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How does a trait measured in an individual plant relate to the trait in a crop stand, where yield and other agronomic attributes are resolved? Seeding rate effects on tillering, grain yield, and yield components of oat at high latitude. Scaling, yield, density-dependence, phenotype, context-dependence. [Extracted from the article]

Oats likely emerged as part of the weedy grass assemblage in early wheat and barley crops. Some Avena species, such as A. fatua and A. sterilis, evolved into aggressive weeds, and the high interspecific competitive ability of cultivated oats (A. sativa) is valued agronomically to facilitate weed control in rotations. We tested the hypothesis, verified in many crops, that high yield of oats is related to low intraspecific competitive ability. Ten contemporary oat varieties, selected for grain yield and milling attributes, where grown in three environments. Response to competition was calculated as 100 x (Yb - Yc)/Yc, where Y is yield measured in border (b) and centre (c) rows. The same definition was used to calculate response to competition of yield components (biomass, harvest index, grain number, grain weight) and the components of grain number (panicle number and grains per panicle). Yield response to competition was affected by all three sources of variation, i.e. environment, variety and variety x environment interaction. The interaction demonstrates the plasticity of yield response to intraspecific competition; for example, the response to competition of variety Mortlock varied from 9% to 71% among environments. This plasticity in yield response to competition was partially related to variety-dependent responses to competition for biomass and harvest index, number of panicles and number of grains per panicle. We did not find the expected negative association between yield and variety-dependent response to competition. We discuss how this lack of correlation could relate to sampling issues, i.e. a limitation in the range of environments and varieties explored in this study, or reflect a legitimate feature of oat crops arising from early and contemporary selective pressures. [ABSTRACT FROM AUTHOR]

Robust associations between yield and crop growth rate in a species-specific critical developmental window have been demonstrated in many crops. In this study we focus on genotype-driven variation in crop growth rate and its association with chickpea yield under drought. We measured crop growth rate using Normalised Difference Vegetative Index (NDVI) in 20 diverse chickpea lines, after calibration of NDVI against biomass accounting for morphological differences between Kabuli and Desi types. Crops were grown in eight environments resulting from the combination of seasons, sowing dates and water supply, returning a yield range from 152 to 366 g m −2 . For both sources of variation – environment and genotype – yield correlated with crop growth rate in the window 300 °Cd before flowering to 200 °Cd after flowering. In the range of crop growth rate from 0.07 to 0.91 g m −2 °Cd −1 , the relationship was linear with zero intercept, as with other indeterminate grain legumes. Genotype-driven associations between yield and crop growth rate were stronger under water stress than under favourable conditions. Despite this general trend, lines were identified with high crop growth rate in both favourable and stress conditions. We demonstrate that calibrated NDVI is a rapid, inexpensive screening tool to capture a physiologically meaningful link between yield and crop growth rate in chickpea. [ABSTRACT FROM AUTHOR]

Natural selection does not lead to optimal solutions due to trade-offs and environmental variation, genetic and developmental constraints, and historical contingency. In this paper we propose that constraints like these also often apply to the improvement of both crop varieties and management practices, creating a dual biological and agronomic barrier for the optimisation of crops. We discuss constraints on optimisation of 1) crop ancestors, by natural selection, 2) crop traits, by artificial selection and biotechnology, and 3) crop management. We outline how trade-offs and environmental variation make single-factor optimisation (e.g. “optimum leaf angle” or “optimal fertiliser rate”) impossible. Definitions of “optimal” that recognise trade-offs and variability can help, but we argue there are major constraints on even those forms of optimality. Optimality theory may be useful to formulate null hypotheses, however, as divergence between actual traits and theoretical optima can highlight constraints that are biologically interesting and agronomically relevant. Understanding the nature and size of these constraints can help us map more likely pathways for future improvements in agriculture. [ABSTRACT FROM AUTHOR]

