High-throughput Phenotyping and Genomic Selection:The Frontiers of Crop Breeding Converge

Genomic selection (GS) and high-throughput phenotyping have recently been captivating the interest of the crop breeding com-munity from both the public and private sectors world-wide.Both approaches promise to revolutionize the prediction of complex traits,including growth,yield and adaptation to stress.Whereas high-throughput phenotyping may help to improve understanding of crop physiology,most powerful techniques for high-throughput field phenotyping are empirical rather than analytical and compa-rable to genomic selection.Despite the fact that the two method-ological approaches represent the extremes of what is understood as the breeding process (phenotype versus genome),they both consider the targeted traits (e.g.grain yield,growth,phenology,plant adaptation to stress) as a black box instead of dissecting them as a set of secondary traits (i.e.physiological) putatively related to the target trait.Both GS and high-throughput phenotyping have in common their empirical approach enabling breeders to use genome profile or phenotype without understanding the underlying biology.This short review discusses the main aspects of both approaches and focuses on the case of genomic selection of maize flowering traits and near-infrared spectroscopy (NIRS) and plant spectral reflectance as high-throughput field phenotyping methods for complex traits such as crop growth and yield.     

Experimental and modelling studies of drought‐adaptive root architectural traits in wheat (Triticum aestivum L.)

Abstract

This paper presents an interdisciplinary approach to crop improvement that links physiology with plant breeding and simulation modelling to enhance the selection of high‐yielding, drought‐tolerant varieties. In a series of field experiments in Queensland, Australia, we found that the yield of CIMMYT wheat line SeriM82 ranged from 6% to 28% greater than the current cultivar Hartog. Physiological studies on the adaptive traits revealed that SeriM82 had a narrower root architecture and extracted more soil moisture, particularly deep in the profile. Results of a simulation analysis of these adaptive root traits with the cropping system model APSIM for a range of rain‐fed environments in southern Queensland indicated a mean relative yield benefit of 14.5% in water‐deficit seasons. Furthermore, each additional millimetre of water extracted during grain filling generated an extra 55 kg ha^?1 of grain yield. Further root studies of a large number of wheat genotypes revealed that wheat root architecture is closely linked to the angle of seminal roots at the seedling stage – a trait which is suitable for large‐scale and cost‐effective screening programmes. Overall, our results suggest that an interdisciplinary approach to crop improvement is likely to enhance the rate of yield improvement in rain‐fed crops.     

试论麦类作物非水力根信号与生活史对策

从植物非水力根信号的生理调节作用和生活史进化解度看,在水分亏缺条件下,自然选择会导致植物产生大量根系以增加对水分的竞争能力,而浅根系则可在干旱来临时,以快速反应的根信号来调节和平衡植株水分状况,度过干旱时期。但是,自然选择压力下的植物特征往往不利于作物籽粒产量这一种群水平上的属性的改善。作物产量的提高过程是一个不断加强的人工选择过程。在作物生产中,作物水环境得到了改善,强大的多年生竞争者基本消失,     

Drought tolerance in rice: morphological and molecular genetic consideration

Rice is one of the most important food crop drastically affected by drought in lowland rice ecosystem. Dissecting out the traits of importance and genomic regions influencing the response of drought tolerance and yield traits on grain yield will aid the breeders to know the genetic mechanism of drought tolerance of rice leads to the development of drought tolerant varieties. Grain yield and its components on drought situation of recombinant inbred population (IR 58821/IR 52561) were investigated under lowland managed stress situation in 2003 and 2004 by given importance to the relative water content. Water deficit resulted in significant effect on phenology and grain yield. Best lines were selected for further varietal development programme. Variability studies showed the traits viz., days to 70% relative water content, leaf rolling, leaf drying, harvest index, biomass yield and grain yield offer high scope for improvement for drought tolerance by way of simple selection technique. Correlation and path analysis indicated that, to harness high yielding combined with drought tolerance breeders should give selection pressure on relative water content, panicle length, grains per panicle, harvest index, biomass yield, root/shoot ratio and root length in positive direction, and low scores of leaf rolling, leaf drying and drought recovery rate. Analysis of quantitative trait loci for drought tolerance, yield and its components allowed the identification of 38 regions associated with both drought tolerant and yield traits. Out of these, 18 were closely linked with DNA markers could be used for marker assisted selection in breeding for drought tolerance in rice. Pleiotropism and G × E effects interaction were noticed in some of the traits. Parent IR 58821 contributed favorable alleles for the entire drought related and most of the yield component traits. Identification of traits of importance and their nature of relationship by morphological and molecular level under lowland condition will be useful to improve drought tolerance of rice.     

