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Biological N2 fixation (BNF) and mineral soil absorption are complementary N sources for soybean growth. BNF is sensitive to nutrient and environmental conditions, though a comprehensive synthesis of the studies underlying these conclusions is currently lacking. Our objective was to conduct a meta-analysis of nutrient and environmental effects on BFN. Articles reporting manipulative treatments and BNF estimations were compiled, and data and metadata were extracted. N fertilization reduced BNF ∼70% relative to the unfertilized control in greenhouse experiments and ∼44% in field experiments. This effect was higher for vegetative than for reproductive stage applications. Fertilization with other nutrients stimulated BNF relative to the unfertilized treatments. Water stress reduced BNF 40% relative to the unstressed control. The negative impact of water stress was larger when stress was applied at vegetative (-70%) compared to reproductive (-30%) stages. Flooding reduced BNF by 40% relative to the non-flooded control with highest effect when applied during vegetative stage (-82%). Increased temperature reduced BNF nitrogenase activity. Carbon dioxide enrichment has a stimulatory effect on BNF. With the exception of N fertilization, soybean and Rhizobium fitness showed a high positive correlation across nutrients and environmental factors. This work summarizes for the first time the impact and relative response of BNF to different nutrient and environmental factors. Results showed an initial critical stage (i.e. vegetative stage) for BNF that is common to most environmental factors negatively affecting BNF. This suggests that common strategies might exist to increase BNF that are independent of the specific identity of the stressor. Also, the positive correlation between host and Rhizobium fitness suggest that strategies to increase BNF potential are the same that those required to increase crop performance. We observed publication bias for some variables, suggesting that effect sizes might be inflated compared to true effect sizes. The only way to solve this problem is to find venues for publication of no significant results when sound experimental approaches and reasonable statistical power can be proved.

Soybean production is challenged to increase yield while maintaining seed protein concentration levels. Variability in protein concentration has been poorly described in many regions, and management options targeting their levels are not available. Our objectives were (i) to describe soybean seed protein in the central production systems of Argentina, (ii) to explore spatial patterns across the region, (iii) to quantify the importance of management and environment on protein concentration, and (iv) to explore correlations between seed protein concentration, seed yield, and total canopy N uptake. We sampled 1721 farm fields across the region, and conducted 52 cultivar trials from 2012 to 2016. Average seed protein concentration was 36.6 and 37.6% for soybean grown as a single crop in the season and as a second crop after a winter one, respectively. Seed protein concentration showed a spatial pattern in soybean as single crop, allowing us to identify well-delimited areas with contrasting protein values. Regression trees of farm fields explained 38 and 48% of total variation in seed protein for soybean as single and second crop, respectively. Relative to this explained variation, management variables accounted for ∼73% of seed protein concentration variation in both crops, where cultivar selection was the most relevant (71.5 and 68.9% for soybean as single and second crop, respectively), followed by planting date (2.5%), and maturity group (1.8%). Despite its minor importance compared to management practices, temperature and rainfall were also associated to protein variability. The response to temperature differed between soybeans as single and second crop. Reduced rainfall was associated to increased seed protein concentration only for soybean as a second crop. Cultivar trials identified consistent high seed protein cultivars. Sites with high yield and quality were evident, but results showed clear cultivar trade-off correlations between yield and protein concentration. Results highlight the need to investigate ways to improve total canopy N uptake for allowing both yield and seed protein to increase simultaneously.

Fil: Di Mauro, Guido. Universidad Nacional de Rosario; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

ABSTRACTThe timing of crop development has significant impacts on management decisions and subsequent yield formation. A large intercontinental dataset recording the timing of soybean developmental stages was used to establish ensembling approaches that leverage both knowledge-based, human-defined models of soybean phenology and data-driven, machine-learned models to achieve accurate and interpretable predictions. We demonstrate that the knowledge-based models can improve machine learning by generating expert-engineered features. The collection of knowledge-based and data-driven models was combined via super learning to both improve prediction and identify the most performant models. Stacking the predictions of the component models resulted in a mean absolute error of 4.41 and 5.27 days to flowering (R1) and physiological maturity (R7), providing an improvement relative to the benchmark knowledge-based model error of 6.94 and 15.53 days, respectively, in cross-validation. The hybrid intercontinental model applies to a much wider range of management and temperature conditions than previous mechanistic models, enabling improved decision support as alternative cropping systems arise, farm sizes increase and changes in the global climate continue to accelerate.

Drought is currently a major constraint to soybean [Glycine max (L.) Merr.] production worldwide and is becoming more widespread due to increased aridity and warmer temperatures in the context of global climate change. In this context, breeding for soybean varieties more tolerant to drought stress is critical and requires efficient screening techniques. To find traits associated with drought tolerance at the vegetative stage that are still present at the reproductive stage, we evaluated morphological, physiological, and biochemical traits in two soybean genotypes contrasting in their response to drought stress. Under drought stress at the vegetative stage, the tolerant A 5009 RG genotype showed higher proline and chlorophyll contents, and early activation of the enzymatic antioxidant system compared with well-watered plants. On the other hand, the sensitive ADM 50048 genotype increased malondialdehyde (oxidative damage marker) and nonenzymatic antioxidant response under stress. Manipulative field assays under contrasting levels of water availability at the reproductive stage mimicked the biochemical patterns observed in the greenhouse tests for the sensitive and tolerant genotypes. A principal component analysis of parameters from vegetative and reproductive stages revealed proline and chlorophyll contents as drought tolerance traits in soybean. We found those traits useful to classify 14 genotypes from the Instituto Nacional de Tecnologia Agropecuaria (INTA) germplasm bank, identifying two new drought-tolerant genotypes (PI 548510 and PI 200492). We propose proline and chlorophylls as useful tools to classify soybean genotypes according to their drought responses in early developmental stages, potentially reducing breeding times. Fil: Guzzo, Maria Carla. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Unidad de Estudios Agropecuarios - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Unidad de Estudios Agropecuarios; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Instituto de Fisiología y Recursos Genéticos Vegetales; Argentina Fil: Costamagna, Carla Antonella. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Unidad de Estudios Agropecuarios - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Unidad de Estudios Agropecuarios; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Instituto de Fisiología y Recursos Genéticos Vegetales; Argentina Fil: Salloum, Maria Soraya. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Unidad de Estudios Agropecuarios - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Unidad de Estudios Agropecuarios; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Instituto de Fisiología y Recursos Genéticos Vegetales; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias; Argentina Fil: Monteoliva, Mariela Inés. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Unidad de Estudios Agropecuarios - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Unidad de Estudios Agropecuarios; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Instituto de Fisiología y Recursos Genéticos Vegetales; Argentina. Oklahoma University; Estados Unidos Fil: Luna, Celina Mercedes. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Unidad de Estudios Agropecuarios - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Unidad de Estudios Agropecuarios; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigaciones Agropecuarias. Instituto de Fisiología y Recursos Genéticos Vegetales; Argentina