Owing to the high contents of oil and protein, soybean [Glycine max L. (Merr.)] seeds are energy-rich. Genetic improvement of soybean yield is linked to the dilution in seed protein; changes in low-energy compounds such as carbohydrates have received less attention. We tested the hypothesis that selection for yield in soybean has shifted seed composition in favor of low-energy compounds. We characterized the dynamics of three seed fractions (protein, oil, and residual) during seed filling in seven commercial soybean varieties released between 1980 and 2013 in a two-year field experiment. Genetic gain in yield was linked with reduced seed energy cost: seed protein concentration decreased by 0.06% yr−1, oil remained stable, and the residual fraction increased by 0.09% yr−1. Seed accumulation of the residual fraction revealed a shorter lag phase and a longer effective filling period for the modern relative to the older soybean varieties. Seed energetic requirement significantly decreased − 0.06% yr−1. Resource availability per seed during the seed-filling period was not limiting across all tested varieties. Further research is warranted on the biochemical nature of the residual fraction, and on the implications of seed composition for future breeding, soybean meal and end uses. [ABSTRACT FROM AUTHOR]

The nitrogen economy of the crop is a critical driver of biomass and grain production, and its importance is reflected in a large, worldwide research effort to link nitrogen, growth and yield. Particular research questions require measurement of specific traits, hence the need to quantify multiple, often complementary traits including crop nitrogen uptake, nitrogen use efficiency and its components, nitrogen concentration in the crop and its parts, down to relevant enzymes (e.g. nitrate reductase) and other products of gene expression. Nitrogen uptake, however, is co-regulated by both soil nitrogen availability and crop biomass accumulation; hence, crop nitrogen uptake or shoot nitrogen concentration reflect univocally crop nitrogen status only if comparisons are made at similar biomass. Although the allometric relationships between biomass and nitrogen uptake have been established for over two decades, many studies still report results in terms of nominal treatments, e.g. high vs low nitrogen, which are uninformative; curves relating yield and fertiliser rate, which are of local interest but provide little insight on the underlying processes and have low generic value; and nitrogen-related traits that are incomplete or inadequate to quantify crop nutrition status. Often, the allometric relationships between nitrogen and biomass are overlooked. In this opinion paper, we summarise the already well established concepts of dilution curves and nitrogen nutrition index, outline the standard partitioning of nitrogen use efficiency, and highlight the confounded effects in nitrogen use efficiency when the allometric relationship between nitrogen uptake and biomass is ignored. A sample of recent papers is used to survey the most common approaches to characterise nitrogen related traits. We illustrate the application of dilution curves and nitrogen nutrition index in the assessment and interpretation of crop responses to agronomic practices and comparisons of wheat cultivars and maize hybrids. [ABSTRACT FROM AUTHOR]

Abstract: In a worldwide context of agricultural intensification, cropping systems in Mediterranean-type environments have been reducing the frequency of long-fallow in a shift to continuous cropping. The focus of this paper is the short summer fallow between successive winter grain crops in environments of South Australia with winter-dominant rainfall. Our aims were to (i) estimate wheat yield response to summer rainfall, (ii) explain yield responses in terms of capture and efficiency in the use of radiation and water, (iii) explore the incidence of the amount and disposition of stubble on storage of summer rainfall, and (iv) assess the interaction between summer rainfall and nitrogen supply. We used trickle irrigation to manipulate summer water supply in two factorial experiments combining water and stubble treatments, and three factorials combining water and nitrogen supply. Addition of 50–100mm of water increased soil water content at sowing between 8 and 46mm compared to controls that only received the background summer rainfall (10–74mm). Yield gain from additional water input in summer declined from 1.1tha−1 to zero when the yield of controls increased from 2.0 to 7.8tha−1. Where yield response was related to a single resource, water or nitrogen, capture of radiation and water were major drivers of growth and yield response. Where yield response was related to the interaction between water and nitrogen, both capture and efficiency in the use of resources were important. Amount (0–5tha−1) and disposition of stubble (standing or flat) did not affect the amount or distribution of water in soils, and had no effect on grain yield. High nitrogen rate was critical to capture the benefits of additional summer water and reciprocally high water supply was required to capture the benefits of nitrogen fertilisation; this highlights the resource co-limitation for wheat production in these environments. In a water×nitrogen factorial, crops with either low nitrogen or no additional summer water supply had radiation use efficiency ∼1.6gMJ−1, biomass per unit evapotranspiration ∼33kgha−1 mm−1 and yield per unit evapotranspiration ∼15kgha−1 which increased to 1.9gMJ−1, 40kgha−1 mm−1 and 18kgha−1 mm−1 in crops with both additional water and high nitrogen. Across experiments and treatments, grain number accounted for 88% of the variation in yield. Grain number was proportional to crop growth rate between stem elongation and anthesis; crops with high nitrogen produced 116±5.0 grains per unit crop growth rate and their low nitrogen counterparts 99±4.6. Building evidence from Mediterranean environments worldwide highlights the importance of grain number as the main source of variation in yield and therefore the critical period between stem elongation and anthesis where both water and nitrogen supply are critical. [Copyright &y& Elsevier]