Analysis of Maize (Zea mays L.) Seedling Roots with the High-Throughput Image Analysis Tool ARIA (Automatic Root Image Analysis)

The maize root system is crucial for plant establishment as well as water and nutrient uptake. There is substantial genetic and phenotypic variation for root architecture, which gives opportunity for selection. Root traits, however, have not been used as selection criterion mainly due to the difficulty in measuring them, as well as their quantitative mode of inheritance. Seedling root traits offer an opportunity to study multiple individuals and to enable repeated measurements per year as compared to adult root phenotyping. We developed a new software framework to capture various traits from a single image of seedling roots. This framework is based on the mathematical notion of converting images of roots into an equivalent graph. This allows automated querying of multiple traits simply as graph operations. This framework is furthermore extendable to 3D tomography image data. In order to evaluate this tool, a subset of the 384 inbred lines from the Ames panel, for which extensive genotype by sequencing data are available, was investigated. A genome wide association study was applied to this panel for two traits, Total Root Length and Total Surface Area, captured from seedling root images from WinRhizo Pro 9.0 and the current framework (called ARIA) for comparison using 135,311 single nucleotide polymorphism markers. The trait Total Root Length was found to have significant SNPs in similar regions of the genome when analyzed by both programs. This high-throughput trait capture software system allows for large phenotyping experiments and can help to establish relationships between developmental stages between seedling and adult traits in the future.     

Environmental Response and Genomic Regions Correlated with Rice Root Growth and Yield under Drought in the OryzaSNP Panel across Multiple Study Systems

The rapid progress in rice genotyping must be matched by advances in phenotyping. A better understanding of genetic variation in rice for drought response, root traits, and practical methods for studying them are needed. In this study, the OryzaSNP set (20 diverse genotypes that have been genotyped for SNP markers) was phenotyped in a range of field and container studies to study the diversity of rice root growth and response to drought. Of the root traits measured across more than 20 root experiments, root dry weight showed the most stable genotypic performance across studies. The environment (E) component had the strongest effect on yield and root traits. We identified genomic regions correlated with root dry weight, percent deep roots, maximum root depth, and grain yield based on a correlation analysis with the phenotypes and aus, indica, or japonica introgression regions using the SNP data. Two genomic regions were identified as hot spots in which root traits and grain yield were co-located; on chromosome 1 (39.7–40.7 Mb) and on chromosome 8 (20.3–21.9 Mb). Across experiments, the soil type/ growth medium showed more correlations with plant growth than the container dimensions. Although the correlations among studies and genetic co-location of root traits from a range of study systems points to their potential utility to represent responses in field studies, the best correlations were observed when the two setups had some similar properties. Due to the co-location of the identified genomic regions (from introgression block analysis) with QTL for a number of previously reported root and drought traits, these regions are good candidates for detailed characterization to contribute to understanding rice improvement for response to drought. This study also highlights the utility of characterizing a small set of 20 genotypes for root growth, drought response, and related genomic regions.     

Evaluating spatial within plot crop variability for different management practices with an optical sensor?

Subsoil constraints to root growth exacerbate frequent water and nutrient limitations to crop yields in Mediterranean-type environments. Amelioration of subsoil constraints can relieve these limitations by opening root-access to subsoil water and nutrients. However, decisions in subsoil amelioration are hampered by seasonally variable yield responses in these environments. We used the APSIM model to analyse the impact of subsoil constraints on yield and yield variability. The simulated yield data were used to calculate the financial benefits of subsoil amelioration across several scenarios. There was a strong yield-dependence on accessible soil water governed by root depth. Root depth development was limited to a minimum of either the effect of subsoil constraints or the weather-dependent depth of the soil wetting front. Insufficient rainfall in dry years or in a drier region often resulted in shallow soil wetting fronts and correspondingly shallow roots even in the absence of subsoil compaction. In these situations, there is little response to subsoil amelioration. Positive yield responses and positive financial returns to subsoil amelioration are therefore greater in good rainfall years and are more likely in a wetter region. A yield response to amelioration is also greater in coarser textured sand than finer textured sandy loam in an average rainfall season because the same amount of rainfall results in a deeper wetting front in sand than in sandy loam. Hence, roots in a sand are required to grow deeper compared to a sandy loam to access the same amount of water and therefore benefited more from subsoil amelioration in an average rainfall year. In wet years, sands leach more nitrate than sandy loam, which decreases yields and the response to subsoil amelioration in sands is more than in the sandy loam. Environmental threats occur along with yield loss when roots cannot access subsoil water. These include increased nitrate leaching and deep drainage due to unused water remaining in the soil profile. By allowing roots to access deep soil water, ameliorating subsoil is expected to yield financial gains in average to good rainfall seasons and decrease the environmental risk of drainage and leaching loss. The financial gains are expected to offset potential financial losses in dry and dry finish seasons especially in coarser textured soils and wetter environment. Responsible Editor: Jan Vos.     

