Stay-green alleles individually enhance grain yield in sorghum under drought by modifying canopy development and water uptake patterns

* Stay-green is an integrated drought adaptation trait characterized by a distinct green leaf phenotype during grain filling under terminal drought. We used sorghum (Sorghum bicolor), a repository of drought adaptation mechanisms, to elucidate the physiological and genetic mechanisms underpinning stay-green. * Near-isogenic sorghum lines (cv RTx7000) were characterized in a series of field and managed-environment trials (seven experiments and 14 environments) to determine the influence of four individual stay-green (Stg1–4) quantitative trait loci (QTLs) on canopy development, water use and grain yield under post-anthesis drought. * The Stg QTL decreased tillering and the size of upper leaves, which reduced canopy size at anthesis. This reduction in transpirational leaf area conserved soil water before anthesis for use during grain filling. Increased water uptake during grain filling of Stg near-isogenic lines (NILs) relative to RTx7000 resulted in higher post-anthesis biomass production, grain number and yield. Importantly, there was no consistent yield penalty associated with the Stg QTL in the irrigated control. * These results establish a link between the role of the Stg QTL in modifying canopy development and the subsequent impact on crop water use patterns and grain yield under terminal drought.

Heat stress effects on grain sorghum productivity-biology and modelling

Heat stress can cause sterility in sorghum and the anticipated increased frequency of high temperature events implies increasing risk to sorghum productivity in Australia. Here we summarise our research on specific varietal attributes associated with heat stress tolerance in sorghum and evaluate how they might affect yield outcomes in production environments by a crop simulation analysis. We have recently conducted a range of controlled environment and field experiments to study the physiology and genetics of high temperature effects on growth and development of sorghum. Sorghum seed set was reduced by high temperature effects (>36-38oC) on pollen germination around flowering, but genotypes differed in their tolerance to high temperature stress. Effects were quantified in a manner that enabled their incorporation into the APSIM sorghum crop model. Simulation analysis indicated that risk of high temperature damage and yield loss depended on sowing date, and variety. While climate trends will exacerbate high temperature effects, avoidance by crop management and genetic tolerance seems possible.

Effective Use in Livestock Feeds of Mouldy and Weather-damaged Grain Containing Mycotoxins-Case Histories and Economic Assessments Pertaining to Pig and Poultry Industries of Queensland

Mould growth in field crops or stored grain reduces starch and lipid content, with consequent increases in fibre, and an overall reduction in digestible energy; palatability is often adversely affected. If these factors are allowed for, and mycotoxin concentrations are low, there are sound economic reasons for using this cheaper grain. Mycotoxins are common in stock feed but their effects on animal productivity are usually slight because either the concentration is too low or the animal is tolerant to the toxin. In Australia, aflatoxins occur in peanut by-products and in maize and sorghum if the grain is moist when stored. Zearalenone is found in maize and in sorghum and wheat in wetter regions. Nivalenol and deoxynivalenol are found in maize and wheat but at concentrations that rarely affect pigs, with chickens and cattle being even more tolerant. Other mycotoxins including cyclopiazonic acid, T-2 toxin, cytochalasins and tenuazonic acid are produced by Australian fungi in culture but are not found to be significant grain contaminants. Extremely mouldy sorghum containing Alternaria and Fusarium mycotoxins decreased feed conversion in pigs and chickens by up to 14%. However, E moniliforme- and Diplodia maydis-infected maize produced only slight reductions in feed intake by pigs and Ustilago- infected barley produced no ill effects. Use of these grains would substantially increase profits if the grain can be purchased cheaply.

Rejection by pigs of mouldy grain containing deoxynivalenol

Weaner pigs on a farm near Beaudesert in south eastern Queensland refused to eat feed comprised largely of wheat and barley. Older pigs consumed small amounts and some prepubertal gilts subsequently displayed enlarged and reddened vulvas. Wheat, barley and triticale were grown on the farm during 1983, which was unusually and persistently wet. The wheat and triticale were harvested and stored for about 3 weeks with moisture contents above 14% before being fed. Samples of the wheat and triticale contained pale pink grains, which can indicate infection by the fungus Fusariurn grarninearurn Schw. On analysis 2 mycotoxins known to be produced by F. graminearurn were detected, deoxynivalenol (vomitoxin) which causes feed refusal and vomiting, and zearalenone which causes oestrogenic effects. Concentrations of deoxynivalenol in the wheat, triticale and barley were 34, 10, and <0.1 mg/kg respectively. Concentrations of zearalenone were 6.2, 2.8 and 0.1 mg/kg respectively. Subsequently, F. grarninearurn was isolated from grains and crop residues. Although the wet weather contributed to F. grarninearurn infection of the crops before harvest, most of the toxins probably developed during storage.