Over the last decade, society witnessed the largest expansion of agricultural land planted with drought tolerant (DT) maize (Zea maysL.) Dedicated efforts to drought breeding led to development of DT maize. Here we show that after two decades of sustained breeding efforts the rate of crop improvement under drought is in the range 1.0-1.6% yr−1, which is higher than rates (0.7% yr−1) reported prior to drought breeding. Prediction technologies that leverage biological understanding and statistical learning to improve upon the quantitative genetics framework will further accelerate genetic gain. A review of published and unpublished analyses conducted on data including 138 breeding populations and 93 environments between 2009 and 2019 demonstrated an average prediction skill (r) improvement around 0.2. These methods applied to pre-commercial stages showed accuracies higher that current statistical approaches (0.85 vs. 0.70). Improvement in hybrid and management choice can increase water productivity. Digital gap analyses are applicable at field scale suggesting the possibility of transition from evaluating hybrids to designing genotype x management (GxM) technologies for target cropping systems in drought prone areas. Due to the biocomplexity of drought, research and development efforts should be sustained to advance knowledge and iteratively improve models.HighlightCrop improvement rate in maize increased after implementation of drought breeding efforts. Harnessing crop, quantitative genetics and gap models will enable the transition from genetic evaluation to crop design.

The main physiological processes associated with soybean [Glycine max (L.) Merr.] genetic yield progress in central temperate Argentina are largely unknown. This knowledge is critical to identify opportunities to accelerate yield gains via trait‐based hybridization. Our objectives were to: (a) evaluate the influence of biomass accumulation vs. harvest index (HI) in explaining genetic progress, and (b) assess the role of radiation and/or N capture and use efficiency (RUE and NUE, respectively) as drivers of biomass accumulation. We tested 173 cultivars released from 1980 to 2014 in two high‐yielding environments. Additionally, a crop modeling exercise was performed to demonstrate the physiological perception that any genetic increase in RUE would only translate into more yield if there is enough water for the realization of that RUE. Observed genetic progress was 42 kg ha−1 yr−1, or ∼1% yr−1, and was mostly explained by increased aboveground biomass accumulation. This higher biomass of modern cultivars was associated with increased RUE and total N uptake. This suggests that, if residual genetic variation is still present in current soybean cultivars, future genetic improvements should focus on further improving N uptake to increase RUE. Increases in RUE are associated with increased stomatal conductance and water use. Therefore, it would be expected that genetic progress is faster in environments with increased rainfall. Our modeling exercise was consistent with this hypothesis and showed that soybean genetic progress simulated in different locations within a rainfall gradient was positively associated with cumulative seasonal precipitation.

Assessing yield gap (Yg) is required to identify opportunities for future yield increases. Central Argentina is one of the most productive soybean regions in the world. In this region, soybean is planted after a winter fallow period (from now on soybean as single crop) or after the harvest of a winter crop (from now on soybean as second crop). Information regarding options for obtaining even higher yields is limited. The objectives of this paper are: i) to estimate Yg of soybean as single or second crop, ii) to identify management and environmental variables associated with soybean Yg variability, and iii) to assess the spatial distribution of soybean Yg. A farmers’ survey with ∼22,500 field observations from 2003 to 2015 was compiled. Water-limited yield potential (Ywlim) was estimated as the 95th percentile of actual farmers’ yield (Ya) across years. Yield gap was the difference between Ywlim and Ya, expressed as a percentage of Ywlim. Factors associated with Yg were evaluated using regression trees. Ordinary kriging was used to explore spatial patterns of Yg. Average Ywlim were 5095 and 4337 kg ha−1 for single and second crop, respectively. Average Yg were 28.7 and 33.5% for single and second crop, respectively. Yield gap showed a wide range of variation. Management accounted for 66 and 91% of explained variation in Yg for single and second crop, respectively. Gap closing for single crop was associated with earlier planting and maize as previous crop. Gap closing for second crop was associated with foliar fungicide utilization, P fertilization, and earlier planting. Single crop Yg was spatially auto-correlated, whereas no auto-correlation was observed for second crop. The spatial structure of single crop was represented by an exponential model, with 81% of total variation explained by the spatial structure and a maximum range of auto-correlation of approximately 120 km. This result is consistent with the observed spatial auto-correlation of variables explaining Yg in single crop. Our approximation allowed the characterization of the magnitude, possible explaining factors, and spatial dependence of soybean Yg in one of the most productive regions in the world. Although average gaps are relatively small compared to those in other regions, there are still opportunities for future yield improvements. Fil: Di Mauro, Guido. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Cipriotti, Pablo Ariel. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Métodos Cuantitativos y Sistemas de Información; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Gallo, Santiago. Asociación Argentina de Consorcios Regionales de Experimentación Agrícola; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina

Plant density and row spacing are management practices that farmers use to maximize soybean (Glycine max (L.) Merr.) yield. Other canopy aspects related to seedling establishment like spatial (spacing among plants) and temporal (timing of plant emergence) within-row plant-to-plant variability have received less attention. Negative yield effects of non-uniform stands have been reported for maize (Zea mays subsp. Mays L.), but it is commonly accepted that soybean plants compensate for any plant-to-plant growth difference. Planting quality is becoming relevant because small yield effects can have a high economic return due to high soybean prices and low implementation costs. The objectives were to assess the impact of non-uniform spatial and temporal seedling establishment on soybean yields and to identify putative mechanisms. Results showed that, across planting dates and plant densities, there was no yield loss due to increased spatial plant-to-plant variation in a later maturity group cultivar (MG IV), but reduced yield in an earlier one (MG III). Contrarily, non-uniform temporal distributions significantly reduced yield across cultivars. Having a poor spatial distribution had no effect on the average plant growth rate or its plant-to-plant variability. A poor temporal distribution did not affect individual average plant growth rate but variability was increased. This increased variability determined that some individuals had higher plant growth but reduced seed number because of reduced reproductive partitioning. Novel findings can be summarized as: (i) the higher relative importance of temporal vs. spatial non-uniform canopies in determining soybean yield reductions, (ii) the influence of reduced seed set at higher plant growth rates to mechanistically explain yield reductions in non-uniform temporal canopies; (iii) the concept that the yield effect of non-uniform temporal canopies cannot be overcome by increased plant densities. Future research needs to better understand the interactions between cultivar reproductive characteristics and susceptibility to non-uniform temporal canopies. Fil: Masino, Alejandra. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Buenos Aires; Argentina Fil: Rugeroni, Pablo. Syngenta Agro; Argentina Fil: Borras, Lucas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina

Biological nitrogen fixation (BNF) in soybean [Glycine max (L.) Merr.] represents, on average, 60% of total nitrogen (N) uptake. Nitrogen dilution curves link aboveground crop N concentration (%N) to biomass accumulation (W). It has been reported that BNF is an energy-intensive process that might reduce biomass production per unit of captured N (physiological N use efficiency or NUE). This increased energy cost could lead to a more attenuated N (i.e. less efficient) dilution curve. However, there are no reports comparing N dilution curves for soybean crops differing in N source. Our objectives were to: (i) evaluate the impact of BNF on soybean N dilution curves and how it influences NUE, and (ii) establish independent N dilution curves for soil and atmospheric N. Our working hypothesis is that relying on BNF attenuates the N dilution curve and reduces NUE. The experiment consisted of a control and a fertilized treatment, 0 and 600 kg N ha−1 respectively, applied to four soybean genotypes in order to establish two differential BNF situations. While the control and fertilized treatments had differential N accumulation from BNF, ∼70% and ∼16%, respectively, there were no differences observed in seed yield (∼5000 kg ha−1), NUE (∼36 kg kg−1) and only slight differences in total N uptake (∼365 kg N ha−1 in fertilized treatment compared to ∼389 kg h−1 in the control treatment). Results suggest that reliance on BNF for N does not influence substantially the attenuation of the N dilution curve and has no impact on NUE. The N dilution parameter (“b”) ranged from −0.128 to −0.218 among cultivars and fertilization treatments. The less negative values (more attenuated curve) corresponded to the fertilized plots likely associated with luxury N consumption. Interestingly, dilution curves from soil mineral N showed the typical dilution pattern, while N derived from the atmosphere followed a concentration pattern as the crop developed. This most likely reflects the continuous N flux from BNF to the plant as opposed to the decreasing soil mineral N supply. Recognizing these concentration/dilution curves for atmospheric and soil N has three immediate implications. First, the atmospheric N concentration curve might indicate an upper benchmark for evaluating symbiosis performance during crop development. Second, the concentration pattern observed for BNF could potentially help to reverse the observed decline in seed protein concentration in modern soybean cultivars. Third, the N concentration/dilution curves for the individual N sources could be incorporated into crop models for estimating BNF at different crop biomass levels during soybean development. Fil: Santachiara, Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias; Argentina Fil: Salvagiotti, Fernando. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Santa Fe. Estación Experimental Agropecuaria Oliveros; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Gerde, Jose Arnaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina

Estimation of crop nutrient demand, seed nutrient removal, and nutrient use efficiency (yield to nutrient uptake ratio) are crucial for pursuing both balanced nutrition and more sustainable farming systems. However, the estimation of the nutrient requirements as the nutrient uptake per unit of seed yields is impaired in many situations due to the narrow variation of the dataset used to obtain these values or by the overgeneralization of considering a constant value for the nutrient demand at varying yield levels. Past studies focused on other crops and using linear models for estimation of the nutrient requirements, but not yet for soybeans (Glycine max L.). The aims of this research study were to: (i) quantify nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) requirements in soybean and (ii) compare linear and non-linear (spherical) models in their relationship between plant and seed nutrient content all relative to seed yield at varying probabilities utilizing quantile regression. A large dataset from different studies conducted between 2009–2018 period, including data of seed yield, total biomass at physiological maturity, and N, P, K, and S uptake. Soybean seed yield ranged from 955 to 6525 kg ha−1, aboveground biomass from 1990 to 15,814 kg ha−1, and harvest index from 0.16 to 0.57. On average, nutrient uptake was 261 kg N ha−1, 25 kg P ha−1, 133 kg K ha−1, and 16 kg S ha−1 (N:P:K:S ratio = 17:1.6:8.5:1), while nutrient content in seeds averaged 191 kg N ha−1, 17 kg P ha−1, 54 kg K ha−1, and 9 kg S ha−1 (N:P:K:S ratio = 21:1.8:5.8:1). The spherical model described better than the linear model the relationship between plant nutrient uptake or nutrient content in seeds with seed yield in soybean, and thus, nutrient requirements per unit of yield decreased as seed yield increased. A relationship between nutrient internal efficiency and seed yield for the different percentiles as determined by the non-linear quantile regression offered probabilistic values for estimating nutrient uptake in soybean, providing useful information for obtaining more reliable estimates of nutrient balances at the system-level.