Varietal differences in shoot and rooting parameters of pearl millet on sandy soils in Niger

Root parameters are important traits for the acquisition of nutrients and water under resource-limited conditions. In order to investigate the extent of varietal differences in rooting parameters in pearl millet, we compared a total of eight pearl millet varieties in two experiments (ridging and traditional sowing) over four years at the ICRISAT Sahelian Centre, Niger. We found substantial genotypic variation for root length density (RLD), root dry matter, and total root length (RL), but not for specific root length, depth of rooting, or partitioning of roots between topsoil and subsoil (>20 cm depth). RL showed a highly dynamic pattern over the growing season. RL and shoot dry matter were positively correlated over P supplies and contrasting levels of field productivity. The relationship between RL and grain yield as well as harvest index were less clear. The root fraction responded to variation in soil productivity, increasing from roughly 20% under high productivity to more than 40% at low productivity, but we found no evidence of varietal differences in this trait despite pronounced differences in maturity and plant stature. Identification of pearl millet varieties suited for growth under low input conditions in south-west Niger can potentially make use of existing genotypic variability in root parameters, but we suggest that, due to high variability for root traits, indirect selection for shoot parameters (e.g., number of stems) is more promising than direct selection for RLD or RL.     

Deciphering genetic basis of developmental and agronomic traits by integrating high-throughput optical phenotyping and genome-wide association studies in wheat

Dissecting the genetic basis of complex traits such as dynamic growth and yield potential is a major challenge in crops. Monitoring the growth throughout growing season in a large wheat population to uncover the temporal genetic controls for plant growth and yield-related traits has so far not been explored. In this study, a diverse wheat panel composed of 288 lines was monitored by a non-invasive and high-throughput phenotyping platform to collect growth traits from seedling to grain filling stage and their relationship with yield-related traits was further explored. Whole genome re-sequencing of the panel provided 12.64 million markers for a high-resolution genome-wide association analysis using 190 image-based traits and 17 agronomic traits. A total of 8327 marker-trait associations were detected and clustered into 1605 quantitative trait loci (QTLs) including a number of known genes or QTLs. We identified 277 pleiotropic QTLs controlling multiple traits at different growth stages which revealed temporal dynamics of QTLs action on plant development and yield production in wheat. A candidate gene related to plant growth that was detected by image traits was further validated. Particularly, our study demonstrated that the yield-related traits are largely predictable using models developed based on i-traits and provide possibility for high-throughput early selection, thus to accelerate breeding process. Our study explored the genetic architecture of growth and yield-related traits by combining high-throughput phenotyping and genotyping, which further unravelled the complex and stage-specific contributions of genetic loci to optimize growth and yield in wheat.     

Early vertical distribution of roots and its association with drought tolerance in tropical maize

Background and aims

Selection for deep roots to improve drought tolerance of maize (Zea mays L.) requires presence of genetic variation and suitable screening methods.

Methods

We examined a diverse set of 33 tropical maize inbred lines that were grown in growth columns in the greenhouse up to the 2-, 4-, and 6-leaf stage and in the field in Mexico. To determine length of roots from different depths at high throughput, we tested an approach based on staining roots with methylene blue and measuring the amount of absorbed dye as proxy measure for root length.

Results

Staining provided no advantage over root weights that are much easier to measure and therefore preferable. We found significant genotypic variation for all traits at the 6-leaf stage. For development rates between the 2-leaf and the 6-leaf stage, genotypes only differed for rooting depth and the number of crown roots. Positive correlations of leaf area with root length and rooting depth indicated a common effect of plant vigor. However, leaf area in growth columns was negatively related to grain yield under drought (r?=??0.50).

Conclusion

The selection for deeper roots by an increase in plant vigor likely results in a poorer performance under drought conditions. The proportion of deep roots was independent of other traits but showed a low heritability and was not correlated to field performance. An improved screening protocol is proposed to increase throughput and heritability for this trait.

