The frequency of the black suppressed type was maximum (45.63%), whereas the white isolate displayed lowest frequency (6.96%) in the natural population. Twenty RAPD (random amplified polymorphic DNA) primers were used to observe the variability among the identified groups of B. sorokininana. From each group, eight random isolates were investigated. A total of 143 bands were amplified, out of which 107 (74.83%) were polymorphic and 36 (25.17%) were monomorphic. On an average, the total numbers of bands generated per primer were 7.15, of which 5.35 and 1.80 were polymorphic and monomorphic, respectively. Dendrograms based on molecular polymorphism unveiled a considerable amount of diversity among the isolates. Specific DNA bands were identified for selected isolates. The distinct markers appeared to be potential enough to be employed as genetic fingerprints for future strain identification and classification. The study indicated that the RAPD primers provide an easy, rapid, and simple technique for the preliminary assessment of genetic diversity among the fungal isolates.

Delivering rust resistant wheat to farmers: a step towards increased food security

An important step towards reducing the vulnerability of wheat in Africa and Asia to the Ug99 race of the stem rust pathogen is the substitution of current susceptible varieties with superior resistant varieties. In the 2008–2009 cropping season both seed multiplication and dissemination of Ug99 resistant varieties were initiated in Afghanistan, Bangladesh, Egypt, Ethiopia, Iran, India, Nepal and Pakistan. Ug99 resistant varieties must occupy about 5% of the area sown to wheat in each country to ensure sufficient seed to displace current popular varieties. Because of the underdeveloped seed industry and small farm sizes in most of these countries, various strategies are being applied for rapid multiplication and dissemination of resistant varieties. Approaches being used include pre-release seed multiplication while candidate resistant lines are being tested in national evaluation trials and farmer participatory selection. Resistant varieties are already released in Afghanistan, Bangladesh, Egypt and Pakistan and more varieties are expected to be released in 2010 in these and other countries. Our results show that some new Ug99 resistant lines have yield superiority over dominant local varieties. Activities and progress in seed multiplication using existing and new Ug99 resistant varieties are discussed.

Role of melanin in release of extracellular enzymes and selection of aggressive isolates of Bipolaris sorokiniana in barley.

Abstract Eighteen barley isolates of Bipolaris sorokiniana belonging to wild and clonal type of black, mixed and white subpopulations were quantitatively assayed for their melanin content and aggressiveness with respect to production of some of the extracellular enzymes such as cellulase, pectinase, amylase and protease. Cellulase and pectinase constituted major portion of the enzymes recovered from the black, mixed and white isolates. Enzyme production and aggressiveness were relatively higher in melanin devoid or low melanin isolates. The melanin deficient isolates were also differentiated from black and mixed isolates on the basis of variation in internal transcribed spacer region of the ribosomal DNA. Higher enzyme productions positively correlated with area under disease progress curve (AUDPC) and lesion development. Melanin content was negatively correlated with extracellular enzymes and aggressiveness of the isolates. Based on melanin content, lesion size, AUDPC and extracellular enzymes, the isolates were grouped in two major clusters (I and II) with further division of cluster II into two sub-clusters (II-A and II-B). The results appears to indicate a possible role of melanin in release of extracellular enzymes and hence in evolution and selection of aggressive isolates of B. sorokiniana in barley.