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 at (1) characterising lentil response to photoperiod with a focus on five loci: the lentil ELF3 ortholog Sn, two loci linked to clusters of lentil FT orthologs and two loci without candidates in chromosomes 2 and 5 (exp. 1: 36 lines, short and long day in phytotron); (2) establishing phenology-yield relationship (exp. 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 yr-1 or 1.5% yr-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 % yr-1 in the field, and -0.42 % yr-1 (short day) and -0.99 % yr-1 (long day) 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.

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.

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.

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.

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.

Aphid Resistance: An Overlooked Ecological Dimension of Nonstructural Carbohydrates in Cereals

Nonstructural carbohydrates in cereals have been widely investigated from physiological, genetic, and breeding perspectives. Nonstructural carbohydrates may contribute to grain filling, but correlations with yield are inconsistent and sometimes negative. Here we ask if there are hidden functions of nonstructural carbohydrates, advance an ecological dimension to this question, and speculate that high concentration of nonstructural carbohydrates may challenge the osmotic homeostasis of aphids, thus providing a working hypothesis that connects nonstructural carbohydrates with aphid resistance in cereals. In the light of this proposition, the amount and concentration of nonstructural carbohydrates should be regarded as functionally different traits, with amount relevant to the carbon economy of the crop and concentration playing an osmotic role. We conclude with suggestions for experiments to test our hypothesis.

The transgenerational effects of solar short-UV radiation differed in two accessions of Vicia faba L. from contrasting UV environments

BACKGROUND AND AIMS

UVB radiation can rapidly induce gene regulation leading to cumulative changes for plant physiology and morphology. We hypothesized that a transgenerational effect of chronic exposure to solar short UV modulates the offspring's responses to UVB and blue light, and that the transgenerational effect is genotype dependent.

METHODS

We established a factorial experiment combining two Vicia faba L. accessions, two parental UV treatments (full sunlight and exclusion of short UV, 290-350 nm), and four offspring light treatments from the factorial combination of UVB and blue light. The accessions were Aurora from southern Sweden, and ILB938 from Andean region of Colombia and Ecuador.

KEY RESULTS

The transgenerational effect influenced morphological responses to blue light differently in the two accessions. In Aurora, when UVB was absent, blue light increased shoot dry mass only in plants whose parents were protected from short UV. In ILB938, blue light increased leaf area and shoot dry mass more in plants whose parents were exposed to short UV than those that were not. Moreover, when the offspring was exposed to UVB, the transgenerational effect decreased in ILB938 and disappeared in Aurora. For flavonoids, the transgenerational effect was detected only in Aurora: parental exposure to short UV was associated with a greater induction of total quercetin in response to UVB. Transcript abundance was higher in Aurora than in ILB938 for both CHALCONE SYNTHASE (99-fold) and DON-GLUCOSYLTRANSFERASE 1 (19-fold).

CONCLUSIONS

The results supported both hypotheses. Solar short UV had transgenerational effects on progeny responses to blue and UVB radiation, and they differed between the accessions. These transgenerational effects could be adaptive by acclimation of slow and cumulative morphological change, and by early build-up of UV protection through flavonoid accumulation on UVB exposure. The differences between the two accessions aligned with their adaptation to contrasting UV environments.

Changes in the Phenotype of Winter Wheat Varieties Released Between 1920 and 2016 in Response to In-Furrow Fertilizer: Biomass Allocation, Yield, and Grain Protein Concentration

