Inter-virus relationships in mixed infections and virus-drought relationships in plants: a quantitative review.

Inter-virus relationships in mixed infections and virus-drought relationships are important in agriculture and natural vegetation. In this quantitative review, we sampled published factorial experiments to probe for relationships against the null hypothesis of additivity. Our sample captured antagonistic, additive and synergistic inter-virus relationships in double infections. Virus-drought relationships in our sample were additive or antagonistic, reinforcing the notion that viruses have neutral or positive effects on droughted plants, or that drought enhances plant tolerance to viruses. Both inter-virus and virus-drought relationships vary with virus species, host plant to the level of cultivar or accession, timing of infection, plant age and trait and growing conditions. The trait-dependence of these relationships has implications for resource allocation in plants. Owing to lagging theories, more experimental research in these fields is bound to return phenomenological outcomes. Theoretical work can advance in two complementary directions. First, the effective theory models the behaviour of the system without specifying all the underlying causes that lead to system state change. Second, mechanistic theory based on a nuanced view of the plant phenotype that explicitly considers downward causation; the influence of the plant phenotype on inter-virus relations and vice versa; the impact of timing, intensity and duration of drought interacting with viruses to modulate the plant phenotype; both the soil (moisture) and atmospheric (vapour pressure deficit) aspects of drought. Theories should scale in time, from short term to full growing season, and in levels of organisation up to the relevant traits: crop yield in agriculture and fitness in nature.

Genetics of phenological development and implications for seed yield in lentil.

Understanding phenology, its genetics and agronomic consequences, is critical for crop adaptation. Here we aim to (i) characterize lentil response to photoperiod with a focus on five loci: the lentil ELF3 orthologue Sn, two loci linked to clusters of lentil FT orthologues, and two loci without candidates in chromosomes 2 and 5 (Experiment 1: 36 lines, short and long days in a phytotron), and (ii) establish the phenology-yield relationship (Experiment 2: 25 lines, 11 field environments). A vintage perspective, where we quantify time trends in phenotype over three decades of breeding, links both experiments. Yield increased linearly from older to newer varieties at 29 kg ha-1 year-1 or 1.5% year-1, correlated negatively with flowering time in both winter- and summer-rainfall regimes, and decoupled from biomass in favourable environments. Time to flowering shortened from older to newer varieties at -0.56% year-1 in the field, and -0.42% year-1 (short days) and -0.99% year-1 (long days) in the phytotron. Early-flowering lines of diverse origin carried multiple early alleles for the five loci, indicating that at least some of these loci affect phenology additively. Current germplasm primarily features the early-flowering haplotype for an FTb cluster region, hence the potential to increase phenological diversity with yield implications.

Wheat Oxylipins in Response to Aphids, CO2 and Nitrogen Regimes.

Wheat is critical for food security, and is challenged by biotic stresses, chiefly aphids and the viruses they transmit. The objective of this study was to determine whether aphids feeding on wheat could trigger a defensive plant reaction to oxidative stress that involved plant oxylipins. Plants were grown in chambers with a factorial combination of two nitrogen rates (100% N vs. 20% N in Hoagland solution), and two concentrations of CO2 (400 vs. 700 ppm). The seedlings were challenged with Rhopalosiphum padi or Sitobion avenae for 8 h. Wheat leaves produced phytoprostanes (PhytoPs) of the F1 series, and three types of phytofurans (PhytoFs): ent-16(RS)-13-epi-ST-Δ14-9-PhytoF, ent-16(RS)-9-epi-ST-Δ14-10-PhytoF and ent-9(RS)-12-epi-ST-Δ10-13-PhytoF. The oxylipin levels varied with aphids, but not with other experimental sources of variation. Both Rhopalosiphum padi and Sitobion avenae reduced the concentrations of ent-16(RS)-13-epi-ST-Δ14-9-PhytoF and ent-16(RS)-9-epi-ST-Δ14-10-PhytoF in relation to controls, but had little or no effect on PhytoPs. Our results are consistent with aphids affecting the levels of PUFAs (oxylipin precursors), which decreased the levels of PhytoFs in wheat leaves. Therefore, PhytoFs could be postulated as an early indicator of aphid hosting for this plant species. This is the first report on the quantification of non-enzymatic PhytoFs and PhytoPs in wheat leaves in response to aphids.

Explaining pre-emptive acclimation by linking information to plant phenotype.