Backgrounds and aims: Soybean yield depends on total N uptake, N use efficiency, and harvest index. Nitrogen uptake relays on biological fixation (BNF) and soil absorption. Usually, BNF is considered a yield-related process. However, there is limited information on whether maximizing percent BNF (%BNF) is actually required to maximize N uptake and yield. Methods: Seventy cultivars were evaluated for total N uptake, N use efficiency, and harvest index. Biological N fixation was determined in a subset of cultivars. The harvest index of N derived from atmosphere and from soil was also assessed. Results: Yield was positively associated with total N uptake. Highest N uptake was not linked to increased %BNF. An inverse relationship between the amount of BNF (kgBNF) and soil N absorption was observed. Harvest index of N derived from BNF was 85%, while it was 77% for N derived from soil. Conclusions: Highest total N uptake was attained by different combinations of kgBNF and mineral soil N absorption. This showed that maximizing %BNF is not required to maximize yield. High %BNF played a pivotal role in determining neutral soil N balance. This is so even though N derived from BNF was more partitioned to seeds than N derived from soil. Fil: Santachiara, Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Borras, Lucas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Salvagiotti, Fernando. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Santa Fe; Argentina Fil: Gerde, Jose Arnaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina

Soybean [Glycine max (L.) Merr.] maturity groups (MGs) exhibit large variation in time to maturity, and are adapted to different latitudes. There is a range of MGs that have similar yield potential at most production regions, especially at temperate and tropical environments. We tested whether similar yields in contrasting MGs are achieved through different physiological processes. Our objectives were: (i) to characterize biomass accumulation, reproductive partitioning and seed set efficiency, and (ii) to analyze the role of N and radiation in biomass accumulation during the seed set period in contrasting MGs. Three MG III and V cultivars were tested in two growing environments. No significant yield or seed number differences were detected despite MG V had 20 d longer growth cycle. MG III had a longer seed set period (R1–R5), received more incident radiation during that period, and captured more N compared to the MG V. However, these cultivars had lower nitrogen use efficiency (NUE) (g biomass per g N uptake) and less radiation interception efficiency compared to MG V. These contrasting physiological processes determined similar biomass accumulation during R1 to R5. No differences between MGs were observed in biomass partitioning to reproductive structures and seed set efficiency during this period. Results showed contrasting MGs have different strategies for reaching similar yield, evidencing physiological processes that could be specifically tailored by breeders for either MG III (radiation interception efficiency, NUE) or MG V (duration of R1–R5 period, N uptake rate) cultivars for further yield increases. Fil: Santachiara, Gabriel. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Departamento de Producción Vegetal; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Borras, Lucas. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Departamento de Producción Vegetal; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rotundo, José Luis. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Departamento de Producción Vegetal; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Negative impacts of increased temperature on maize yield are anticipated using simulation models. However, some temperature functions are parameterized with partial information. There is limited information on photosynthesis response to high temperature in modern maize hybrids. Improved photosynthesis-temperature functions are key for realistic yield simulations. Our experiment was aimed at building a functional relationship between photosynthesis and air temperature exploring temperature ranges relevant for global warming simulations. Maize hybrids from cold, temperate, and subtropical regions were included in the study to assess genetic adaptation. Results showed a trilinear response to temperature with an optimum of 40 °C. No genetic adaptation was observed among the diverse set of hybrids evaluated. Results contrast with common temperature-limiting functions indicating a decline in carbon assimilation above 30–33 °C. Our results suggest possible overestimations of negative impacts of global warming on maize yield due to the use of inadequate response functions relating carbon assimilation to temperature.

Maize (Zea Mays L.) yield responsiveness to nitrogen (N) fertilization depends on the yield under non-limiting N supply as well as on the inherent productivity under zero N fertilizer (Y0). Understanding the driving factors and developing predictive algorithms for Y0 will enhance the optimization of N fertilization in maize. Using a random forest algorithm, we analyzed data from 679 maize N fertilization studies (1031 Y0 observations) conducted between 1999–2019 in the United States and Canada. Predictability of Y0 was assessed while identifying determinant factors such as soil, crop management, and weather. The inclusion of weather variables as predictors improved the model efficiency (ME) from 51 up to 64 %, and reduced the root mean square error (RMSE) from 2.5 to 2.0 Mg ha−1, 34 to 27 % in relative terms (RRMSE). The most relevant predictors of Y0 were previous crop, irrigation, and soil organic matter (SOM), while the most influential weather data was linked to the radiation per unit of thermal time (Q quotient) around flowering and spring precipitations. The crop rotation effect resulted in Alfalfa (Medicago sativa L.) as the previous crop with the highest Y0 level (IQR = 11.5–15.0 Mg ha−1) as compared to annual legumes (IQR = 5.6–10.0 Mg ha−1) and other previous crops (IQR = 3.6–7.8 Mg ha−1). The Q quotient around flowering positively affected Y0, while spring precipitations and extreme temperature events during grain filling showed a negative association to Y0. Overall, these results reinforce the concept that yields are controlled not only by soil N supply but also by factors modifying plant demand and ability to capture N. Lastly, we foresee a promising future for the use of machine learning to address both prediction and interpretation of maize yield to obtain more reliable N guidelines.

Genetic progress is assessed to estimate its contribution to on-farm yield increases and to identify traits that have been improved over some period of time. Although Argentina is a major soybean [Glycine max (L.) Merr.] producer, there is limited information about genetic progress in this system. Argentinean soybean cultivars were developed from US commercial cultivars. Because the genetic base of US cultivars is narrow, it would be expected that genetic progress in Argentina to be slower than in the United States. We assessed the genetic gain for yield and related traits in cultivars released in Argentina from 1980 to 2015. One hundred and eighty-one cultivars belonging to maturity groups (MGs) III, IV, and V were evaluated in three environments in the northern pampas from Argentina. Genetic gain in yield was 43 kg ha−1 yr−1 and was not different across MGs. Relative genetic gain was 1.1% yr−1, similar to reports from the United States or Brazil. Newer cultivars from MGs III and IV had increased days to maturity, while cultivars from MG V showed the opposite trend. Vegetative period was also reduced in newer cultivars from MGs IV and V. Seed protein concentration was reduced over the years. Genetic progress explained 50% of total on-farm yield increase. Results from this experiment showed that breeding programs in Argentina were able to attain a similar genetic gain to the United States even though the starting parents were only a few US cultivars selected from an already narrow genetic base. Fil: de Felipe, Matías. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Gerde, Jose Arnaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina

Soybean seed oil concentration and fatty acid (FA) composition are relevant traits defining oil quality and economic value for industry. In spite of the importance of FA composition for soybean value, a detailed description of FA profile and accumulation during seed filling in contrasting soybean varieties and management conditions is currently lacking. This study aims to: i) evaluate whether genetic progress in soybean yield indirectly affected FA profile, ii) assess the influence of N fertilization on FA profile, iii) characterize the temporal dynamics of FA accumulation during seed filling, and iv) evaluate the relative contribution of FA accumulation rate vs. duration on final FA seed content. Seven soybean varieties released between 1980 and 2013 were field grown under control (0 kg N ha−1) and fertilized (670 kg N ha−1 added) treatments. In addition to seed yield, total seed oil and FA contents during seed filling were recorded by sampling seeds from the beginning of seed formation to maturity. Genetic progress was evident in seed yield with an increase of 31.3 kg ha year−1. Total oil concentration (g oil kg−1) slightly decreased across year of release for the control. Fatty acid concentration (g FA 100 g oil−1) profile was not associated with year of release but affected by varieties. Variation in total oil and FA contents (mg seed−1) across varieties and N treatments was mostly associated to longer filling but was also affected by accumulation rates. Interestingly, individual FA concentration at maturity was independent from FA content. Our results indicate past breeding efforts have not modified soybean FA profile. The fact that changes in FA contents were not translated in changes in FA concentration indicates that seed control of FA profile is narrow. Disentangling the physiological mechanism associated with FA homeostasis warrants further examination.

