[Show abstract][Hide abstract] ABSTRACT: Drought during grain filling is a common challenge for sorghum production in north-eastern Australia, central-western India, and sub-Saharan Africa. We show that the stay-green drought adaptation trait enhances sorghum grain yield under post-anthesis drought in these three regions. A positive relationship between stay-green and yield was generally found in breeding trials in north-eastern Australia that sampled 1668 unique hybrid combinations and 23 environments. Physiological studies in Australia also found that introgressing four individual stay-green (Stg1–4) quantitative trait loci (QTLs) into a senescent background reduced water demand before flowering and hence increased water supply during grain filling, resulting in higher grain yield relative to the senescent control. Studies in India found that various Stg QTLs affected both transpiration and transpiration efficiency, although these effects depended on the interaction between genetic background (S35 and R16) and individual QTLs. The yield variation unexplained by harvest index was related to transpiration efficiency in S35 (R2 = 0.29) and R16 (R2 = 0.72), and was related to total water extracted in S35 (R2 = 0.41) but not in R16. Finally, sixty-eight stay-green enriched lines were evaluated in six countries in sub-Saharan Africa during the 2013/14 season. Analysis of the data from Kenya indicates that stay-green and grain size were positively correlated at two sites: Kiboko (high yielding, r2=0.25) and Masongaleni (low yielding, r2=0.37). Together, these studies suggest that stay-green is a beneficial trait for sorghum production in the semi-arid tropics and is a consequence of traits altering the plant water budget.
[Show abstract][Hide abstract] ABSTRACT: Breeders have successfully improved maize (Zea mays L.) grain yield for the conditions of the US corn-belt over the past 80 years, with the past 50 years utilizing single-cross hybrids. Long-term improvement for grain yield under water-limited conditions has also been reported. Grain yield under water-limited conditions depends on water use, water use efficiency, and harvest index. It has been hypothesized that long-term genetic gain for yield could be due, in part, to increased water capture from the soil. This hypothesis was tested using a set of elite single-cross hybrids that were released by DuPont Pioneer between 1963 and 2009. Eighteen hybrids were grown in the field during 2010 and 2011 growing seasons at Woodland, CA, USA. Crops grew predominantly on stored soil water and drought stress increased as the season progressed. Soil water content was measured to 300cm depth throughout the growing season. Significant water extraction occurred to a depth of 240-300cm and seasonal water use was calculated from the change in soil water over this rooting zone. Grain yield increased significantly with year of commercialization, but no such trend was observed for total water extraction. Therefore, the measured genetic gain for yield for the period represented by this set of hybrids must be related to either increased efficiency of water use or increased carbon partitioning to the grain, rather than increased soil water uptake.
Preview · Article · Oct 2015 · Journal of Experimental Botany
[Show abstract][Hide abstract] ABSTRACT: Significant genotypic differences in tolerance of pollen germination and seed set to high temperatures have been shown in sorghum. However, it is unclear whether differences were associated with variation in either the threshold temperature above which reproductive processes are affected, or in the tolerance to increased temperature above that threshold. The objectives of this study were to (a) dissect known differences in heat tolerance for a range of sorghum genotypes into differences in the threshold temperature and tolerance to increased temperatures, (b) determine whether poor seed set under high temperatures can be compensated by increased seed mass, and (c) identify whether genotypic differences in heat tolerance in a controlled environment facility (CEF) can be reproduced in field conditions. Twenty genotypes were grown in a CEF under four day/night temperatures (31.9/21.0 °C, 32.8/21.0 °C, 36.1/21.0 °C, and 38.0/21.0 °C), and a subset of six genotypes was grown in the field under four different temperature regimes around anthesis. The novelty of the findings in this study related to differences in responsiveness to high temperature—genotypic differences in seed set percentage were found for both the threshold temperature and the tolerance to increased maximum temperature above that threshold. Further, the response of seed set to high temperature in the field study was well correlated to that in the CEF (R2 = 0.69), although the slope was significantly less than unity, indicating that heat stress effects may have been diluted under the variable field conditions. Poor seed set was not compensated by increased seed mass in either CEF or field environments. Grain yield was thus closely related to seed set percentage. This result demonstrates the potential for development of a low-cost field screening method to identify high-temperature tolerant varieties that could deliver sustainable yields under future warmer climates.
