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Summary1 The 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. 2 We 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. 3 Litter 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 seedsoil contact) showed reduced seedling emergence and growth. In the field, litter did not have a direct effect but emergence was promoted by burial. 4 Integrating 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. 5 Inconsistent 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.

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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.

1153 RESEARCH S oybean [Glycine max (L.) merr.] seed yield has been increasing between 0.5 and 1% per year during the last 20 years in the United States (Specht et al., 1999). Approximately 50% of this increase is due to genetic improvement while the other 50% is attributed to better management practices (Duvick, 2005;Egli, 2008;Specht et al., 1999). Future yield increases must continue to come, at least partially, from genetic improvement.A more rapid rate of genetic gain could be achieved through increased interaction between crop physiologists and plant breeders (Cooper and Hammer, 2005;Duvick, 2005;Hammer et al., 2006). Traditional breeding of autogamous species is an empirical ABSTRACT 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.

Over the last decade, society witnessed the largest expansion of agricultural land planted with drought tolerant (DT) maize (Zea mays L.) 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.

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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 (A ) and leaf nitrogen (LN ) for cultivated and wild species. The relationship between A and LN was evaluated using a frontier analysis (90 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.