     

Identification of QTLs associated with agronomic performance under nitrogen-deficient conditions using chromosome segment substitution lines of a wild rice relative, <Emphasis Type="Italic">Oryza rufipogon</Emphasis>

Improved root system architecture can enhance agronomic performance by increasing water and nitrogen (N) acquisition efficiency. However, little is known about interaction between root system architecture and agronomic performance under field environments. To gain a better understanding about the genetic basis of these relationships, we evaluated a set of chromosome segment substitution lines (CSSLs) derived from crosses between a tropical japonica rice cultivar ‘Curinga’ and a wild species Oryza rufipogon accession IRGC105491. Root system architectural traits were investigated using the CSSLs at 40 days old seedlings using the root basket method under hydroponic conditions, and agronomic performances were also tested under field conditions with different N treatments. Agronomic performances were computed as the ratio of a trait value under low to high N treatments, including grain yield and biomass yield as nitrogen-deficiency tolerance (NDT) traits. Root architecture and NDT trait QTLs were mapped using 238 SNP marker loci. A total of 13 QTLs for root system architectural, NDT and morpho-physiological traits were identified on chromosomes 1, 3, 4, 5, 7, 8, 9, 10 and 12. Interestingly, a QTL for deeper root number was identified the region of SNP markers between id1012330 and id1021697 on chromosome 1 under hydroponic conditions overlapped with a QTL for NDT trait of relative grain yield (qRGY1). These results suggest that deeper root trait is helpful to maintain grain yield under nitrogen-deficient conditions. The QTL associated root architecture could potentially be used in future rice-breeding efforts to increase agronomic performance under nitrogen-deficient conditions.     

不同抗旱性花生品种的根系形态发育及其对干旱胁迫的响应

为明确不同抗旱性花生品种的根系形态发育特征,探讨其根系形态发育特征对不同土壤水分状况的响应机制,在防雨棚旱池内进行土柱栽培试验,研究抗旱型品种“花育22号”、“唐科8号”和干旱敏感型品种“花育23号”3个不同抗旱性花生品种根系形态发育特征及其对干旱胁迫的响应.结果表明:抗旱型品种根系较发达,具有较大的根系生物量、总根长、总根系表面积.干旱胁迫使抗旱型品种根系总表面积和体积增加,而干旱敏感型品种则相反.干旱胁迫显著增加抗旱型品种“花育22号”20 cm以下土层内根长密度分布比例及根系表面积和体积,但“唐科8号”相应根系性状仅在20-40 cm土层内增加;干旱胁迫使干旱敏感型品种“花育23号”40 cm以下土层内各根系性状升高,但未达显著水平且其深层土壤内各根系性状增加幅度小于“花育22号”.花生根系总长、总表面积及0-20 cm土层内根系性状与产量间呈显著或极显著正相关.土壤水分亏缺条件下,花生主要通过增加深层土壤内根长、根系表面积和体积等形态特性,优化空间分布构型,以调节植株对水分的利用.     

Quantitative trait loci (QTLs) for water use and crop production traits co-locate with major QTL for tolerance to water deficit in a fine-mapping population of pearl millet (<Emphasis Type="Italic">Pennisetum glaucum</Emphasis> L. R.Br.)

Key message

Four genetic regions associated with water use traits, measured at different levels of plant organization, and with agronomic traits were identified within a previously reported region for terminal water deficit adaptation on linkage group 2. Close linkages between these traits showed the value of phenotyping both for agronomic and secondary traits to better understand plant productive processes.

Abstract

Water saving traits are critical for water stress adaptation of pearl millet, whereas maximizing water use is key to the absence of stress. This research aimed at demonstrating the close relationship between traits measured at different levels of plant organization, some putatively involved in water stress adaptation, and those responsible for agronomic performance. A fine-mapping population of pearl millet, segregating for a previously identified quantitative trait locus (QTL) for adaptation to terminal drought stress on LG02, was phenotyped for traits at different levels of plant organization in different experimental environments (pot culture, high-throughput phenotyping platform, lysimeters, and field). The linkages among traits across the experimental systems were analysed using principal component analysis and QTL co-localization approach. Four regions within the LG02-QTL were found and revealed substantial co-mapping of water use and agronomic traits. These regions, identified across experimental systems, provided genetic evidence of the tight linkages between traits phenotyped at a lower level of plant organization and agronomic traits assessed in the field, therefore deepening our understanding of complex traits and then benefiting both geneticists and breeders. In short: (1) under no/mild stress conditions, increasing biomass and tiller production increased water use and eventually yield; (2) under severe stress conditions, water savings at vegetative stage, from lower plant vigour and fewer tillers in that population, led to more water available during grain filling, expression of stay-green phenotypes, and higher yield.