Impact of resistance on the efficacy of binary combinations of spinosad, chlorpyrifos-methyl and s-methoprene against five stored-grain beetles

Laboratory experiments were conducted to determine the efficacy of spinosad (a biopesticide), chlorpyrifos-methyl (an organophosphorus compound (OP)) and s-methoprene (a juvenile hormone analogue) applied alone and in binary combinations against five stored-grain beetles in wheat. There were three strains of Rhyzopertha dominica, and one strain each of Sitophilus oryzae, Tribolium castaneum, Oryzaephilus surinamensis and Cryptolestes ferrugineus. These strains were chosen to represent a range of possible resistant genotypes, exhibiting resistance to organophosphates, pyrethroids or methoprene. Treatments were applied at rates that are registered or likely to be registered in Australia. Adults were exposed to freshly treated wheat for 2 weeks, and the effects of treatments on mortality and reproduction were determined. No single protectant or protectant combination controlled all insect strains, based on the criterion of >99% reduction in the number of live F1 adults relative to the control. The most effective combinations were spinosad at 1 mg kg-1+chlorpyrifos-methyl at 10 mg kg-1 which controlled all strains except for OP-resistant O. surinamensis, and chlorpyrifos-methyl at 10 mg kg-1+s-methoprene at 0.6 mg kg-1 which controlled all strains except for methoprene-resistant R. dominica. The results of this study demonstrate the difficulty in Australia, and potentially other countries which use protectants, of finding protectant treatments to control a broad range of pest species in the face of resistance development.

Root-lesion nematodes (Pratylenchus thornei and P. neglectus): A review of recent progress in managing a significant pest of grain crops in northern Australia

Two species of root-lesion nematode (predominantly Pratylenchus thornei but also P. neglectus) are widespread pathogens of wheat and other crops in Australia's northern grain belt, a subtropical region with deep, fertile clay soils and a summer-dominant rainfall pattern. Losses in grain yield from P. thornei can be as high as 70% for intolerant wheat cultivars. This review focuses on research which has led to the development of effective integrated management programs for these nematodes. It highlights the importance of correct identification in managing Pratylenchus species, reviews the plant breeding work done in developing tolerant and resistant cultivars, outlines the methods used to screen for tolerance and resistance, and discusses how planned crop sequencing with tolerant and partially resistant wheat cultivars, together with crops such as sorghum, sunflower, millets and canaryseed, can be used to reduce nematode populations and limit crop damage. The declining levels of soil organic matter in cropped soils are also discussed with reference to their effect on soil health and biological suppression of root-lesion nematodes.

An overview of integrated pest management (IPM) in north-eastern Australian grain farming systems: Past, present and future prospects

The authors overview integrated pest management (IPM) in grain crops in north-eastern Australia, which is defined as the area north of latitude 32°S. Major grain crops in this region include the coarse grains (winter and summer cereals), oilseeds and pulses. IPM in these systems is complicated by the diversity of crops, pests, market requirements and cropping environments. In general, the pulse crops are at greatest risk, followed by oilseeds and then by cereal grains. Insecticides remain a key grain pest management tool in north-eastern Australia. IPM in grain crops has benefited considerably through the increased adoption of new, more selective insecticides and biopesticides for many caterpillar pests, in particular Helicoverpa spp. and loopers, and the identification of pest-crop scenarios where spraying is unnecessary (e.g. for most Creontiades spp. populations in soybeans). This has favoured the conservation of natural enemies in north-eastern Australia grain crops, and has arguably assisted in the management of silverleaf whitefly in soybeans in coastal Queensland. However, control of sucking pests and podborers such as Maruca vitrata remains a major challenge for IPM in summer pulses. Because these crops have very low pest-damage tolerances and thresholds, intervention with disruptive insecticides is frequently required, particularly during podfill. The threat posed by silverleaf whitefly demands ongoing multi-pest IPM research, development and extension as this pest can flare under favourable seasonal conditions, especially where disruptive insecticides are used injudiciously. The strong links between researchers and industry have facilitated the adoption of IPM practices in north-eastern Australia and augers well for future pest challenges and for the development and promotion of new and improved IPM tactics.

Host plant resistance in grain crops and prospects for invertebrate pest management in Australia: An overview

An integrated pest management (IPM) approach that relies on an array of tactics is adopted commonly in response to problems with pesticide-based production in many agricultural systems. Host plant resistance is often used as a fundamental component of an IPM system because of the generally compatible, complementary role that pest-resistant crops play with other tactics. Recent research and development in the resistance of legumes and cereals to aphids, sorghum midge resistance, and the resistance of canola varieties to mite and insect pests have shown the prospects of host plant resistance for developing IPM strategies against invertebrate pests in Australian grain crops. Furthermore, continuing advances in biotechnology provide the opportunity of using transgenic plants to enhance host plant resistance in grains.