We review mechanisms for pre-emptive acclimation in plants and propose a conceptual model linking developmental and evolutionary ecology with the acquisition of information through sensing of cues and signals. The idea is that plants acquire much of the information in the environment not from individual cues and signals but instead from their joint multivariate properties such as correlations. If molecular signalling has evolved to extract such information, the joint multivariate properties of the environment must be encoded in the genome, epigenome, and phenome. We contend that multivariate complexity explains why extrapolating from experiments done in artificial contexts into natural or agricultural systems almost never works for characters under complex environmental regulation: biased relationships among the state variables in both time and space create a mismatch between the evolutionary history reflected in the genotype and the artificial growing conditions in which the phenotype is expressed. Our model can generate testable hypotheses bridging levels of organization. We describe the model and its theoretical bases, and discuss its implications. We illustrate the hypotheses that can be derived from the model in two cases of pre-emptive acclimation based on correlations in the environment: the shade avoidance response and acclimation to drought.

Genetic basis and adaptive implications of temperature-dependent and temperature-independent effects of drought on chickpea reproductive phenology.

Water deficit often hastens flowering of pulses partially because droughted plants are hotter. Separating temperature-independent and temperature-dependent effects of drought is important to understand, model, and manipulate phenology. We define a new trait, drought effect on phenology (DEP), as the difference in flowering time between irrigated and rainfed crops, and use FST genome scanning to probe for genomic regions under selection for this trait in chickpea (Cicer arietinum). Owing to the negligible variation in daylength in our dataset, variation in phenology with sowing date was attributed to temperature and water; hence, genomic regions overlapping for early- and late-sown crops would associate with temperature-independent effects and non-overlapping genomic regions would associate with temperature-dependent effects. Thermal-time to flowering was shortened with increasing water stress, as quantified with carbon isotope composition. Genomic regions on chromosomes 4-8 were under selection for DEP. An overlapping region for early and late sowing on chromosome 8 revealed a temperature-independent effect with four candidate genes: BAM1, BAM2, HSL2, and ANT. The non-overlapping regions included six candidate genes: EMF1, EMF2, BRC1/TCP18, BZR1, NPGR1, and ERF1. Modelling showed that DEP reduces the likelihood of drought and heat stress at the expense of increased likelihood of cold stress. Accounting for DEP would improve genetic and phenotypic models of phenology.

Evolutionary and ecological perspectives on the wheat phenotype.

Technologies, from molecular genetics to precision agriculture, are outpacing theory, which is becoming a bottleneck for crop improvement. Here, we outline theoretical insights on the wheat phenotype from the perspective of three evolutionary and ecologically important relations-mother-offspring, plant-insect and plant-plant. The correlation between yield and grain number has been misinterpreted as cause-and-effect; an evolutionary perspective shows a striking similarity between crop and fishes. Both respond to environmental variation through offspring number; seed and egg size are conserved. The offspring of annual plants and semelparous fishes, lacking parental care, are subject to mother-offspring conflict and stabilizing selection. Labile reserve carbohydrates do not fit the current model of wheat yield; they can stabilize grain size, but involve trade-offs with root growth and grain number, and are at best neutral for yield. Shifting the focus from the carbon balance to an ecological role, we suggest that labile carbohydrates may disrupt aphid osmoregulation, and thus contribute to wheat agronomic adaptation. The tight association between high yield and low competitive ability justifies the view of crop yield as a population attribute whereby the behaviour of the plant becomes subordinated within that of the population, with implications for genotyping, phenotyping and plant breeding.

Partial-film mulch returns the same gains in yield and water use efficiency as full-film mulch with reduced cost and lower pollution: a meta-analysis.

BACKGROUND: Plastic film mulch is widely used to improve crop yield and water use efficiency (WUE, yield per unit evapotranspiration) in semi-arid regions. It is commonly applied as partial-film mulch (PM: at least 50% soil cover) or full-film mulch (FM: complete soil cover). The PM has lower economic and environmental cost; hence it would be a superior technology provided it delivers similar gains in yield and WUE in relation to FM. RESULTS: To solve contradictory results from individual studies, we compared FM and PM in a meta-analysis of 100 studies with 1881 comparisons (685 for wheat; 1196 for maize). Compared with bare ground, FM and PM both increased yield of wheat (20-26%) and maize (37-52%), and WUE of wheat (16-20%) and maize (38-48%), with statistically undistinguishable differences between PM and FM. The increases in crop yield and WUE were stronger at elevation > 1000 m, with annual precipitation<400 mm, and on loess soil, especially for maize. CONCLUSIONS: We concluded that partial-film mulch could replace full-film mulch to return similar yield and WUE improvement, with reduced cost and environmental pollution. © 2021 Society of Chemical Industry.