     

Inheritance of deeper root length and grain yield in half-diallel durum wheat (Triticum durum) crosses

Inheritance, heterosis and combining ability of deeper root length (DRL) and grain yield (GY) were investigated in durum wheat populations obtained from half‐diallel crossings among five parental lines differing in their DRL and GY. The study was conducted with the final objective of identifying parent lines to be used in a breeding programme to develop drought‐tolerant wheat varieties. General combining ability and specific combining ability effects were significant for both traits; however, additive gene effects were predominant over non‐additive effects. Partial dominance was ambidirectional for DRL and unidirectional for GY. Lines INRAT69 and Omrabia conferred DRL whereas Omrabia and Khiar transmitted high GY to their respective progenies. In the studied material, both characters were controlled mainly by dominant alleles, but they could also be attributed to recessive alleles although less frequently. Both broad‐sense and strict‐sense heritabilities were high for DRL, confirming the importance of additive gene effects, whereas strict‐sense heritability for GY was average, indicating the importance of interaction effects as compared with the additive effects; this could mean reduced selection efficiency for the latter trait. Thus, the expected genetic progress per cycle of selection will be lower for GY compared with DRL. Omrabia should be included in the breeding programme as a parent so that while maintaining high GY, resulting progeny should be better able to resist drought through DRL.     

Optimization modeling of plant root architecture for water and phosphorus acquisition

An optimization model is presented that examines the relationship between root architecture and multiple resource acquisition, specifically water and phosphorus in spatially heterogeneous environments. The basal root growth angle of an individual common bean plant, which determines the orientation and localization of the bulk of the root system, was modeled as the decision variable. The total payoff to the plant, the benefit obtained from water and phosphorus acquisition, minus the costs of spatial competition between roots, is given as a function of the (x,y) coordinates of the basal root in two-dimensional Cartesian space. We obtained a general solution and applied it to four unique environmental cases which are as follows: (1) the case of uniformly distributed water and phosphorus; (2) the case of localized shallow phosphorus; (3) the case of localized deep water; and (4) the case of shallow phosphorus and deep water. The general solution states that the optimal basal root growth angle will occur at the point where the total rate of change in the value of the resources acquired equals the total rate of change in cost that results from locating the root deeper in the soil. An optimizing plant locates its roots deeper in the soil profile until the marginal benefit exactly equals the marginal cost. The model predicts that the basal root angle of an optimizing plant will be shallower for Case 2 and deeper for Case 3, relative to the basal root angle obtained in the case of uniformly distributed water and phosphorus. The optimal basal root angle for Case 4 will depend on the marginal rate of substitution of water availability for phosphorus availability that occurs with depth. Empirical observations of bean root architecture in the greenhouse and in the field confirm model results and are discussed. In addition, the potential importance of phenotypic plasticity and phenotypic variation are discussed in relation to optimization of traits and adaptation to spatially heterogeneous environments.     

Exploitation of yield stability in barley

Key message

Analyses of registration trials of winter barley suggested that yield and yield stability can be enhanced by developing hybrid instead of line varieties.

Abstract

Yield stability is central to cope with the expected increased frequency of extreme weather conditions. The objectives of our study were to (1) examine the dimensioning of field trials needed to precisely portray yield stability of individual winter barley (Hordeum vulgare L.) genotypes, (2) compare grain yield performance and yield stability of two-rowed lines with those of six-rowed lines and hybrids, and (3) investigate the association of various agronomic traits with yield stability. Static and dynamic yield stability as well as grain yield performance was determined in five series of 3-year registration trials of winter barley in Germany. Each series included 4 or 5 six-rowed hybrids, 40–46 six-rowed inbred lines, as well as 42–49 two-rowed inbred lines. The genotypes were evaluated in 10–45 environments, i.e. year-by-location combinations. We found that precise assessment of yield stability of individual genotypes requires phenotyping in at least 40 test environments. Therefore, selection for yield stability is not usually feasible since the required number of test environments exceeds the common capacity of barley breeding programs. Also, indirect improvement of yield stability by means of agronomic traits seemed not possible since there was no constant association of any agronomic trait with yield stability. We found that compared with line varieties, hybrids showed on average higher grain yield performance combined with high dynamic yield stability. In conclusion, breeding hybrid instead of line varieties may be a promising way to develop high yielding and yield stable varieties.     