Full-text · Article · Feb 2015 · Field Crops Research
[Show abstract][Hide abstract] ABSTRACT: Stay-green sorghum plants exhibit greener leaves and stems during the grain-filling period under water-limited conditions
compared with their senescent counterparts, resulting in increased grain yield, grain mass, and lodging resistance. Stay-green
has been mapped to a number of key chromosomal regions, including Stg1, Stg2, Stg3, and Stg4, but the functions of these individual quantitative trait loci (QTLs) remain unclear. The objective of this study was to
show how positive effects of Stg QTLs on grain yield under drought can be explained as emergent consequences of their effects on temporal and spatial water-use
patterns that result from changes in leaf-area dynamics. A set of four Stg near-isogenic lines (NILs) and their recurrent parent were grown in a range of field and semicontrolled experiments in southeast
Queensland, Australia. These studies showed that the four Stg QTLs regulate canopy size by: (1) reducing tillering via increased size of lower leaves, (2) constraining the size of the
upper leaves; and (3) in some cases, decreasing the number of leaves per culm. In addition, they variously affect leaf anatomy
and root growth. The multiple pathways by which Stg QTLs modulate canopy development can result in considerable developmental plasticity. The reduction in canopy size associated
with Stg QTLs reduced pre-flowering water demand, thereby increasing water availability during grain filling and, ultimately, grain
yield. The generic physiological mechanisms underlying the stay-green trait suggest that similar Stg QTLs could enhance post-anthesis drought adaptation in other major cereals such as maize, wheat, and rice.
Full-text · Article · Nov 2014 · Journal of Experimental Botany
[Show abstract][Hide abstract] ABSTRACT: Key message
A QTL model for the genetic control of tillering in sorghum is proposed, presenting new opportunities for sorghum breeders to select germplasm with tillering characteristics appropriate for their target environments.
Tillering in sorghum can be associated with either the carbon supply–demand (S/D) balance of the plant or an intrinsic propensity to tiller (PTT). Knowledge of the genetic control of tillering could assist breeders in selecting germplasm with tillering characteristics appropriate for their target environments. The aims of this study were to identify QTL for tillering and component traits associated with the S/D balance or PTT, to develop a framework model for the genetic control of tillering in sorghum. Four mapping populations were grown in a number of experiments in south east Queensland, Australia. The QTL analysis suggested that the contribution of traits associated with either the S/D balance or PTT to the genotypic differences in tillering differed among populations. Thirty-four tillering QTL were identified across the populations, of which 15 were novel to this study. Additionally, half of the tillering QTL co-located with QTL for component traits. A comparison of tillering QTL and candidate gene locations identified numerous coincident QTL and gene locations across populations, including the identification of common non-synonymous SNPs in the parental genotypes of two mapping populations in a sorghum homologue of MAX1, a gene involved in the control of tiller bud outgrowth through the production of strigolactones. Combined with a framework for crop physiological processes that underpin genotypic differences in tillering, the co-location of QTL for tillering and component traits and candidate genes allowed the development of a framework QTL model for the genetic control of tillering in sorghum.
Full-text · Article · Aug 2014 · Theoretical and Applied Genetics
[Show abstract][Hide abstract] ABSTRACT: Agricultural systems models worldwide are increasingly being used to explore options and solutions for the food security, climate change adaptation and mitigation and carbon trading problem domains. APSIM (Agricultural Production Systems sIMulator) is one such model that continues to be applied and adapted to this challenging research agenda. From its inception twenty years ago, APSIM has evolved into a framework containing many of the key models required to explore changes in agricultural landscapes with capability ranging from simulation of gene expression through to multi-field farms and beyond. Keating et al. (2003) described many of the fundamental attributes of APSIM in detail. Much has changed in the last decade, and the APSIM community has been exploring novel scientific domains and utilising software developments in social media, web and mobile applications to provide simulation tools adapted to new demands. This paper updates the earlier work by Keating et al. (2003) and chronicles the changing external challenges and opportunities being placed on APSIM during the last decade. It also explores and discusses how APSIM has been evolving to a “next generation” framework with improved features and capabilities that allow its use in many diverse topics.
Full-text · Article · Aug 2014 · Environmental Modelling and Software
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Tillering determines the plant size of sorghum (Sorghum bicolor) and an understanding of its regulation is important to match genotypes to prevalent growing conditions in target production environments. The aim of this study was to determine the physiological and environmental regulation of variability in tillering among sorghum genotypes, and to develop a framework for this regulation. Diverse sorghum genotypes were grown in three experiments with contrasting temperature, radiation and plant density to create variation in tillering. Data on phenology, tillering, and leaf and plant size were collected. A carbohydrate supply/demand (S/D) index that incorporated environmental and genotypic parameters was developed to represent the effects of assimilate availability on tillering. Genotypic differences in tillering not explained by this index were defined as propensity to tiller (PTT) and probably represented hormonal effects. Genotypic variation in tillering was associated with differences in leaf width, stem diameter and PTT. The S/D index captured most of the environmental effects on tillering and PTT most of the genotypic effects. A framework that captures genetic and environmental regulation of tillering through assimilate availability and PTT was developed, and provides a basis for the development of a model that connects genetic control of tillering to its phenotypic consequences.