High seed protein concentration (HP) in soybeans [Glycine max (L.) Merr.] is attained by increases in protein content in large seed genotypes (HP large seed) or by reductions in oil and carbohydrates contents in small seed genotypes (HP small seed). We hypothesized that these alternative strategies impact seed development, component accumulation, and gene expression differently. We compared a standard protein commercial genotype with two HP genotypes having contrasting seed size. The HP large seed genotype exhibited the fastest rate and longest period of seed growth and reserves accumulation compared with the HP small genotype. Seed development of these contrasting genotypes was normalized using a moisture depletion framework. Expression levels of some of the genes involved in protein and oil synthesis were lower in the HP small seed genotype compared with the other genotypes. No difference in gene expression was observed between the commercial and the HP large seed genotypes, suggesting a role for assimilate supply controlling high protein concentration based on this strategy. Our results indicate that seed development and gene expression are not necessarily associated with high seed protein concentration per se; a better understanding of seed composition requires acknowledging the contrasting strategies, in terms of seed size, to attain high seed protein concentration. Fil: Poeta, Florencia Belen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Ochogavía, Ana Claudia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Permingeat, Hugo Raúl. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Laboratorio de Biología Molecular; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina

Year-to-year variability in field seedling emergence in arid rangelands poses questions about the viability of perennial grasses threatened by overgrazing. There are practical difficulties in estimating long-term seedling emergence for perennial species with erratic behavior (high temporal variability). We present a novel approach combining seed germination studies, modeling, and field experiments to estimate long-term germination probability using Poa ligularis as a perennial grass model. Additional objectives were to evaluate: (i) germination requirements of different P. ligularis populations, (ii) field emergence behavior at two extreme locations within the distributional range in the southern region, and (iii) the influence of microsite or local soil on field emergence. Germination parameters were obtained from 8 populations collected across a 12° latitude transect. Germination parameters were combined with a soil water model to estimate long-term germination (used as a proxy of field emergence) in the southernmost location. Germination requirements differed across populations and correlated with environmental conditions of collection sites, demonstrating ecological significance. Field experiments showed very low emergence. Simulations for the southern location indicated erratic emergence. Fifty percent of the years had nil simulated field emergence and the magnitude of the events, when occurred, was small. The magnitude of these events depended on the population, demonstrating an additional genetic constraint. Erratic field emergence arises from the mismatch between environmental conditions and germination requirements. Knowing favorable climatic conditions triggering seed germination (and thus emergence) events are key for opportunistic management of species with this erratic behavior.

Crop production increase needed to satisfy a growing world population depends, at least partially, on increasing current genetic gain in yield. Theory proposes that increased genetic gain can be attained using diverse high-yielding parents. Physiological traits, compared to molecular or morphological markers, are hypothesized to better estimate parental diversity. A trait-based hybridization approach will require assessing diversity for physiological traits. Here, we phenotyped several traits using two physiological frameworks to assess diversity in 25 and 65 elite cultivars from Argentina (ARG) and the United States (USA), respectively. First, we identified the highest-yielding cultivar clusters across two environments in each country. These cultivars had the highest N uptake at both ARG and USA. Therefore, there was no genotypic diversity for total N uptake within each cluster. For other traits, the highest yielding clusters did not show the highest values. There was residual diversity within ARG and USA highest yielding clusters in the temporal pattern of N uptake, N use efficiency, and N harvest index. Stacking these traits in one cultivar could potentially increase yield by 13%. The possibility of such stacking, however, depends on the nature of the phenotypic correlation among traits. We demonstrated that several trade-off correlations between phenotyped traits, thought to hinder stacking, are actually not biologically based Fil: Rotundo, José Luis. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Borras, Lucas. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: De Bruin, Jason. University of Iowa; Estados Unidos Fil: Pedersen, Palle. University of Iowa; Estados Unidos

Developing high protein (HP) cultivars is often precluded by the inverse relationship between protein and yield. We hypothesized that attaining HP concentration based on contrasting seed size impacts crop growth and development differently. We screened 97 soybean genotypes and found lines with HP concentration (~450 g kg−1) associated with (i) increased protein content (mg seed−1) in large seed genotypes, and (ii) reduced oil and carbohydrate contents in small seed ones. Then, we evaluated different growth traits in a subset of three HP large and three HP small seed genotypes, as well as in three high-yielding genotypes with average seed size and protein concentration. High-yielding genotypes showed higher leaf area duration and harvest index when compared with HP genotypes, regardless of seed size. High protein large seed was associated with more assimilate availability per seed during seed filling, while HP small seed showed higher leaf area at the beginning of seed fill, more canopy biomass production, and very low levels of assimilate per seed. Results show that selecting for seed protein concentration can impact crop growth and development differently, depending on the strategy used for selection in terms of seed size. These findings, if utilized for parental selection, might contribute to eliminating negative correlations between seed protein and yield, since these strategies may be under different genetic control and/or determine different biophysical constraints. Fil: Poeta, Florencia Belen. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Departamento de Producción Vegetal; Argentina Fil: Borras, Lucas. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Departamento de Producción Vegetal; Argentina Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Departamento de Producción Vegetal; Argentina