Plant breeding has increased the yield of winter wheat (Triticum aestivum L.) over decades, and the rate of genetic gain has been faster under high fertility in some countries. However, this response is not universal, and limited information exists on the physiological traits underlying the interaction between varieties and fertilization. Thus, our objectives were to identify the key shifts in crop phenotype in response to selection for yield and quality, and to determine whether historical and modern winter wheat varieties respond differently to in-furrow fertilizer. Factorial field experiments combined eight winter wheat varieties released between 1920 and 2016, and two fertilizer practices [control versus 112 kg ha^-1 in-furrow 12 -40-0-10-1 (N-P-K-S-Zn)] in four Kansas environments. Grain yield and grain N-removal increased nonlinearly with year of release, with greater increases between 1966 and 2000. In-furrow fertilizer increased yield by ~300 kg ha^-1 with no variety × fertility interaction. Grain protein concentration related negatively to yield, and the residuals of this relationship were unrelated to year of release. Yield increase was associated with changes in thermal time to critical growth stages, as modern varieties had shorter vegetative period and longer grain filling period. Yield gains also derived from more kernels m^-2 resultant from more kernels head^-1. Historical varieties were taller, had thinner stems, and allocated more biomass to the stem than semidwarf varieties. Yield gains resulted from increases in harvest index and not in biomass accumulation at grain filling and maturity, as shoot biomass was similar among varieties. The allometric exponent (i.e., the slope between log of organ biomass and log of shoot biomass) for grain increased with, and for leaves was unrelated to, year of release. The ability of modern varieties to allocate more biomass to the kernels coupled to an early maturity increased grain yield and grain N-removal over time. However, increases in grain yield were greater than increases in grain N-removal, reducing grain protein concentration in modern varieties.

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.

Interplay between nitrogen fertilizer and biological nitrogen fixation in soybean: implications on seed yield and biomass allocation

Legumes rely on soil mineral nitrogen (N) and biological N fixation (BNF). The interplay between these two sources is biologically interesting and agronomically relevant as the crop can accommodate the cost of BNF by five non-mutually exclusive mechanisms, whereby BNF: reduces shoot growth and seed yield, or maintains shoot growth and seed yield by enhanced photosynthesis, or reduced root:shoot ratio, or maintains shoot growth but reduces seed yield by reducing the fraction of shoot biomass allocated to seed (harvest index), or reducing concentration of oil and protein in seed. We explore the impact of N application on the seasonal dynamics of BNF, and its consequences for seed yield with emphasis on growth and shoot allocation mechanisms. Trials were established in 23 locations across the US Midwest under four N conditions. Fertilizer reduced the peak of BNF up to 16% in applications at the full flowering stage. Seed yield declined 13 kg ha^-1 per % increase in RAU_R6. Harvest index accounted for the decline in seed yield with increasing BNF. This indicates the cost of BNF was met by a relative change in dry matter allocation against the energetically rich seed, and in favor of energetically cheaper vegetative tissue.

Interplay between nitrogen fertilizer and biological nitrogen fixation in soybean: implications on seed yield and biomass allocation

Legumes rely on soil mineral nitrogen (N) and biological N fixation (BNF). The interplay between these two sources is biologically interesting and agronomically relevant as the crop can accommodate the cost of BNF by five non-mutually exclusive mechanisms, whereby BNF: reduces shoot growth and seed yield, or maintains shoot growth and seed yield by enhanced photosynthesis, or reduced root:shoot ratio, or maintains shoot growth but reduces seed yield by reducing the fraction of shoot biomass allocated to seed (harvest index), or reducing concentration of oil and protein in seed. We explore the impact of N application on the seasonal dynamics of BNF, and its consequences for seed yield with emphasis on growth and shoot allocation mechanisms. Trials were established in 23 locations across the US Midwest under four N conditions. Fertilizer reduced the peak of BNF up to 16% in applications at the full flowering stage. Seed yield declined 13 kg ha-1 per % increase in RAUR6. Harvest index accounted for the decline in seed yield with increasing BNF. This indicates the cost of BNF was met by a relative change in dry matter allocation against the energetically rich seed, and in favor of energetically cheaper vegetative tissue.