Light-mediated self-organization of sunflower stands increases oil yield in the field.

Here, we show a unique crop response to intraspecific interference, whereby neighboring sunflower plants in a row avoid each other by growing toward a more favorable light environment and collectively increase production per unit land area. In high-density stands, a given plant inclined toward one side of the interrow space, and the immediate neighbors inclined in the opposite direction. This process started early as an incipient inclination of pioneer plants, and the arrangement propagated gradually as a "wave" of alternate inclination that persisted until maturity. Measurements and experimental manipulation of light spectral composition indicate that these responses are mediated by changes in the red/far-red ratio of the light, which is perceived by phytochrome. Cellular automata simulations reproduced the patterns of stem inclination in field experiments, supporting the proposition of self-organization of stand structure. Under high crop population densities (10 and 14 plants per m2), as yet unachievable in commercial farms with current hybrids due to lodging and diseases, self-organized crops yielded between 19 and 47% more oil than crops forced to remain erect.

Responses of flavonoid profile and associated gene expression to solar blue and UV radiation in two accessions of Vicia faba L. from contrasting UV environments.

Blue light and UV radiation shape a plant's morphology and development, but accession-dependent responses under natural conditions are unclear. Here we tested the hypothesis that two faba bean (Vicia faba L.) accessions adapted to different latitudes and altitudes vary in their responses to solar blue and UV light. We measured growth, physiological traits, phenolic profiles and expression of associated genes in a factorial experiment combining two accessions (Aurora, a Swedish cultivar adapted to high latitude and low altitude; ILB938, from the Andean region of Colombia and Ecuador, adapted to low latitude and high altitude) and four filter treatments created with plastic sheets: 1. transparent as control; 2. attenuated short UV (290-350 nm); 3. attenuated UV (290-400 nm); 4. attenuated blue and UV light. In both accessions, the exclusion of blue and UV light increased plant height and leaf area, and decreased transcript abundance of ELONGATED HYPOCOTYL 5 (HY5) and TYROSINE AMINOTRANSFERASE 3 (TAT3). Blue light and short UV induced the accumulation of epidermal and whole-leaf flavonoids, mainly quercetins, and the responses in the two accessions were through different glycosides. Filter treatments did not affect kaempferol concentration, but there were more tri-glycosides in Aurora and di-glycosides in ILB938. Furthermore, fewer quercetin glycosides were identified in ILB938. The transcript abundance was consistently higher in Aurora than in ILB938 for all seven investigated genes: HY5, TAT3, CHALCONE SYNTHASE (CHS), CHALCONE ISOMERASE (CHI), DON-GLUCOSYLTRANSFERASE 1 (DOGT1), ABA INSENSITIVE 2 (ABI2), AUXIN-INDUCIBLE 2-27 (IAA5). The two largest differences in transcript abundance between the two accessions across treatments were 132-fold in CHS and 30-fold in DOGT1 which may explain the accession-dependent glycosylation patterns. Our findings suggest that agronomic selection for adaptation to high altitude may favour phenotypes with particular adaptations to the light environment, including solar UV and blue light.

Hypoxia in grape berries: the role of seed respiration and lenticels on the berry pedicel and the possible link to cell death.

Mesocarp cell death (CD) during ripening is common in berries of seeded Vitis vinifera L. wine cultivars. We examined if hypoxia within berries is linked to CD. The internal oxygen concentration ([O2]) across the mesocarp was measured in berries from Chardonnay and Shiraz, both seeded, and Ruby Seedless, using an oxygen micro-sensor. Steep [O2] gradients were observed across the skin and [O2] decreased toward the middle of the mesocarp. As ripening progressed, the minimum [O2] approached zero in the seeded cultivars and correlated to the profile of CD across the mesocarp. Seed respiration declined during ripening, from a large proportion of total berry respiration early to negligible at later stages. [O2] increased towards the central axis corresponding to the presence of air spaces visualized using X-ray micro-computed tomography (CT). These air spaces connect to the pedicel where lenticels are located that are critical for berry O2 uptake as a function of temperature, and when blocked caused hypoxia in Chardonnay berries, ethanol accumulation, and CD. The implications of hypoxia in grape berries are discussed in terms of its role in CD, ripening, and berry water relations.

Water productivity of rainfed maize and wheat: A local to global perspective.