The relationship between leaf photosynthesis and nitrogen is a critical production function for ecosystem functioning. Cultivated species have been studied in terms of this relationship, focusing on improving nitrogen (N) use, while wild species have been studied to evaluate leaf evolutionary patterns. A comprehensive comparison of cultivated vs wild species for this relevant function is currently lacking. We hypothesize that cultivated species show increased carbon assimilation per unit leaf N area compared with wild species as associated with artificial selection for resource-acquisition traits. - We compiled published data on light-saturated photosynthesis (Amax) and leaf nitrogen (LNarea) for cultivated and wild species. The relationship between Amax and LNarea was evaluated using a frontier analysis (90th percentile) to benchmark the biological limit of nitrogen use for photosynthesis. - Carbon assimilation in relation to leaf N was not consistently higher in cultivated species; out of 14 cultivated species, only wheat, rice, maize and sorghum showed higher ability to use N for photosynthesis compared with wild species. - Results indicate that cultivated species have not surpassed the biological limit on nitrogen use observed for wild species. Future increases in photosynthesis based on natural variation need to be assisted by bioengineering of key enzymes to increase crop productivity. Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Investigaciones en Ciencias Agrarias de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Instituto de Investigaciones en Ciencias Agrarias de Rosario; Argentina Fil: Cipriotti, Pablo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

Soybean has a narrow genetic base thought to limit future yield genetic gains. However, there is no evidence whether this reduction in genetic diversity correlates with diversity loss for any yield trait. We tested how photosynthetic nitrogen use efficiency (leaf photosynthesis per unit nitrogen, NUEp) evolved from the wild relative Glycine soja Siebold & Zucc. to the current Glycine max (L.) Merr. Five populations resulting from different evolutionary bottlenecks were evaluated under field conditions. Populations were wild ancestors, domesticated Asian landraces, North American ancestors, and modern cultivars. Genotypic differences in photosynthesis and leaf nitrogen were evident, creating a significant 3-fold variation in phenotypic NUEp. There was a parallel reduction in molecular marker and phenotypic NUEp diversity after each evolutionary bottleneck. G. soja had three times more NUEp diversity and 25% more average NUEp compared with the elite modern cultivars. Two strategies for increasing NUEp were identified: (i) increases in light saturated photosynthesis (Pmax), and, alternatively, (ii) reductions in leaf nitrogen. A modelling approach showed that NUEp will increase yield only if based on increased Pmax. Our study quantified the genetic potential of exotic germplasm available for trait-directed breeding. Results antagonise the concept that elite germplasm is always superior for any relevant yield trait when compared with undomesticated germplasm. Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias; Argentina Fil: Borras, Lucas. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias; Argentina

Soybean [Glycine max (L.) Merr.] seed composition data are becoming critical for efficient marketing and trade of soybean and soybean products. Previous reports of variation in seed protein and oil across regions within the United States focused on variation associated with state and regional boundaries. We conducted an analysis of an 8-yr survey of seed protein and oil across US states and regions, including a geostatistical approach to better characterize the continuous variation in seed composition on a regional scale. The objectives were (i) to determine the relative importance of temporal and regional variation, (ii) to explore the extent of spatial variability across years, (iii) to evaluate the temporal stability across regions, and (iv) to explore the negative correlation between protein and oil across regions and years. Our results confirmed previous findings showing higher protein concentration in southern states and regions. However, most of the observed variation occurred at scales below these political boundaries. The geostatistical approach indicated a moderate level of spatial dependency for protein and protein plus oil but low spatial autocorrelation for oil. In all cases, year-to-year variation in weather conditions modulated the expression of regional spatial patterns in composition. To fully predict seed composition at a regional scale would require additional information associated with weather and agronomic management. Fil: Rotundo, José Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias; Argentina Fil: Miller Garvin, Jill E.. University of Minnesota; Estados Unidos Fil: Naeve, Seth L.. University of Minnesota; Estados Unidos

There is evidence that soybean yield needs to increase at faster rates to satisfy a growing demand. Because yield is mostly determined by the number of harvested seeds, we used a seed number determination framework for phenotyping a large set of cultivars. This framework incorporates biomass accumulation during the seed set period, biomass partitioning to reproductive structures and seed set efficiency per unit of accumulated reproductive biomass. Our objectives were: (i) identify clusters of cultivars differing in seed number across environments, (ii) describe average differences among clusters for the parameters that determine seed number, (iii) evaluate parameter variation within the highest yielding clusters. Objectives were independently tested at central USA and central Argentina (ARG) using cultivars from each region (61 at USA and 25 at ARG). Clusters differing in seed number across environments were identified. Seed number was associated with rapid growth rates (USA only), increased partitioning and greater seed set efficiency. In spite of these differences between clusters, there was significant residual variation among genotypes within highest yielding clusters for most physiological parameters. Our study provided evidence there is no unique physiological pathway for achieving high yields in either environment. This indicated developing targeted segregating populations using high yielding parents with contrasting physiological strategies is feasible. It provides an opportunity for testing yield increases using ideotype construction by pyramiding desirable physiological parameters.

The concentrations of protein and oil in soybean (Glycine max L.) seeds are important factors determining the quality of soybean meal and its competitiveness in international markets. Studies aimed at understanding genetic factors and/or environmental conditions that distinguish high levels of protein and oil accumulation from those that result in deficient levels typically analyze mature seeds. This approach inevitably confounds maternal, zygotic, and environmental effects on seed component accumulation. We have attempted to resolve these effects by manipulating assimilate supply per seed and quantifying the impact directly on the rate and duration of protein, oil, and residual (predominately carbohydrate) accumulation. Genotypes varying in final seed protein concentration were grown in the field and exposed to treatments to increase (depodding) or decrease (shading) assimilate supply per seed during seed filling. Depodding increased the rate of protein, oil, and residual accumulation. Protein accumulation was most responsive, resulting in a dramatic increase in final protein concentration, particularly in the low-protein genotypes. Shading decreased oil accumulation with less impact on protein and residual content, leading to a reduction in final oil concentration. Rates of protein accumulation explained most of the variation in final contents rather than duration. For each seed component, a single hyperbolic function described the relationship between assimilate supply per seed and the rate of component accumulation for all genotypes and treatments. These results underscore the importance of evaluating genetic and environmental effects directly on seed component accumulation, and confirm the predominant influence of assimilate supply per seed on soybean seed composition. Theoretical modeling showed that further increases in seed protein concentration cannot rely on more assimilates supply and that future increases will depend on increases in resource use efficiency and/or changes in partitioning.