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 Stress Scatters Nitrogen Dilution Curves in Wheat

Nitrogen dilution curves relate a crop's critical nitrogen concentration (%N_c) to biomass (W) according to the allometric model %N_c = a W ^-b . This model has a strong theoretical foundation, and parameters a and b show little variation for well-watered crops. Here we explore the robustness of this model for water stressed crops. We established experiments to examine the combined effects of water stress, phenology, partitioning of biomass, and water-soluble carbohydrates (WSC), as driven by environment and variety, on the %N_c of wheat crops. We compared models where %N_c was plotted against biomass, growth stage and thermal time. The models were similarly scattered. Residuals of the %N_c - biomass model at anthesis were positively related to biomass, stem:biomass ratio, Δ¹³C and water supply, and negatively related to ear:biomass ratio and concentration of WSC. These are physiologically meaningful associations explaining the scatter of biomass-based dilution curves. Residuals of the thermal time model showed less consistent associations with these variables. The biomass dilution model developed for well-watered crops overestimates nitrogen deficiency of water-stressed crops, and a biomass-based model is conceptually more justified than developmental models. This has implications for diagnostic and modeling. As theory is lagging, a greater degree of empiricism might be useful to capture environmental, chiefly water, and genotype-dependent traits in the determination of critical nitrogen for diagnostic purposes. Sensitivity analysis would help to decide if scaling nitrogen dilution curves for crop water status, and genotype-dependent parameters are needed.

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.

Metabolic effects of elevated temperature on organic acid degradation in ripening Vitis vinifera fruit

Berries of the cultivated grapevine Vitis vinifera are notably responsive to temperature, which can influence fruit quality and hence the future compatibility of varieties with their current growing regions. Organic acids represent a key component of fruit organoleptic quality and their content is significantly influenced by temperature. The objectives of this study were to (i) manipulate thermal regimes to realistically capture warming-driven reduction of malate content in Shiraz berries, and (ii) investigate the mechanisms behind temperature-sensitive malate loss and the potential downstream effects on berry metabolism. In the field we compared untreated controls at ambient temperature with longer and milder warming (2-4 °C differential for three weeks; Experiment 1) or shorter and more severe warming (4-6 °C differential for 11 days; Experiment 2). We complemented field trials with control (25/15 °C) and elevated (35/20 °C) day/night temperature controlled-environment trials using potted vines (Experiment 3). Elevating maximum temperatures (4-10 °C above controls) during pre-véraison stages led to higher malate content, particularly with warmer nights. Heating at véraison and ripening stages reduced malate content, consistent with effects typically seen in warm vintages. However, when minimum temperatures were also raised by 4-6 °C, malate content was not reduced, suggesting that the regulation of malate metabolism differs during the day and night. Increased NAD-dependent malic enzyme activity and decreased phosphoenolpyruvate carboxylase and pyruvate kinase activities, as well as the accumulation of various amino acids and γ-aminobutyric acid, suggest enhanced anaplerotic capacity of the TCA cycle and a need for coping with decreased cytosolic pH in heated fruit.

Improvement of crop yield in dry environments: benchmarks, levels of organisation and the role of nitrogen

Crop yield in dry environments can be improved with complementary approaches including selecting for yield in the target environments, selecting for yield potential, and using indirect, trait- or genomic-based methods. This paper (i) outlines the achievements of direct selection for yield in improving drought adaptation, (ii) discusses the limitations of indirect approaches in the context of levels of organization, and (iii) emphasizes trade-offs and synergies between nitrogen nutrition and drought adaptation. Selection for yield in the water- and nitrogen-scarce environments of Australia improved wheat yield per unit transpiration at a rate of 0.12kg ha(-1) mm(-1) yr(-1); for indirect methods to be justified, they must return superior rates of improvement, achieve the same rate at lower cost or provide other cost-effective benefits, such as expanding the genetic basis for selection. Slow improvement of crop adaptation to water stress using indirect methods is partially related to issues of scale. Traits are thus classified into three broad groups: those that generally scale up from low levels of organization to the crop level (e.g. herbicide resistance), those that do not (e.g. grain yield), and traits that might scale up provided they are considered in a integrated manner with scientifically sound scaling assumptions, appropriate growing conditions, and screening techniques (e.g. stay green). Predicting the scalability of traits may help to set priorities in the investment of research efforts. Primary productivity in arid and semi-arid environments is simultaneously limited by water and nitrogen, but few attempts are made to target adaptation to water and nitrogen stress simultaneously. Case studies in wheat and soybean highlight biological links between improved nitrogen nutrition and drought adaptation.