Water productivity (WP) is a robust benchmark for crop production in relation to available water supply across spatial scales. Quantifying water-limited potential (WPw) and actual on-farm (WPa) WP to estimate WP gaps is an essential first step to identify the most sensitive factors influencing production capacity with limited water supply. This study combines local weather, soil, and agronomic data, and crop modeling in a spatial framework to determine WPw and WPa at local and regional levels for rainfed cropping systems in 17 (maize) and 18 (wheat) major grain-producing countries representing a wide range of cropping systems, from intensive, high-yield maize in north America and wheat in west Europe to low-input, low-yield maize systems in sub-Saharan Africa and south Asia. WP was calculated as the quotient of either water-limited yield potential or actual yield, and simulated crop evapotranspiration. Estimated WPw upper limits compared well with maximum WP reported for field-grown crops. However, there was large WPw variation across regions with different climate and soil (CV = 29% for maize and 27% for wheat), which cautions against the use of generic WPw benchmarks and highlights the need for region-specific WPw. Differences in simulated evaporative demand, crop evapotranspiration after flowering, soil evaporation, and intensity of water stress around flowering collectively explained two thirds of the variation in WPw. Average WP gaps were 13 (maize) and 10 (wheat) kg ha-1 mm-1, equivalent to about half of their respective WPw. We found that non-water related factors (i.e., management deficiencies, biotic and abiotic stresses, and their interactions) constrained yield more than water supply in ca. half of the regions. These findings highlight the opportunity to produce more food with same amount of water, provided limiting factors other than water supply can be identified and alleviated with improved management practices. Our study provides a consistent protocol for estimating WP at local to regional scale, which can be used to understand WP gaps and their mitigation.

Estimating yield gaps at the cropping system level.

Yield gap analyses of individual crops have been used to estimate opportunities for increasing crop production at local to global scales, thus providing information crucial to food security. However, increases in crop production can also be achieved by improving cropping system yield through modification of spatial and temporal arrangement of individual crops. In this paper we define the cropping system yield potential as the output from the combination of crops that gives the highest energy yield per unit of land and time, and the cropping system yield gap as the difference between actual energy yield of an existing cropping system and the cropping system yield potential. Then, we provide a framework to identify alternative cropping systems which can be evaluated against the current ones. A proof-of-concept is provided with irrigated rice-maize systems at four locations in Bangladesh that represent a range of climatic conditions in that country. The proposed framework identified (i) realistic alternative cropping systems at each location, and (ii) two locations where expected improvements in crop production from changes in cropping intensity (number of crops per year) were 43% to 64% higher than from improving the management of individual crops within the current cropping systems. The proposed framework provides a tool to help assess food production capacity of new systems (e.g. with increased cropping intensity) arising from climate change, and assess resource requirements (water and N) and associated environmental footprint per unit of land and production of these new systems. By expanding yield gap analysis from individual crops to the cropping system level and applying it to new systems, this framework could also be helpful to bridge the gap between yield gap analysis and cropping/farming system design.

Phenotypic plasticity and its genetic regulation for yield, nitrogen fixation and δ13C in chickpea crops under varying water regimes.

We measured yield components, nitrogen fixation, soil nitrogen uptake and carbon isotope composition (δ(13)C) in a collection of chickpea genotypes grown in environments where water availability was the main source of yield variation. We aimed to quantify the phenotypic plasticity of these traits using variance ratios, and to explore their genetic basis using FST genome scan. Fifty-five genes in three genomic regions were found to be under selection for plasticity of yield; 54 genes in four genomic regions for the plasticity of seeds per m(2); 48 genes in four genomic regions for the plasticity of δ(13)C; 54 genes in two genomic regions for plasticity of flowering time; 48 genes in five genomic regions for plasticity of nitrogen fixation and 49 genes in three genomic regions for plasticity of nitrogen uptake from soil. Plasticity of yield was related to plasticity of nitrogen uptake from soil, and unrelated to plasticity of nitrogen fixation, highlighting the need for closer attention to nitrogen uptake in legumes. Whereas the theoretical link between δ(13)C and transpiration efficiency is strong, the actual link with yield is erratic due to trade-offs and scaling issues. Genes associated with plasticity of δ(13)C were identified that may help to untangle the δ(13)C-yield relationship. Combining a plasticity perspective to deal with complex G×E interactions with FST genome scan may help understand and improve both crop adaptation to stress and yield potential.

Independent genetic control of maize (Zea mays L.) kernel weight determination and its phenotypic plasticity.