[Show abstract][Hide abstract] ABSTRACT: Climatic variability in dryland production environments (E) generates variable yield and crop production risks. Optimal combinations of genotype (G) and management (M) depend strongly on E and thus vary among sites and seasons. Traditional crop improvement seeks broadly adapted genotypes to give best average performance under a standard management regime across the entire production region, with some subsequent manipulation of management regionally in response to average local environmental conditions. This process does not search the full spectrum of potential G × M × E combinations forming the adaptation landscape. Here we examine the potential value (relative to the conventional, broad adaptation approach) of exploiting specific adaptation arising from G × M × E. We present an in-silico analysis for sorghum production in Australia using the APSIM sorghum model. Crop design (G × M) is optimised for subsets of locations within the production region (specific adaptation) and is compared with the optimum G across all environments with locally modified M (broad adaptation). We find that geographic subregions that have frequencies of major environment types substantially different from that for the entire production region show greatest advantage for specific adaptation. Although the specific adaptation approach confers yield and production risk advantages at industry scale, even greater benefits should be achievable with better predictors of environment-type likelihood than that conferred by location alone.
Full-text · Article · Jan 2014 · Crop and Pasture Science
[Show abstract][Hide abstract] ABSTRACT: Presence of the dw3 sorghum dwarfing gene had negative effects on grain yield in some genetic backgrounds and environments. In a previous study we showed that this was due to a significant reduction in shoot biomass (mainly via reduced stem mass), which in turn negatively affected grain size. The current study examines whether shoot biomass was reduced via effects of dw3 on traits associated with resource capture, such as leaf area index (LAI), light interception (LI), and canopy extinction coefficient (k) or with resource use efficiency, such as radiation use efficiency (RUE). Three pairs of near-isogenic sorghum lines differing only in the presence or absence of the dwarfing allele dw3 (3-dwarfs vs 2-dwarfs) were grown in large field plots. Biomass accumulation and LI were measured for individual canopy layers to examine canopy characteristics of tall and short types. Similar to the previously reported effects on grain yield, the effects of dw3 on RUE, LI and k varied among genetic backgrounds and environments. Interactions between dw3 and genetic background, but also interactions with environment are likely to have modulated the extent to which RUE, LI, or k contributed to biomass differences between tall and short sorghum.
Full-text · Article · Aug 2013 · Field Crops Research
[Show abstract][Hide abstract] ABSTRACT: Sorghum (Sorghum bicolor (L.) Moench) is grown as a dryland crop in semiarid subtropical and tropical environments where it is often exposed to high temperatures around flowering. Projected climate change is likely to increase the incidence of exposure to high temperature, with potential adverse effects on growth, development and grain yield. The objectives of this study were to explore genetic variability for the effects of high temperature on crop growth and development, in vitro pollen germination and seed-set. Eighteen diverse sorghum genotypes were grown at day : night temperatures of 32 : 21 degrees C (optimum temperature, OT) and 38 : 21 degrees C (high temperature, HT during the middle of the day) in controlled environment chambers. HT significantly accelerated development, and reduced plant height and individual leaf size. However, there was no consistent effect on leaf area per plant. HT significantly reduced pollen germination and seed-set percentage of all genotypes; under HT, genotypes differed significantly in pollen viability percentage (17-63%) and seed-set percentage (7-65%). The two traits were strongly and positively associated (R-2 = 0.93, n = 36, P < 0.001), suggesting a causal association. The observed genetic variation in pollen and seed-set traits should be able to be exploited through breeding to develop heat-tolerant varieties for future climates.
Full-text · Article · Jan 2013 · Functional Plant Biology
[Show abstract][Hide abstract] ABSTRACT: Positive correlations between plant height and grain yield have been reported for sorghum. The introduction of stay-green in sorghum, and the associated reduction in lodging, has opened the possibility to exploit this positive association. The aim of this study was to analyse the direct effects of the dwarfing gene dw3 (and therefore plant height) on shoot biomass, grain yield, and yield components in pairs of 3-dwarf genotypes and their isogenic 2-dwarf tall mutants. Isogenic pairs with different genetic backgrounds were grown in three field experiments under nutrient and water non-limiting conditions. Tall mutants were significantly taller and produced more shoot and stem biomass than their shorter counterparts. Generally, tall types yielded more grain than short types, but significant interactions between experiment, genetic background and stature affected the consistency of the results. dw3 only affected grain size and not grain number. Increased grain mass of tall types was associated with significantly greater stem mass per grain at anthesis and greater shoot biomass per grain accumulated between anthesis and maturity. The increased biomass of tall plants was therefore important for increased grain yield under optimum conditions. Potential implications of increased biomass production for drought adaptation are discussed.