Phenotypic plasticity of stem water potential correlates with crop load in horticultural trees

Conceptual models accounting for the influence of source:sink ratio on water relations of trees are theoretically relevant from a physiological perspective and practically important for irrigation scheduling. Midday stem water potential of horticultural trees often declines with increasing crop load but the actual response depends on environmental, management and plant factors. Here we advance a quantitative synthesis of the response of stem water potential to crop load from the perspective of phenotypic plasticity, defined as 'the amount by which the expression of individual characteristics of a genotype are changed by different environments'. Data sets of stem water potential for contrasting crop loads were compiled for apple (Malus domestica L. Borkh.), olive (Olea europea L.), peach (Prunus persica L.), pear (Pyrus communis L.) and plum (Prunus domestica L.). Phenotypic plasticity of stem water potential was calculated as the slope of the linear regression between stem water potential for each crop load and the environmental mean of stem water potential across crop loads. Regression lines for trees with different crop load diverged with decreasing environmental mean stem water potential. For the pooled data, plasticity of stem water potential was a linear function of relative crop load. This represents a significant shift in perspective: the effect of crop load on the trait per se (stem water potential) is environmentally contingent, but the effect of crop load on the plasticity of the trait is not. We conclude that research on the effects of crop load on tree water relations would return more robust results if plant traits are considered from the dual perspective of the trait per se and its plasticity.

Contradictions in host plant resistance to pests: spider mite (Tetranychus urticae Koch) behaviour undermines the potential resistance of smooth-leaved cotton (Gossypium hirsutum L.)

BACKGROUND

Two-spotted spider mites (Tetranychus urticae Koch) oviposit near leaf veins or in leaf folds on the undersides of cotton (Gossypium hirsutum L.) leaves where the humid boundary layer offers protection from desiccation. The authors predicted that the boundary layer of glabrous cotton leaves should be shallower than that of hairy leaves, providing some resistance to mites. The dynamics of mite populations, leaf damage, leaf gas exchange and crop yield on two leaf hair isolines (smooth versus hairy) in two genetic backgrounds was assessed.

RESULTS

Mite colonies developed faster on the hairy leaf isolines, but leaf damage per mite was higher in smooth leaf isolines, indicating more intense damage. A 50% reduction in photosynthesis on the hairy isolines required 1.8 times more mites than smooth leaves. The yield of cotton was reduced in + mite treatments, but the magnitude of reduction was similar for hairy and smooth isolines.

CONCLUSION

Paradoxically, the relative inhospitality of glabrous leaves may have induced mites to concentrate in protected leaf sections, causing more localised and more severe damage, negating the yield benefits from fewer mites. These results highlight interactions between leaf microenvironment, pest behaviour and plant productivity that may have implications for other instances of plant resistance.

The interplay between the effectiveness of the grass-endophyte mutualism and the genetic variability of the host plant

Neotyphodium endophytic fungi, the asexual state of Epichloë species, protect cool-season grasses against stresses. The outcomes of Neotyphodium-grass symbioses are agronomically relevant as they may affect the productivity of pastures. It has been suggested that the mutualism is characteristic of agronomic grasses and that differential rates of gene flow between both partners’ populations are expected to disrupt the specificity of the association and, thus, the mutualism in wild grasses. We propose that compatibility is necessary but not sufficient to explain the outcomes of Neotyphodium-grass symbiosis, and advance a model that links genetic compatibility, mutualism effectiveness, and endophyte transmission efficiency. For endophytes that reproduce clonally and depend on allogamous hosts for reproduction and dissemination, we propose that this symbiosis works as an integrated entity where gene flow promotes its fitness and evolution. Compatibility between the host plant and the fungal endophyte would be high in genetically close parents; however, mutualism effectiveness and transmission efficiency would be low in fitness depressed host plants. Increasing the genetic distance of mating parents would increase mutualism effectiveness and transmission efficiency. This tendency would be broken when the genetic distance between parents is high (out-breeding depression). Our model allows for testable hypotheses that would contribute to understand the coevolutionary origin and future of the endophyte-grass mutualism.