Abstract: A data base including grain yield, nitrogen uptake (N u) and phosphorus uptake (P u) was compiled to investigate the N:P stoichiometry of cereal (n =759), grain legume (n =413) and oilseed species (n =421). Actual ratios and slopes of functions relating nitrogen uptake and phosphorus uptake were used to characterise N:P stoichiometry. The focus is on variability in N:P ratios of field crops, and N:P stoichiometry of crops achieving maximum yield. N:P ratios varied between ∼20, the maximum for legumes and oilseed crops, and ∼1.5, the minimum for cereals. By definition, N:P ratios are a direct function of N uptake and an inverse function of P uptake. The expected association between N:P ratio and N uptake was not evident except for grain legumes whereas the expected relationship between N:P and P uptake was verified for all three crop types. This highlights the role of P uptake as the main source of variability in N:P stoichiometry. The relationship between N uptake (N u) and P uptake (P u) for crops achieving maximum yield in each experiment was markedly non-linear. Functions of the form N u = N max P u/( + P u) described the way in which N:P stoichiometry scaled with grain yield. Maximum uptake of nitrogen (N max) was similar for all three crop types (≈240kgNha−1), but it was achieved via different avenues, i.e. high yield and low grain protein concentration for cereals compared to lower yield and higher grain protein concentration in oilseed and legume crops. Phosphorus uptake at half N max () ranged from 27kgPha−1 for oilseeds to 14kgPha−1 for legumes. The N:P ratio at was 4.5 for oilseed crops, 5.6 for cereals and 8.7 for legumes. For cereals and oilseeds, over 40% of crops attaining maximum yield had N:P ratios in a relatively narrow range between 4 and 6. Variation in grain protein concentration was a major source of instability in N:P ratios of legumes. Being at the base of agro-ecological food webs, the N:P stoichiometry of crops has implications for both decomposers and consumers, and stoichiometric concepts might be of interest in fertiliser management and modelling. Variability in N:P stoichiometry related to plant storage products, however, restricts its application. [Copyright &y& Elsevier]