Seasonal incidence of Stenodiplosis sorghicola (Coquillett) (Diptera: Cecidomyiidae) and its parasitoids on Sorghum halepense (L.) Pers. in south-eastern Queensland, Australia

To quantify the role of Johnson grass, Sorghum halepense, in the population dynamics of the sorghum midge, Stenodiplosis sorghicola, patterns of flowering of Johnson grass and infestation by sorghum midge were studied in two different climatic environments in the Lockyer Valley and on the Darling Downs in south-eastern Queensland for 3 years. Parasitism levels of S. sorghicola were also recorded. In the Lockyer Valley, Johnson grass panicles were produced throughout the year but on the Darling Downs none were produced between June and September. In both areas, most panicle production occurred between November and March and infestation by S. sorghicola was the greatest during this period. The parasitism levels were between 20% and 50%. After emergence from winter diapause, one to two generations of S. sorghicola developed on S. halepense before commercial grain sorghum crops were available for infestation. Parasitoids recorded were: Aprostocetus diplosidis, Eupelmus australiensis and two species of Tetrastichus. Relationships between sorghum midge population growth rate and various environmental and population variables were investigated. Population size had a significant negative effect (P < 0.0001) on population growth rate. Mortality due to parasitism showed a significant positive density response (P < 0.0001). Temperature, rainfall, open pan evaporation, degree-days and host availability showed no significant effect on population growth rate. Given the phenology of sorghum production in south-eastern Queensland, Johnson grass provides an important bridging host, sustaining one to two generations of sorghum midge. Critical studies relating population change and build-up in sorghum to sorghum midge populations in Johnson grass are yet to be performed.

Weed species richness, density and relative abundance on farms in the subtropical grain region of Australia

Weed management is one of the most important economic and agronomic issues facing farmers in Australia's grain regions. Weed species occurrence and abundance was monitored between 1997 and 2000 on 46 paddocks (sites) across 18 commercial farms located in the Northern Grain Region. The sites generally fell within 4 disjunct regions, from south to north: Liverpool Plains, Moree, Goondiwindi and Kingaroy. While high species richness was found (139 species or species groups), only 8 species occurred in all 4 regions and many (56 species) only occurred at 1 site or region. No species were observed at every site but 7 species (Sonchus spp., Avena spp., Conyza spp., Echinochloa spp., Convolvulus erubescens, Phalaris spp. and Lactuca serriola) were recorded on more than 70% of sites. The average number of species observed within crops after treatment and before harvest was less than 13. Species richness tended to be higher in winter pulse crops, cotton and in fallows, but overall was similar at the different sampling seasons (summer v. winter). Separate species assemblages associated with the Goondiwindi and Kingaroy regions were identified by correspondence analysis but these appeared to form no logical functional group. The species richness and density was generally low, demonstrating that farmers are managing weed populations effectively in both summer and winter cropping phases. Despite the apparent adoption of conservation tillage, an increase in opportunity cropping and the diversity of crops grown (13) there was no obvious effect of management practices on weed species richness or relative abundance. Avena spp. and Sonchus spp. were 2 of the most dominant weeds, particularly in central and southern latitudes of the region; Amaranthus spp. and Raphanus raphanistrum were the most abundant species in the northern part of the region. The ubiquity of these and other species shows that continued vigilance is required to suppress weeds as a management issue.

Survey of weed flora and management relative to cropping practices in the north-eastern grain region of Australia

The main weeds and weed management practices undertaken in broad acre dryland cropping areas of north-eastern Australia have been identified. The information was collected in a comprehensive postal survey of both growers and agronomists from Dubbo in New South Wales (NSW) through to Clermont in central Queensland, where 237 surveys were returned. A very diverse weed flora of 105 weeds from 91 genera was identified for the three cropping zones within the region (central Queensland, southern Queensland and northern NSW). Twenty-three weeds were common to all cropping zones. The major common weeds were Sonchus oleraceus, Rapistrum rugosum, Echinochloa spp. and Urochloa panicoides. The main weeds were identified for both summer and winter fallows, and sorghum, wheat and chickpea crops for each of the zones, with some commonality as well as floral uniqueness recorded. More genera were recorded in the fallows than in crops, and those in summer fallows exceeded the number in winter. Across the region, weed management relied heavily on herbicides. In fallows, glyphosate and mixes with glyphosate were very common, although the importance of the glyphosate mix partner differed among the cropping zones. Use and importance of pre-emergence herbicides in-crop varied considerably among the zones. In wheat, more graminicides were used in northern NSW than in southern Queensland, and virtually none were used in central Queensland, reflecting the differences in winter grass weed flora across the region. Atrazine was the major herbicide used in sorghum, although metolachlor was also used predominantly in northern NSW. Fallow and inter-row cultivation were used more often in the southern areas of the region. Grazing of fallows was more prominent in northern NSW. High crop seeding rates were not commonly recorded indicating that growers are not using crop competition as a tool for weed management. Although many management practices were recorded overall, few growers were using integrated weed management, and herbicide resistance has been and continues to be an issue for the region.