Early planting (late April to early May) is recommended for increasing soybean [Glycine max (L.) Merr.] yield but a full understanding of the physiological response is lacking. Carbon dioxide exchange rate (CER) was measured on soybean planted between 18 April and 29 May to determine if CER could resolve this yield difference. During the season CER was measured twice weekly on individual leaves. Total carbon fixation, dry matter accumulation, and radiation use efficiency (RUE) were calculated for the growth periods R1 to R3 and R3 to R6. During the early vegetative stages CER (before flowering) declined for all planting dates and reached a minimum of 0.566 (late planting) to 0.629 (optimal planting) mg CO2 m−2 s−1 by approximately R2 (late flowering) followed by a linear rate increase to a maximum of 1.23 mg CO2 m−2 s−1 by R5 to R6 (early seed fill). The optimal first week of May planting increased main-stem node number, average CER during R1 to R6, total carbon fixation, and dry matter but did not increase seed yield. Overall, yield and seed number were positively related to CER between R3 and R6. These data demonstrate that (i) CER was controlled by plant growth stage and was independent of the date that growth stage occurred and (ii) early planting did not consistently increase the CER rate during the R3 to R6 growth stage, limiting yield improvement. Factors other than CER are more likely to explain the reported positive yield response to early planting.

The soybean cyst nematode (Heterodera glycines Ichinohe) is the main yield limiting pathogen of soybean [Glycine max (L.) Merr.] in the USA. Resistant cultivars are the most efficient management tool today. Our research studied the physiological basis of yield differences between H. glycines-susceptible and H. glycines- resistant cultivars developed from the Hartwig, PI 88788, and Peking sources of resistance at two locations in Iowa during 2005 and 2006. Supplementing resistance with chemical control may improve soybean yield and/or nematode control, so nematicide application {aldicarb[2-methyl-2 (methylthio) propionaldehyde O-(methylcarbamoyl) oxime]} was included as an experimental factor. Aldicarb increased total plant biomass by 9% during R1-R5 soybean growth stages, but there was no increase in seed yield. Yields of the resistant cultivars were greater than those of the susceptible cultivars, except for the Peking source. Compared with the susceptible cultivars, cultivars with H. glycines resistance from PI 88788 had a 13% increase in yield associated with a 15% increase in growth during R1-R5. In cultivars with resistance from Hartwig, a 6% increase in yield was associated with a 4% increase in R1-R5 duration and increased seed-set efficiency. This work demonstrates that yield increases due to resistance to H. glycines can be attained by different physiological mechanisms associated with the different resistance sources and probably are controlled by different genes. This opens the possibility of pyramiding genes conferring resistance by different mechanisms. © Crop Science Society of America. Fil: Rotundo, José Luis. University of Iowa; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Tylka, G.L.. Syngenta Crop Protection; Estados Unidos Fil: Pedersen, P.. University of Iowa; Estados Unidos

Lack of adequate soil moisture during seed filling limits seed size in soybean [Glycine max (L.) Merr.], generally by shortening the duration of seed fill. Seed growth rate typically remains largely unaffected. The smaller seeds often have increased seed protein concentration due to less detrimental effect on final protein accumulation relative to other major seed components. Such changes in seed composition despite stable rates of seed filling suggest compensation may occur among component accumulation rates or the duration of accumulation of individual seed components is affected differentially by water stress. We examined these possibilities by evaluating the rates and durations of protein, oil, and residual accumulation in two soybean lines subjected to limited soil moisture during seed filling. Results indicated that both responses (rate and duration of accumulation) to water deficit can occur. The rates of accumulation of the different seed components often compensated, resulting in a constant seed growth rate, typically favoring protein accumulation. To our knowledge, this is the first demonstration of a differential response among rates and durations of seed component accumulation when soil moisture becomes limiting. The relative stability of protein accumulation likely reflects the availability of remobilizable N late in seed filling and the dependence of oil accumulation on current photosynthesis.

Developing soybean cultivars with high seed protein concentration has been hampered by the negative correlation between seed protein and seed yield. While previous in vitro studies have documented the impact of assimilate supply to the seed in determining seed protein, in planta studies generally have failed to link seed protein accumulation directly with assimilate supply per plant during seed filling. It may be possible to reconcile this apparent contradiction by expressing the relationship between seed protein and assimilate supply in planta on a per seed basis. We evaluated the association between seed composition and assimilate supply per seed in closely related experimental lines varying in seed protein concentration and in several elite varieties from the Iowa State University breeding program. High seed protein content was associated with greater leaf area per seed at R5.5, which was a consequence of fewer seeds set per plant. The more favorable source/sink ratio provided greater assimilate per seed during grain filling, but limited the yield potential of the high protein lines because of reduced seed set. Depodding during grain filling increased seed size of low protein lines and increased seed protein concentration to levels comparable to those in the untreated high protein lines. Seed size was far less responsive to depodding in the high protein lines. These results suggest that high protein lines maintain assimilate supply per seed at or near saturating levels during seed filling. Improving seed protein levels in high yielding varieties will require increasing assimilate supply per seed without sacrificing seed numbers.

The value of commodity soybean depends on the concentration of protein and oil in the seeds. While seed composition is primarily genetically determined, environmental conditions during seed development also affect seed component accumulation, and can result in protein and/or oil deficits for processing. To understand the general environmental effects on soybean composition, we conducted a meta-analysis of published data quantifying the effect of water stress, temperature, and/or nitrogen supply on seed protein and oil accumulation and their final concentrations. The meta-analysis showed that water stress reduced the content (mg per seed) of protein, oil and residual seed fractions. Protein accumulation, however, was less affected than were oil and residual accumulation, resulting in an increase in final protein concentration (% dry weight). Growth at high temperature also increased protein concentration in a manner similar to that observed for water stress. But in neither case was the increase in protein concentration due to an increase in protein synthesis per se. Increasing nitrogen supply to seeds cultured in vitro and to plants grown hydroponically increased both final seed protein concentration and content. But the magnitude of seed component response to experimental manipulation under field conditions was far less than that observed in the Uniform Soybean Regional Field Tests. Greater knowledge of the physiological processes that regulate these responses is essential to predict when and where future protein deficits might occur. Limitations of the meta-analysis approach and implications for future research on soybean seed composition are discussed.