Undamaged cotton plants yield more if their neighbour is damaged: implications for pest management

Understanding the compensatory responses of crops to pest damage is important in developing pest thresholds. Compensation for pest damage in crops can occur at the plant level, where the architecture, growth dynamics and allocation patterns of damaged plants are altered, allowing them to recover or, at the crop level, where differential damage between plants may alter plant-to-plant interactions. We investigated growth and yield of cotton (Gossypium hirsutum L.) following non-uniform manual defoliation of seedlings. This partially replicates real pest damage and is valuable in understanding crop-level responses to damage because it can be inflicted precisely. Damage distributions included damaging 0, 25, 50, 75 or 100% of the plants. Damage intensity for the damaged plants was varied by removing 100 or 75% of each true leaf when plants had two, four and six true leaves. At the crop level, yield loss increased as the proportion of plants damaged and intensity of damage per damaged plant increased. Neighbour interactions occurred; undamaged plants with damaged neighbours grew larger and yielded better than undamaged plants with undamaged neighbours, while the converse applied for damaged plants with undamaged neighbours. Neighbour interactions were influenced by the intensity of damage and were stronger when 100% of the leaf area was removed than when 75% was removed. At the crop level, when compared with yield estimates based on yield of plants from uniformly damaged or undamaged plots, these interactions resulted in higher yield than expected (+8%). This suggests that damage distribution may have to be considered in studies where artificial or real pest damage is inflicted uniformly on plants.

Irrigated Shiraz vines (Vitis vinifera) upregulate gas exchange and maintain berry growth in response to short spells of high maximum temperature in the field

We tested the hypotheses that (i) a short period of high maximum temperature disrupts gas exchange and arrests berry growth and sugar accumulation in irrigated Shiraz vines (Vitis vinifera L.), and (ii) the magnitude of these effects depend on the phenological window when stress occur. Using a system combining passive heating (greenhouse effect) and active cooling (fans) to control daytime temperature, we compared vines heated to a nominal maximum of 40°C for three consecutive days and untreated controls. Maximum air temperature in heated treatments was 7.3°C (2006-07) and 6.5°C (2007-08) above ambient. Heat episodes were aligned with the beginning of a weekly irrigation cycle and applied in one of four phenological windows, namely post-fruit set, pre-veraison, veraison and pre-harvest. Heating systems did not affect relative humidity, hence vapour pressure deficit (VPD) was increased in the heated treatments and tracked the daily cycle of temperature. Heat did not affect the dynamics of berry growth and sugar accumulation, except for a 16% reduction in berry size and sugar content in vines heated shortly after fruit set in 2006-07. Vines upregulated stomatal conductance and gas exchange in response to heat. Stomatal conductance, photosynthesis and transpiration at a common VPD were consistently higher in heated vines than in controls. We suggest that stomatal behaviour previously described as part of Shiraz anisohydric syndrome may be adaptive in terms of heat tolerance at the expense of short-term transpiration efficiency.

Suboptimal temperature favors reserve formation in biennial carrot (Daucus carota) plants

In response to suboptimal temperatures, temperate annual plants often increase root:shoot ratios, build-up carbohydrates and display typical morphological and anatomical changes. We know less about the responses of biennials such as carrot. As a model plant, carrot has the additional feature of two functionally and morphologically distinct root parts: the taproot, which stores carbohydrate and other compounds, and the fibrous root system involved in acquisition of water and nutrients. Here, we analyze the effects of temperature (12 vs 25°C) on growth, carbohydrate accumulation and whole-plant morphology in two carrot cultivars. Our working hypothesis is that suboptimal temperature favors active formation of reserve structures, rather than passive accumulation of storage carbohydrates. In comparison with plants grown at 25°C, plants grown at 12°C had: (1) higher fibrous root:shoot ratio (13%) , (2) thicker (10-15%) and smaller (up to two- to three-fold) leaves, (3) lower leaf cuticular permeance (two- to four-fold), (4) higher taproot:shoot ratio (two-fold), (5) higher phloem:xylem ratios in taproot (two- to six-fold), (6) unchanged percentage dry matter content (%DMC) in leaves, petioles or fibrous roots and (7) higher %DMC in taproot (20%). However, %DMC of individual taproot tissues (phloem and xylem) was unaffected by temperatures and was consistently higher in the phloem (up to 30%). Therefore, the higher %DMC of whole taproots at 12°C was attributed solely to the increased development of phloem tissue. Carrot, therefore, shares many of the most conspicuous elements of temperate plant responses to low temperatures. Consistently with our hypothesis, however, carrots grown at suboptimal temperature promoted reserve structures, rather than the increase in carbohydrate concentration typical of most temperate annual species and woody perennials.