Full-text · Article · Jan 2013 · Field Crops Research
[Show abstract][Hide abstract] ABSTRACT: Genotypic variability in root system architecture has been associated with root angle of seedlings and water extraction patterns of mature plants in a range of crops. The potential inclusion of root angle as a selection criterion in a sorghum breeding program requires (1) availability of an efficient screening method, (2) presence of genotypic variation with high heritability, and (3) an association with water extraction pattern. The aim of this study was to determine the feasibility for inclusion of nodal root angle as a selection criterion in sorghum breeding programs. A high-throughput phenotypic screen for nodal root angle in young sorghum plants has recently been developed and has been used successfully to identify significant variation in nodal root angle across a diverse range of inbred lines and a mapping population. In both cases, heritabilities for nodal root angle were high. No association between nodal root angle and plant size was detected. This implies that parental inbred lines could potentially be used to asses nodal root angle of their hybrids, although such predictability is compromised by significant interactions. To study effects of nodal root angle on water extraction patterns of mature plants, four inbred lines with contrasting nodal root angle at seedling stage were grown until at least anthesis in large rhizotrons. A consistent trend was observed that nodal root angle may affect the spatial distribution of root mass of mature plants and hence their ability to extract soil water, although genotypic differences were not significant. The potential implications of this for specific adaptation to drought stress are discussed. Results suggest that nodal root angle of young plants can be a useful selection criterion for specific drought adaptation, and could potentially be used in molecular breeding programs if QTLs for root angle can be identified.
Full-text · Article · Oct 2012 · European Journal of Agronomy
[Show abstract][Hide abstract] ABSTRACT: For annual and perennial crops, mathematical models have been developed to describe tissue nitrogen (N) dilution during crop growth and to estimate the plant N status applying the N nutrition index (NNI), the ratio between the actual tissue N concentration ([N]) and the tissue N concentration needed to obtain the maximum instantaneous crop growth rate (critical tissue N concentration, [N]c). The relationship between shoot [N]c and shoot dry matter (DM, t ha−1) can be described by an allometric power equation: [N]c = a DM−b, where a and b are crop-specific parameters. Critical N dilution curves (CNDC) have been determined for several C3 crops but not specifically for sunflower (Helianthus annuus L.). The objectives of this work were to (i) determine and validate the N dilution curves for critical, minimum, and maximum [N] for sunflower from the juvenile stages to the end of flowering, (ii) compare the critical curve with published CNDCs for other C3 crops, and (iii) estimate the range of variation of NNI for different levels of N fertilization and irrigation. A wide range of field experiments from Argentina, Australia, France, Italy, and Spain was used to establish the dilution curve for sunflower and to independently validate it. The fitted CNDC [N]c = 4.53 DM−0.42 yielded lower values for [N]c than references used until now for diagnosis and decision making in sunflower. The value of parameter a was generally similar to that of other C3 species, but the value for parameter b differed. This was possibly associated with species differences in dry mass partitioning, and justified the development of a sunflower-specific CNDC. A preliminary reference curve for maximum [N] suggested an evolution from the juvenile stages to the end of flowering similar to that of [N]c. Minimum [N], in contrast, appeared to be more constant over time. Relationships between relative grain yield and NNI across a range of locations indicated that in general, maximum grain yield was reached around NNI = 0.8, although at one location this was around NNI = 1.0. The CNDC can provide useful applications for crop modeling, N status diagnosis, and N fertilization decision.
Full-text · Article · Sep 2012 · Field Crops Research
[Show abstract][Hide abstract] ABSTRACT: Increased leaf appearance rate (LAR) could reduce preanthesis water use of sorghum [Sorghum bicolor (L.) Moench] by restricting plant size via reduced tillering. The aim of this paper was to assess LAR as a potential pathway for adaptation to postanthesis drought stress. Four hybrids with different LAR were grown in four semicontrolled experiments under well-watered conditions or postanthesis drought stress and in two irrigated field experiments. Observations included leaf area dynamics, transpiration, transpiration efficiency (TE), leaf N, biomass, and grain yield. ATx642 hybrids (0.0306 leaf degrees C d(-1)) had signifi cantly greater LAR than AQL39 hybrids (0.0279 leaf degrees C d(-1)) and this increased early main shoot vigor. Under low plant density, this reduced tiller number and hence leaf area and biomass around anthesis. As hybrids had similar TE and differed little in phenology, this can reduce preanthesis water use. Water availability at flag leaf determined grain number per plant (adjusted R(2) = 0.80, p < 0.01) and hence grain yield. However, the effect of increased LAR on reduced plant size was temperature dependent. Under high temperature, genotypic differences in tillering were reduced and main shoot leaf number increased more in hybrids with greater LAR. This increased responsiveness of leaf number could increase plant size and water use at anthesis. Hence, greater LAR may confer drought adaptation only in specific environments unless it is also associated with critical aspects of biomass partitioning.