• Modern maize hybrids sown at low densities tiller in non-restrictive environments. • Light quality and plant growth rate at early stages govern tiller emission. • Tiller fertility would be determined by shoot growth rate around flowering. • Reproductive plasticity is needed to stabilize yields in ultra-low density settings. • The impact of tillering on crop water and nitrogen economies is hypothesized. Selection for maize (Zea mays L.) grain yield in high-yielding environments at high population densities has favored a compact phenotype tolerant to crowding stress, bearing a single, well-grained ear. However, by contributing to vegetative and reproductive plasticity (i.e., multiple shoots and ears per plant, respectively), tillering may be adaptive in environments with low and variable availability of resources, chiefly water and nutrients, where crops are sown at low plant population density. In this work we present a robust, new conceptual framework for vegetative and reproductive plasticity in maize with direct agronomic applications, combining original data from new experiments and data reviewed from the literature. First, we describe production systems where tillering in maize would be relevant in terms of grain yield. Next, we discuss possible masked effects of genetic selection at high plant densities on tillering and present novel experimental results showing genotypic variation of tillering in modern maize hybrids and genotype x environment x management effects (plant density x location x sowing date) on tillering expression. We follow with a two-part framework to analyze tillering and prolificacy. In the first part (from axillary buds to tillers), we integrate the early effects of the light environment (through photomorphogenesis) and carbon balance on tillering emission, and discuss the environmental factors (temperature, photoperiod, radiation, water, nitrogen) that modulate tiller emission and tiller growth. In the second part (from tillers to kernels), we summarize the functional relationships governing kernel set on the ears of main shoot (apical and sub-apical ears) and tillers, focusing on the growth rate of shoot cohorts, rather than the whole plant. We then provide examples of the diverse patterns of contribution of multiple shoots to crop grain yield for maize husbandry in low-yielding environments. Finally, we address the effect of tillering on resource capture and use efficiency of maize crops by discussing its relationship with biomass and grain yield and provide supportive experimental data. We conclude with identification of knowledge gaps leading to testable hypotheses. [ABSTRACT FROM AUTHOR]

• Excel x Kaspa RILs screened in dry, hot environment (yield 149 g m-2) and a wet, mild environment (477 g m-2). • Ranking of RILs was consistent between environments for pod wall ratio, the weight ratio between the pod wall and the whole pod including seeds. • A QTL for flowering time on chromosome 2 co-located with a QTL for a number of yield related traits including pod wall ratio. Trait-based selection to improve stress adaptation can complement direct selection for yield provided the trait meets six criteria: (1) it must be genetically correlated with yield in the target environments; (2) it should be less affected by the GxE interaction than yield; (3) it should not be associated with low yield in favourable conditions; (4) it must show genetic variation; (5) it must be genetically stable, persistent across generations and relevant in different genetic backgrounds, and (6) it must lend itself to rapid, cost-effective and reliable quantification. Against these criteria, this study focused on pod wall ratio, defined as the weight ratio between the pod wall and the whole pod including seeds. We explored the phenotypic and genetic associations of this trait with yield components in a field pea population of 102 recombinant inbred lines (RILs) from a cross between Excel and Kaspa grown in two environments: Roseworthy 2015 (average yield 149 g m−2) and Turretfield 2016 (477 g m−2). All three sources of variation, environment, line and their interaction, affected yield (all P < 0.0001). The ranking of RILs for yield varied between environments, reflecting the strong G x E. The ranking of RILs for pod wall ratio was maintained between environments. Broad-sense heritability was higher for pod wall ratio (≥ 0.83) than for yield (≤ 0.59). There was a narrow range of pod wall ratio, 0.07 to 0.25 at Roseworthy 2015 compared to 0.09 to 0.52 in Turretfield 2016. Pod wall ratio correlated with seeds per pod and harvest index in Roseworthy 2015, and with yield, seeds per pod and harvest index in Turretfield 2016. A QTL for flowering time on linkage group 2 co-located with a QTL for a number of traits including pod wall ratio, days to pod emergence, seeds per pod, seeds per m2, shoot biomass at harvest and grain yield. These markers are potentially valuable for breeding after validation in independent populations. [ABSTRACT FROM AUTHOR]