Do plant parts compete for resources? An evolutionary viewpoint

Simultaneously growing sinks are thought to compete for plant resources. Negative correlations, for example between grain number and stem mass in cereals, indeed resemble competition; but is the notion of intra-plant competition evolutionarily justified? Here we review intra-plant competition in light of two aspects of evolutionary biology: (a) major transitions that led to the reorganization of evolutionary individuals (e.g. isolated DNA molecules and independent cells) into new units of adaptation (e.g. chromosomes and multicellular organisms) with associated constraints to intra-individual conflict; and (b) genomic conflicts within individual plants with implications for resource allocation. Against this background, we look at apparent competition among genetically identical plant parts, and conclude that plants might use competition-like mechanisms to allocate resources, but only to the extent that these proximate mechanisms enhance overall plant fitness. In dealing with apparent competition among genetically different plant structures, we emphasize developing seeds attached to the same maternal plant, and the determination of yield components in annual crops. We propose that competition-like mechanisms among genetically different plant parts have been strongly shaped by the evolution of genomic conflict between parent and offspring, between female and male parents, and among siblings. By defining the number and potential size of grain simultaneously and before fertilization, a strong maternal control of resource allocation is exerted that favours uniform offspring size and partially counteracts genomic conflict.

Benchmarking water-use efficiency of rainfed wheat in dry environments

Attainable water-use efficiency relates attainable yield, i.e. the best yield achieved through skilful use of available technology, and seasonal evapotranspiration (ET). For wheat crops in south-eastern Australia, there is a common, often large gap between actual and attainable water-use efficiency. To evaluate whether this gap is only an Australian problem or a general feature of dry environments, we compared water-use efficiency of rainfed wheat in south-eastern Australia, the North American Great Plains, China Loess Plateau, and the Mediterranean Basin. A dataset of published data was compiled (n = 691); water-use efficiency (WUEY/ET) was calculated as the ratio between actual grain yield and seasonal ET. Maximum WUEY/ET was 22 kg grain/ha.mm. Average WUEY/ET (kg grain/ha.mm) was 9.9 for south-eastern Australia, 9.8 for the China Loess Plateau, 8.9 for the northern Great Plains of North America, 7.6 for the Mediterranean Basin, and 5.3 for the southern-central Great Plains; the variation in average WUEY/ET was largely accounted for by reference evapotranspiration around flowering. Despite substantial differences in important factors including soils, precipitation patterns, and management practices, crops in all these environments had similarly low average WUEY/ET, between 32 and 44% of attainable efficiency. We conclude that low water-use efficiency of Australian crops is not a local problem, but a widespread feature of dry environments. Yield gap analysis for crops in the Mallee region of Australia revealed low availability of phosphorus, late sowing, and subsoil chemical constraints as key factors reducing water-use efficiency, largely through their effects on soil evaporation.

A quantitative top-down view of interactions between stresses: theory and analysis of nitrogen - water co-limitation in Mediterranean agro-ecosystems