Summary 1The recovery of rare, threatened plant populations can sometimes be achieved through modification of the disturbance regime. Accumulation of litter is likely to follow a reduction in grazing pressure and we therefore examine its effects on seedling recruitment of a threatened species in a temperate arid rangeland. 2We studied the effects of litter on seed longevity in the field and performed glasshouse and field experiments with natural and plastic litter to assess their physical and biological/chemical nature. Seeds were sown on the soil surface, buried or within the litter layer. Published data on spatial distribution of seeds were used to calculate the net effect of litter on seedling recruitment. 3Litter increased seed longevity. In the glasshouse, litter increased seedling emergence and growth for surface, but not for buried, seeds. Seeds within the litter layer (no seed-soil contact) showed reduced seedling emergence and growth. In the field, litter did not have a direct effect but emergence was promoted by burial. 4Integrating the effects of microsite quality and seed density showed that litter microsites recruited c. 50% of Bromus pictus seedlings, double that expected from its cover (25%). The positive effect of litter on seed density and on seed longevity outweighed the negative effect of litter acting as a mechanical barrier to burial. 5Inconsistent effects of litter on plant recruitment in arid environments may be due to responses being dependent on seed size and shape, and thus may represent indirect effects acting via the probability of burial. Alternatively, studies may report effects only on seed retention or emergence and growth rather than net effects on establishment.

Seed development is partitioned into a ‘lag’ phase, a ‘seed filling’ phase, and a ‘maturation’ phase. Transitions between phases correspond to seed water concentration (WC) values that are fairly consistent within species. For soybean (Glycine max L.), linear seed filling begins at approximately 85% (WCL) and maximum dry weight is attained at approximately 60% (WCM). While such WC values benchmark the progress of seed development, their utility for establishing onset and duration of individual seed chemical component accumulation is not known. Our objectives were (i) to determine WCL and WCM for seed protein, oil, and residual (mostly carbohydrates), (ii) to assess stability across genotypes and environments, and (iii) to investigate their relationship with the duration of accumulation. The WCL and WCM for oil, protein, and residual were significantly different. Values were higher for residual and lower for oil. Since seeds desiccate throughout their development, residual accumulation was initiated first, followed by protein, then by oil. The parameter WCL was more stable across genotypes than was WCM. Genotypes with lower WCM values had a longer duration of component accumulation. Increasing assimilate supply per seed decreased WCL for all seed components, but had little impact on WCM. Our results indicate that a water relations framework can be used to characterize accumulation patterns of individual seed components across genotypes and environments, providing a common basis for modeling the composition of soybean seeds. Fil: Poeta, Florencia Belen. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Cátedra de Cultivo Extensivos Cereales y Oleaginosas; Argentina Fil: Rotundo, José Luis. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Cátedra de Cultivo Extensivos Cereales y Oleaginosas; Argentina Fil: Borras, Lucas. Universidad Nacional de Rosario. Facultad de Ciencias Agrarias. Cátedra de Cultivo Extensivos Cereales y Oleaginosas; Argentina Fil: Westgate, Mark. University of Iowa; Estados Unidos

Nosotros estudiamos respuestas morfologicas y de crecimiento a la sequia para dos subpoblaciones de Bromus pictus provenientes de comunidades con suelos de distinta disponibilidad de agua en la estepa patagonica. Luego de un proceso de aclimatizacion comun a ambas subpoblaciones, individuos de cada subpoblacion fueron sometidos a tres niveles de disponibilidad de agua durante 44 dias. Independientemente del nivel de disponibilidad hidrica, la subpoblacion proveniente de la comunidad con mayor agua disponible exhibio una mayor tasa de crecimiento relativa aerea (RGR) y mayor tamano foliar (area y peso) comparado con la subpoblacion proveniente de la comunidad con menor agua disponible. Estos resultados prueban la evolucion esperada de mayores tasas de crecimiento y tamanos de hoja en aquellos ambientes ricos en recursos. La subpoblacion proveniente de la comunidad con menor agua disponible tambien exhibio una mayor tasa de macollaje y un habito de crecimiento mas postrado que la subpoblacion proveniente de la comunidad con mayor agua disponible. Un mayor numero de macollos pequenos puede ser una estrategia util para una rapida respuesta a los pulsos de agua y para distribuir el riesgo la muerte inducida por sequias. Un habito de crecimiento mas postrado puede tambien presentar ventajas tales como reducir el area expuesta al aire seco. Los resultados obtenidos demuestran que la heterogeneidad del suelo puede promover variacion genetica, aunque no permitieron detectar interacciones estadisticamente significativas entre los factores vinculados con las poblaciones y la disponibilidad de agua. La variacion genetica encontrada entre las poblaciones puede ser util para desarrollar programas de mejoramiento e incrementar el potencial evolutivo de una especie nativa para adaptarse a cambios ambientales

Question: What is the effect of vertical seed distribution (i.e. seeds on soil surface or buried) and the relative importance of vertical and horizontal seed distribution (i.e. seeds near established vegetation or near bare soil) for seedling emergence? Location: Patagonian arid steppe. Methods: To evaluate the effect of vertical seed distribution on seedling emergence, four sowing treatments were used in field and greenhouse experiments. Vertical vs. horizontal seed distribution and the occurrence of seedlings were analysed in the field. Effects of trampling and mechanical drilling on seedling emergence were investigated experimentally in the field. Results: In field and glasshouse experiments buried seeds showed a higher emergence and larger seedling size than seeds lying on the soil surface. The observational field study indicated that natural emergence was not associated with particular microsites or plant cover (horizontal distribution), but that most seedlings emerged from buried positions (vertical distribution). Buried seeds represented less than 10% of the sampled seed cohort but account for almost 60% of the seedlings. The trampling/drilling experiment showed that sheep trampling and mechanical drilling increased seedling emergence compared to non-treated controls. Conclusions: The population dynamics of Bromus pictus is strongly constrained by the vertical position of seeds, which largely affects emergence, and seed horizontal distribution, which largely affects seedling survival. Our results showed that sheep trampling after seed dispersal may be a low-input technique for increasing grass recruitment.