Determining the Causes of Spatial and Temporal Variability of Wheat Yields at Sub-field Scale Using a New Method of Upscaling a Crop Model

Wheat yields in the Mediterranean climate of Western and Southern Australia are often limited by water. Our measurements on a 70 ha growers field showed linear relationships between grain yield and the plant available soil water storage capacity (PAWc) of the top 100 cm of the soil profile. PAWc was linearly related to apparent soil electrical conductivity measured by proximal sensing using electromagnetic induction (EM38). The APSIM wheat model also employs PAWc as one of the systems parameters and simulated linear relationships between PAWc and yield. These relationships were used to transform an EM38-derived PAWc map of the field into yield maps for three major season types (dry, medium and wet) and nitrogen (N) fertiliser management scenarios. The results indicated that the main cause of temporal and spatial yield variability within the field was due to interactions of seasonal rainfall, PAWc and N fertiliser applications. Spatial variability was low in low rainfall years when yields across the field were low and the higher soil water storage capacity sites were often underutilised. With adequate N, spatial variability increased with seasonal rainfall as sites with higher PAWc conserved more water in wet seasons to give higher yield response than sites with low PAWc. The higher yield response of high PAWc sites to rainfall gave rise to larger temporal variability compared with sites with low PAWc. Provision of adequate N is required for the water limited yield potential to be expressed and this increased both spatial and temporal variability. Sites with low PAWc performed poorly irrespective of rainfall and N application. PAWc is inherently low on deep coarse sands; these sites should be considered for a change in land use. Elsewhere, strategic management interventions should aim to improve PAWc through sub-soil amelioration and deep root growth to increase the capital asset of the farm. The resulting increase in yields will occur in favourable seasons and with adequate fertiliser provisions. The largest grain yield response to water and N will be obtained on sites with the highest PAWc and it is at those sites that the greatest profits from fertiliser use could be achieved in wet seasons.     

半干旱区春小麦不同年代品种根系生长冗余的比较实验研究

 研究了半干旱区几个春小麦品种的籽实产量、根量与根茎比的关系。表明在开花期地方品种和尚头较之现代品种有更大的根量和根茎比。产量因降水条件而不同：极端干旱的1995年,大根系品种与现代品种产量无显著差异(p>0.05);在降水分配较为均匀的1996年,无论有、无灌溉条件,具较大根系的地方品种与现代品种的产量均最低(p<0.05),而根量与品种产量及地上生物量呈显著的负相关。表明大根系品种在根系上存在着冗余,减少根系冗余可望成为半干旱区小麦高产育种的一条有效途径。     

Accuracy of genomic selection in European maize elite breeding populations

Genomic selection is a promising breeding strategy for rapid improvement of complex traits. The objective of our study was to investigate the prediction accuracy of genomic breeding values through cross validation. The study was based on experimental data of six segregating populations from a half-diallel mating design with 788 testcross progenies from an elite maize breeding program. The plants were intensively phenotyped in multi-location field trials and fingerprinted with 960 SNP markers. We used random regression best linear unbiased prediction in combination with fivefold cross validation. The prediction accuracy across populations was higher for grain moisture (0.90) than for grain yield (0.58). The accuracy of genomic selection realized for grain yield corresponds to the precision of phenotyping at unreplicated field trials in 3–4 locations. As for maize up to three generations are feasible per year, selection gain per unit time is high and, consequently, genomic selection holds great promise for maize breeding programs.     

黄土高原地区春小麦对有限灌溉的反应及其生理生态基础

从对黄土高原地区有限灌溉条件下作物生理生态反应的众多研究中得出：（1）水分轻度亏缺时,作物可通过根信号物质ABA调节叶片的气孔导度。非水力根信号作用太强,可因降低光合作用而减少干物质生产和影响干物质分配模式而影响产量和水分利用效率,故削弱非水力根信号的作用将有利于提高产量。（2）浅层根系占根系总量比值越高,对干旱越敏感,表现为根信号能力增强;深层根系所占比例越高,越有利于土壤深层水分利用,并可削弱根信号,同理,给土壤中下层补水或采用播种前灌溉,可因为减少了无效蒸发,且削弱根信号而提高水分利用率。（3）本地区有限灌溉的最佳时期由于降水变率较高而变得较为复杂,不同降水年型,最佳灌溉时期差异很大,对有限灌溉进行科学管理还需要做更多的研究工作。