The multiple factors constraining the growth, reproduction, and survival of diverse organisms are often non-additive. Research of interacting factors generally involves conceptual models that are specific for target organism, type of stress, and process. As a complement to this reductionist, bottom-up view, in this review I discuss a quantitative top-down approach to interacting stresses based on co-limitation theory. Firstly, co-limitation theory is revised. Co-limitation is operationally identified when the output response of a biological system (e.g. plant or population growth) to two or more inputs is greater than its response to each factor in isolation. The hypothesis of Bloom, Chapin, and Mooney, that plant growth is maximised when it is equally limited by all resources, is reworded in terms of co-limitation and formulated in quantitative terms, i.e. for a given intensity of aggregate stress, plant growth is proportional to degree of resource co-limitation. Emphasis is placed on the problems associated with the quantification of co-limitation. It is proposed that seasonal indices of nitrogen and water stress calculated with crop simulation models can be integrated in indices accounting for the aggregated intensity of water and nitrogen stress (SWN), the degree of water and nitrogen co-limitation (CWN), and the integrated effect of stress and co-limitation (SCWN = CWN/SWN). The expectation is that plant growth and yield should be an inverse function of stress intensity and a direct function of co-limitation, thus proportional to SCWN. Secondly, the constraints imposed by water and nitrogen availability on yield and water use efficiency of wheat crops are highlighted in case studies of low-input farming systems of south-eastern Australia. Thirdly, the concept of co-limitation is applied to the analysis of (i) grain yield responses to water–nitrogen interactions, and (ii) trade-offs between nitrogen- and water-use efficiency. In agreement with theoretical expectations, measured grain yield is found to be proportional to modelled SCWN. Productivity gains associated with intensification of cropping practices are interpreted in terms of a trade-off, whereby water-use efficiency is improved at the expense of nitrogen-use efficiency, thus leading to a higher degree of resource co-limitation.

Crop rotation effect on wheat grain yield as mediated by changes in the degree of water and nitrogen co-limitation

Theoretically, growth of stressed plants is maximised when all resources are equally limiting. The concept of co-limitation could be used to integrate key factors affected by crop rotation. This paper tested the hypothesis that the effect of crop rotation on the yield of wheat is partially mediated by changes in the degree of co-limitation between nitrogen and water. Four rotations were established on a sodic, supracalcic, red chromosol in a Mediterranean-type environment of southern Australia. Rotations included wheat grown after (a) faba bean harvested for grain, (b) faba bean incorporated as green manure, (c) ryegrass pasture, or (d) medic pasture; barley was grown after wheat in all cases. The response of wheat to the rotations during 3 growing seasons was analysed in terms of nitrogen and water co-limitation, and the response of barley was taken as a measure of the persistence of rotation effects. Daily scalars quantifying water and nitrogen stress effects on tissue expansion were calculated with a crop simulation model. These scalars were integrated in a series of seasonal indices to quantify the intensity of water (SW ) and nitrogen stress (SN ), the aggregated intensity of water and nitrogen stress (SWN ), the degree of water and nitrogen co-limitation (CWN ), and the integrated effect of stress and co-limitation (SCWN 25 CWN/SWN ). The expectation is that grain yield should be inversely proportional to stress intensity and directly proportional to degree of co-limitation, thus proportional to SCWN . Combination of rotations and seasons generated a wide variation in the amount of water and inorganic nitrogen in the 1-m soil profile at the time of wheat sowing. Plant-available water ranged from 33 to 107 mm, and inorganic nitrogen from 47 to 253 kg N/ha. Larger amounts of nitrogen were found after green-manured faba bean, and smaller after grass pasture. There was a consistent effect of rotation on wheat yield and grain protein content, which persisted in subsequent barley crops. Measured grain yield of wheat crops ranged from 2.5 to 4.8 t/ha. It was unrelated to water or nitrogen stresses taken individually, inversely related to the aggregated stress index SWN , and directly related to the CWN index of co-limitation. The combination of stress and co-limitation in a single index SCWN accounted for 65% of the variation in measured crop yield. This is a substantial improvement with respect to the stress effect quantified with SWN , which accounted for 43% of yield variation. It is concluded that rotation effects mediated by changes in the relative availability of water and nitrogen can be partially accounted for by degree of resource co-limitation.