Winter crops are the backbone of Australian agriculture. This study reports the first comparative analysis of the impact of temperature and water stress on yields of wheat, barley, canola, chickpea and field pea across four major production zones in Australia (North, East, South and West) using the 2009-2013 National Variety Trials (NVT). Developmental windows of 100 °Cd centred at flowering were used to sample rainfall, vapour pressure deficit (VPD), potential evapotranspiration, water supply/demand ratio, average minimum (Tmin) and maximum (Tmax) temperature, number of days below 0 °C and above 30 °C, incident radiation, photothermal quotient (PTQ) and PTQ corrected by VPD. Flowering was estimated for a mid-season cultivar for each trial using a simulation model (APSIM). There was a consistent negative association between Tmax and yields in all crops, from early in the season in the South and after flowering in the West. Our study supports that high temperature in the non-stressful range is associated with yield reduction with crop specific effects. Days exceeding 30 °C were unlikely before flowering; wheat and chickpea were sensitive to temperatures above 30 °C from early and late in grain filling respectively. Chickpea was sensitive to low temperatures from flowering. Canola was overall the most sensitive to water stress. Unequivocally, the interaction between temperature and water stress exhibited strong regional differences. In the West, with Mediterranean rainfall pattern, high Tmin before flowering was associated with higher yields in wheat, barley, canola and chickpea, indicating a role in promoting early growth and water use and reducing evaporation. In the North, crops depend on initial soil moisture, and high yields were associated with lower Tmin, likely slowing growth and early water use and lessening terminal stress. These relationships need direct experimental confirmation to show causality but there is room for large scale studies to uncover seasonal and regional patterns and highlight targets for research, breeding and management options aimed at improving yield under climate change. [ABSTRACT FROM AUTHOR]

Donald’s ideotype and empirical evidence in cereal and oilseed crops indicate high yield is associated with less competitive plants. In this study we grew 20 chickpea lines in six environments to investigate the association between yield and intra-specific competitive ability and its genetic underpinnings using Fst genome scan based on whole genome resequencing data. We measured yield and its components and calculated response to competition (RC) as the ratio between the trait in outer rows (relaxed competition) and the trait in inner rows (higher competition). Crop yield correlated negatively with RC for yield, biomass, harvest index, seed number, and pod number. Fst genome scan revealed 14 genomic regions under selection for response to competition of yield, seed number or biomass, and 6 genomic regions under selection for yield in inner or outer canopy rows. Candidate genes in these regions include members of the nitrate-transporter 1 family, patatin and hormone-related genes. The top genomic regions found to be under selection for yield in inner rows and outer rows did not coincide. This genetic architecture provides a mechanistic basis for the observation that phenotypes that are adequate for relaxed competition often perform poorly in dense stands. [ABSTRACT FROM AUTHOR]

• We measure yield and protein of shaded lentil in southern Australia and Chile. • The most sensitive period was pod emergence, between 50–126 °Cd after flowering. • Grain number and biomass accounted for most of the variation in yield. • Grain protein concentration varied according to a bi-linear model. • Protein yield tracks grain yield and is partially buffered by late season protein. Lentil is a cool season grain legume produced primarily in the Mediterranean and temperate regions of the world where water and heat stress in critical developmental windows constrain yield. The effect of stress on yield depends on the timing, intensity and duration of stress; here we focus on timing relative to phenological development. To determine the critical period for grain yield and grain protein, we compared unshaded controls and crops exposed to sequential 10–14 d shading periods using two locally adapted varieties at Roseworthy, south-eastern Australia, and Valdivia, southern Chile. Yield of unstressed controls varied from 1 t ha−1 at Roseworthy to 7 t ha-1 at Valdivia, and grain protein from 22.0 % at Valdivia to 27.6 % at Roseworthy. Irrespective of growing conditions, the most sensitive period was at 50–126 °Cd after flowering, around pod emergence. Grain number and biomass accounted for most of the variation in yield. Grain protein concentration varied according to a bi-linear model, with minor reduction (<10 %) before a developmental threshold (i.e. flowering at Valdivia, ∼ 200 °Cd after flowering at Roseworthy) and a linear increase after the threshold. Protein yield tracked grain yield, but the increased grain protein concentration partially buffered the effect of post-flowering stress. [ABSTRACT FROM AUTHOR]