Pre-anthesis ovary development determines genotypic differences in potential kernel weight in sorghum

Kernel weight is an important factor determining grain yield and nutritional quality in sorghum, yet the developmental processes underlying the genotypic differences in potential kernel weight remain unclear. The aim of this study was to determine the stage in development at which genetic effects on potential kernel weight were realized, and to investigate the developmental mechanisms by which potential kernel weight is controlled in sorghum. Kernel development was studied in two field experiments with five genotypes known to differ in kernel weight at maturity. Pre-fertilization floret and ovary development was examined and post-fertilization kernel-filling characteristics were analysed. Large kernels had a higher rate of kernel filling and contained more endosperm cells and starch granules than normal-sized kernels. Genotypic differences in kernel development appeared before stamen primordia initiation in the developing florets, with sessile spikelets of large-seeded genotypes having larger floret apical meristems than normal-seeded genotypes. At anthesis, the ovaries for large-sized kernels were larger in volume, with more cells per layer and more vascular bundles in the ovary wall. Across experiments and genotypes, there was a significant positive correlation between kernel dry weight at maturity and ovary volume at anthesis. Genotypic effects on meristem size, ovary volume, and kernel weight were all consistent with additive genetic control, suggesting that they were causally related. The pre-fertilization genetic control of kernel weight probably operated through the developing pericarp, which is derived from the ovary wall and potentially constrains kernel expansion.

A consensus genetic map of sorghum that integrates multiple component maps and high-throughput Diversity Array Technology (DArT) markers

Background: Sorghum genome mapping based on DNA markers began in the early 1990s and numerous genetic linkage maps of sorghum have been published in the last decade, based initially on RFLP markers with more recent maps including AFLPs and SSRs and very recently, Diversity Array Technology (DArT) markers. It is essential to integrate the rapidly growing body of genetic linkage data produced through DArT with the multiple genetic linkage maps for sorghum generated through other marker technologies. Here, we report on the colinearity of six independent sorghum component maps and on the integration of these component maps into a single reference resource that contains commonly utilized SSRs, AFLPs, and high-throughput DArT markers. Results: The six component maps were constructed using the MultiPoint software. The lengths of the resulting maps varied between 910 and 1528 cM. The order of the 498 markers that segregated in more than one population was highly consistent between the six individual mapping data sets. The framework consensus map was constructed using a "Neighbours" approach and contained 251 integrated bridge markers on the 10 sorghum chromosomes spanning 1355.4 cM with an average density of one marker every 5.4 cM, and were used for the projection of the remaining markers. In total, the sorghum consensus map consisted of a total of 1997 markers mapped to 2029 unique loci ( 1190 DArT loci and 839 other loci) spanning 1603.5 cM and with an average marker density of 1 marker/0.79 cM. In addition, 35 multicopy markers were identified. On average, each chromosome on the consensus map contained 203 markers of which 58.6% were DArT markers. Non-random patterns of DNA marker distribution were observed, with some clear marker-dense regions and some marker-rare regions. Conclusion: The final consensus map has allowed us to map a larger number of markers than possible in any individual map, to obtain a more complete coverage of the sorghum genome and to fill a number of gaps on individual maps. In addition to overall general consistency of marker order across individual component maps, good agreement in overall distances between common marker pairs across the component maps used in this study was determined, using a difference ratio calculation. The obtained consensus map can be used as a reference resource for genetic studies in different genetic backgrounds, in addition to providing a framework for transferring genetic information between different marker technologies and for integrating DArT markers with other genomic resources. DArT markers represent an affordable, high throughput marker system with great utility in molecular breeding programs, especially in crops such as sorghum where SNP arrays are not publicly available.

Functional dynamics of the nitrogen balance of sorghum: I. N demand of vegetative plant parts.

Stay-green, an important trait for grain yield of sorghum grown under water limitation, has been associated with a high leaf nitrogen content at the start of grain filling. This study quantifies the N demand of leaves and stems and explores effects of N stress on the N balance of vegetative plant parts of three sorghum hybrids differing in potential crop height. The hybrids were grown under well-watered conditions at three levels of N supply. Vertical profiles of biomass and N% of leaves and stems, together with leaf size and number, and specific leaf nitrogen (SLN), were measured at regular intervals. The hybrids had similar minimum but different critical and maximum SLN, associated with differences in leaf size and N partitioning, the latter associated with differences in plant height. N demand of expanding new leaves was represented by critical SLN, and structural stem N demand by minimum stem N%. The fraction of N partitioned to leaf blades increased under N stress. A framework for N dynamics of leaves and stems is developed that captures effects of N stress and genotype on N partitioning and on critical and maximum SLN.