Size-dependent growth and the development of inequality in maize, sunflower and soybean

Links were investigated between allometry of plant growth and dynamics of size structure of well-fertilized, irrigated crops of soybean (Glycine max L.), sunflower (Helianthus annuus L.) and maize (Zea mays L.) grown at standard plant-population densities (D), as in commercial crops (D = 30, 6 and 8.5 plants m-2, respectively), and at high densities (2D). Patterns of size-dependent growth of shoot and seed mass accumulation were distinctly different among species. In soybean and sunflower, non-linear relationships between size and subsequent growth led to strong hierarchical populations in terms of both shoot and seed biomass. Curvilinear (soybean) and sigmoid (sunflower) size-dependent growth determined strongly asymmetrical (soybean) and bimodal (sunflower) frequency distributions of shoot biomass indicating predominantly size asymmetrical competition among individuals. In comparison, a lower plant-to-plant variation coupled with a typical linear allometry of growth to plant size indicated symmetrical two-sided plant interference in maize. Despite the weak development of hierarchies in shoot biomass, a strong inequality in reproductive output developed in crowded populations of maize indicating an apparent breakage of reproductive allometry.

Influence of size of rainfall events on water-driven processes. II. Soil nitrogen mineralisation in a semi-arid environment

Power laws describe the relationships between the number N (s) and the size s of daily rainfall events, i.e. N (s) ~ s–τ, with higher τ corresponding to sites or seasons with greater frequency of small rainfall events. This paper tested the hypothesis that the rate of soil nitrogen mineralisation increases with increasing exponent τ, as affected by both spatial and temporal sources of variation. Rates of nitrogen mineralisation in an uncropped sandy loam soil were calculated using a simulation model with detailed nitrogen and water balances, and long-term weather data for 6 Australian locations in a range of annual rainfall from 260 to 360 mm. Daily rates of mineralisation were calculated using actual rainfall, and variable or fixed temperature and evaporative demand. The annual pattern of mineralisation rate, calculated as a function of rainfall and variable temperature and evaporative demand, was bimodal with peaks in April and November. These peaks disappeared and differences among locations were reduced when the effects of temperature and evaporative demand were removed. Under constant temperature and evaporative demand, mineralisation rates between April and November were 68% greater than rates between December and March. In the former period, characterised by a high frequency of small rainfall events, monthly mineralisation rate was a direct function of the amount of rainfall. In contrast, mineralisation was independent of the amount of rainfall during the period of larger, less frequent rainfall events from December to March. Parameter τ accounted for 75% of the variation in mineralisation rate in the period December–March and it also accounted for a substantial part of the variation between periods.

Canopy nitrogen distribution and the photosynthetic performance of sunflower crops during grain filling - a quantitative analysis

Measurements of the profiles of leaf area and leaf nitrogen were made on five occasions from midflowering to maturity (53, 61, 70, 78 and 83 days after emergence, DAE) in sunflower crops grown at contrasting density (2.4 and 4.8 plants m^-2) and nitrogen supply (0 and 5 g N m^-2 at emergence) in the summer in Buenos Aires, Argentina. As the crops matured, nitrogen was withdrawn unequally from all leaf positions and leaves senesced from the bases of the canopies. A model was used to estimate the daytime net photosynthesis (P_c) of canopies of defined leaf area and nitrogen content under the observed conditions of temperature and irradiance. Comparisons were made between the observed profiles of leaf nitrogen and those that would maximise P_c (the optimal profiles). The observed nitrogen profiles were sub-optimal at mid-flowering, except in the low-density, low-nitrogen treatment. The differences were most marked in the high-nitrogen treatments which held 'excessive' nitrogen in their lower canopies. As the canopies matured and nitrogen was mobilised to the grain, leaf area index and total nitrogen content decreased and optimal profiles changed shape from exponential to linear. During this period observed profiles became more optimal. There was, however, little difference in P_c between observed and optimal profiles. The maximum difference was 3.2% observed in the low-density, high-nitrogen treatment at DAE 53. The comparison of actual and optimal profiles as leaf nitrogen content (mg N) in addition to the more commonly used specific leaf nitrogen (SLN, g N m^-2 leaf) explains this result because relatively large changes inSLN in the small leaves at the top of canopies have little effect on P_c. The study shows that leaf nitrogen content is an appropriate basis for comparison of canopy nitrogen profiles in sunflower.