[Show abstract][Hide abstract] ABSTRACT: Nodal root angle in sorghum influences vertical and horizontal root distribution in the soil profile and is thus relevant to drought adaptation. In this study, we report for the first time on the mapping of four QTL for nodal root angle (qRA) in sorghum, in addition to three QTL for root dry weight, two for shoot dry weight, and three for plant leaf area. Phenotyping was done at the six leaf stage for a mapping population (n = 141) developed by crossing two inbred sorghum lines with contrasting root angle. Nodal root angle QTL explained 58.2% of the phenotypic variance and were validated across a range of diverse inbred lines. Three of the four nodal root angle QTL showed homology to previously identified root angle QTL in rice and maize, whereas all four QTL co-located with previously identified QTL for stay-green in sorghum. A putative association between nodal root angle QTL and grain yield was identified through single marker analysis on field testing data from a subset of the mapping population grown in hybrid combination with three different tester lines. Furthermore, a putative association between nodal root angle QTL and stay-green was identified using data sets from selected sorghum nested association mapping populations segregating for root angle. The identification of nodal root angle QTL presents new opportunities for improving drought adaptation mechanisms via molecular breeding to manipulate a trait for which selection has previously been very difficult.
[Show abstract][Hide abstract] ABSTRACT: Sorghum [Sorghum bicolor (L.) Moench] is a major dryland cereal crop in environments with low and unpredictable rainfall. Root angle at the seedling stage can potentially influence root architecture and hence adaptation to drought. The aims of this study were to investigate the extent of genetic variation in nodal root angle of sorghum and to quantify the general combining ability (GCA) and specific combining ability (SCA) for the trait. A diverse range of inbred lines and hybrids was grown in custom-made containers in a glasshouse. Plants were harvested when six leaves had fully expanded and encompassing angle of first flush of nodal roots (relative to vertical), leaf area, and root and shoot dry weight were determined for each plant. Nodal root angle ranged from 15 to 50 degrees for inbred lines and 14 to 43 degrees for hybrids and had a moderately high heritability (47%) in inbred lines. However, significant interaction between male and female parents in hybrids indicated significant SCA. Variation in nodal root angle was independent of variation in plant size (shoot and root weight and leaf area). Variability in plant size among hybrids was associated with the female parent (p < 0.001) and resulted in significant GCA. Screening and selection for nodal root angle should not be affected by potential differences in plant size between inbred lines and hybrids, although the significant SCA would limit predictability of root angle in hybrids based on the root angle of the parental inbred lines.
[Show abstract][Hide abstract] ABSTRACT: The utility of crop growth and development models for investigating emergent behaviour of complex gene-to-phenotype interactions is limited if such models rely on algorithms that describe key aspects of crop growth and development too simply. While this approach has proven adequate for predicting crop responses to agronomic management and some major genetic factors (e.g. physiological maturity), attempts at connecting these frameworks with drivers of genetics that underpin complex traits have had little success or impact on crop improvement. In further development of the generic APSIM Cereal model we have introduced aspects of physiological development at a more detailed level so that complex phenotypic responses become emergent properties of the model dynamics. At the same time, we intend to retain model simplicity by dealing with crop growth and development processes at a mostly similar level (organ-plant-canopy) to that used in conventional agronomic models. However, we argue that more biologically-grounded quantification of the underpinning processes will provide more robust avenues to link plant response to molecular genetics. There is some evidence for this from a range of studies on adaptation of crops to drought stress, where the generic APSIM cereal template has been employed. In this paper, we present an outline of the template framework and detail the approach to modelling nitrogen dynamics (uptake, allocation, retranslocation) in sorghum. This provides an example of how genetic variation linked to specific attributes of the plant (e.g. height) can generate emergent phenotypic differences in leaf area retention associated with nitrogen. We consider that the generic APSIM Cereal template provides a good framework to develop and enhance the functionality of whole-plant modelling required to help bridge the gene-to-phenotype gap.