Maize kernel weight (KW) is associated with the duration of the grain-filling period (GFD) and the rate of kernel biomass accumulation (KGR). It is also related to the dynamics of water and hence is physiologically linked to the maximum kernel water content (MWC), kernel desiccation rate (KDR), and moisture concentration at physiological maturity (MCPM). This work proposed that principles of phenotypic plasticity can help to consolidated the understanding of the environmental modulation and genetic control of these traits. For that purpose, a maize population of 245 recombinant inbred lines (RILs) was grown under different environmental conditions. Trait plasticity was calculated as the ratio of the variance of each RIL to the overall phenotypic variance of the population of RILs. This work found a hierarchy of plasticities: KDR ≈ GFD > MCPM > KGR > KW > MWC. There was no phenotypic and genetic correlation between traits per se and trait plasticities. MWC, the trait with the lowest plasticity, was the exception because common quantitative trait loci were found for the trait and its plasticity. Independent genetic control of a trait per se and genetic control of its plasticity is a condition for the independent evolution of traits and their plasticities. This allows breeders potentially to select for high or low plasticity in combination with high or low values of economically relevant traits.

The Interaction Between Nitrogen Supply and Light Quality Modulates Plant Growth and Resource Allocation.

Nitrogen availability and light quality affect plant resource allocation, but their interaction is poorly understood. Herein, we analyzed the growth and allocation of dry matter and nitrogen using lettuce (Lactuca sativa L.) as a plant model in a factorial experiment combining three light regimes (100% red light, R; 50% red light + 50% blue light, RB; 100% blue light, B) and two nitrogen rates (low, 0.1 mM N; high, 10 mM N). Red light increased shoot dry weight in relation to both B and RB irrespective of nitrogen supply. Blue light favored root growth under low nitrogen. Allometric analysis showed lower allocation to leaf in response to blue light under low nitrogen and similar leaf allocation under high nitrogen. A difference in allometric slopes between low nitrogen and high nitrogen in treatments with blue light reflected a strong interaction effect on root-to-shoot biomass allocation. Shoot nitrate concentration increased with light exposure up to 14 h in both nitrogen treatments, was higher under blue light with high nitrogen, and varied little with light quality under low nitrogen. Shoot nitrogen concentration, nitrogen nutrition index, and shoot NR activity increased in response to blue light. We conclude that the interaction between blue light and nitrogen supply modulates dry mass and nitrogen allocation between the shoot and root.

Effects of the source:sink ratio on the phenotypic plasticity of stem water potential in olive (Olea europaea L.).

The aims of this work were to quantify (i) the effect of the source:sink ratio on stem water potential (SWP) and (ii) the phenotypic plasticity of SWP and its relationship to oil yield components in olive. Trees with a 3-fold variation in the source:sink ratio (crown volume/fruit number per tree) were monitored in 2007-2008 and 2008-2009 in a fully irrigated orchard in Mendoza, Argentina. The combination of rainfall, irrigation, and evaporative demand led to a steady SWP largely above -1.65 MPa in 2007-2008 and a marked seasonal decline from -1.13 MPa to -2.04 MPa in trees with a medium and low source:sink ratio in 2008-2009. Plasticity was quantified as the slope of the norm of reaction for each trait. Across seasons, trees with a high source:sink ratio had a higher SWP than their counterparts with a medium and low source:sink ratio. Plasticity of SWP was highest in olives with a low source:sink ratio (slope=1.28) and lowest for trees with a high source:sink ratio (slope=0.76). The average SWP for each source:sink ratio and season was unrelated to both the source:sink ratio and yield components. On the other hand, the plasticity of SWP was positively associated with fruit number and negatively associated with the source:sink ratio, fruit weight, and fruit oil weight. The plasticity of the SWP was unrelated to SWP per se. It is concluded that understanding the effect of the source:sink ratio on plant water relations would benefit from a dual perspective considering the trait per se and its plasticity. A dual approach would also allow for more robust plant-based indicators for irrigation.

Symmetric response to competition in binary mixtures of cultivars associates with genetic gain in wheat yield.

The evolution in the definition of crop yield-from the ratio of seed harvested to seed sown to the contemporary measure of mass of seed per unit land area-has favoured less competitive phenotypes. Here we use binary mixtures of cultivars spanning five decades of selection for yield and agronomic adaptation to ask three questions. First, what is the degree of symmetry in the response of yield to neighbour; this is, if an older, more competitive cultivar increases yield by 10% with a less competitive neighbour in comparison to pure stands, would the newer, less competitive cultivar reduce yield by 10% when grown with older neighbour. Lack of symmetry would indicate factors other than competitive ability underly yield improvement. Second, what are the yield components underlying competitive interactions. Third, to what extent are the responses to neighbour mediated by radiation, water and nitrogen. A focus on yield components and resources can help the interpretation of shifts in the crop phenotype in response to selection for yield. The rate of genetic gain in yield over five decades was 24 kg ha-1 year-1 or 0.61% year-1. A strongly symmetrical yield response to neighbour indicates that yield improvement closely associates with a reduction in competitive ability. Response to neighbour was larger for grain number and biomass than for grain weight and allocation of biomass to grain. Under our experimental conditions, competition for radiation was dominant compared to competition of water and nitrogen. High-yielding phenotypes had lower competitive ability for radiation but compensated with higher radiation use efficiency, a measure of canopy photosynthetic efficiency. Genetic and agronomic manipulation of the crop phenotype to reduce competitive ability could further improve wheat yield to meet the challenge of global food security.

Australian Lentil Breeding Between 1988 and 2019 Has Delivered Greater Yield Gain Under Stress Than Under High-Yield Conditions.

The contemporary lentil (Lens culinaris ssp. culinaris) industry in Australia started in the late 1980s. Yield in farmers' fields averages 1.2 t ha-1 nationally and has not increased over three decades. Lack of yield progress can be related to a number of non-mutually exclusive reasons: expansion of lentil to low-yielding environments, lack of genetic gain in yield, lack of progress in agronomic practices, and lack of adoption of superior technologies. The aims of this study were to (i) quantify the genetic gain in lentil yield since 1988, (ii) explore the variation in the expression of genetic gain with the environment, and (iii) identify shifts in crop phenotype associated with selection for yield and agronomic adaptation. We grew a historic collection of 19 varieties released between 1988 and 2019 in eight environments resulting from the factorial combination of two sowing dates, two water regimes, and two seasons. Across environments, yield varied 11-fold from 0.2 to 2.2 t ha-1. The rate of genetic gain averaged 20 kg ha-1 year-1 or 1.23% year-1 across environments and was higher in low-yield environments. The yield increase was associated with substantial shifts in phenology. Newer varieties had a shorter time to flowering and pod emergence, and the rate of change in these traits was more pronounced in slow-developing environments (e.g., earlier sowing). Thermal time from sowing to end of flowering and maturity were shorter in newer varieties, and thermal time from pod emergence to maturity was longer in newer varieties; the rate of change in these traits was unrelated to developmental drivers and correlated with environmental mean yield. Genetic gain in yield was associated with increased grain number and increased harvest index. Despite their shorter time to maturity, newer varieties had similar or higher biomass than their older counterparts because crop growth rate during the critical period increased with the year of release. Genotype-dependent yield increased over three decades in low-yield environments, whereas actual farm yield has been stagnant; this suggests an increasing yield gap requiring agronomic solutions. Genetic improvement in high-yield environments requires improved coupling of growth and reproduction.

The role of plant labile carbohydrates and nitrogen on wheat-aphid relations.

Interactions between plants and herbivores are key drivers of evolution and ecosystem complexity. We investigated the role of plant labile carbohydrates and nitrogen on wheat-aphid relations in a 22 factorial combining [CO2] and nitrogen supply. We measured life history traits (assay 1) and feeding behaviour (assay 2) of bird-cherry oat aphid (Rhopalosiphum padi L.) and English grain aphid (Sitobion avenae F.) forced to feed on single leaf laminae, and reproduction of R. padi in a setting where insects moved freely along the plant (assay 3). Experimental setting impacted aphid traits. Where aphids were constrained to single leaf, high nitrogen reduced their fitness and discouraged phloem feeding. Where aphids could move throughout the plant, high nitrogen enhanced their reproduction. Aphid responses to the interaction between nitrogen and [CO2] varied with experimental setting. The number of R. padi adults varied tenfold with plant growing conditions and correlated negatively with molar concentration of sugars in stem (assay 3). This finding has two implications. First, the common interpretation that high nitrogen favours insect fitness because protein-rich animal bodies have to build from nitrogen-poor plant food needs expanding to account for the conspicuous association between low nitrogen and high concentration of labile carbohydrates in plant, which can cause osmotic stress in aphids. Second, the function of labile carbohydrates buffering grain growth needs expanding to account for the osmotic role of carbohydrates in plant resistance to aphids.