Stimulation of C3 crop yield by rising concentrations of atmospheric carbon dioxide ([CO2]) is widely expected to counteract crop losses that are due to greater drought this century. But these expectations come from sparse field trials that have been biased towards mesic growth conditions. This eight-year study used precipitation manipulation and year-to-year variation in weather conditions at a unique open-air field facility to show that the stimulation of soybean yield by elevated [CO2] diminished to zero as drought intensified. Contrary to the prevalent expectation in the literature, rising [CO2] did not counteract the effect of strong drought on photosynthesis and yield because elevated [CO2] interacted with drought to modify stomatal function and canopy energy balance. This new insight from field experimentation under hot and dry conditions, which will become increasingly prevalent in the coming decades, highlights the likelihood of negative impacts from interacting global change factors on a key global commodity crop in its primary region of production.

Plants grown in elevated [CO2] have lower protein and mineral concentrations compared with plants grown in ambient [CO2]. Dilution by enhanced production of carbohydrates is a likely cause, but it cannot explain all of the reductions. Two proposed, but untested, hypotheses are that (1) reduced canopy transpiration reduces mass flow of nutrients to the roots thus reducing nutrient uptake and (2) changes in metabolite or enzyme concentrations caused by physiological changes alter requirements for minerals as protein cofactors or in other organic complexes, shifting allocation between tissues and possibly altering uptake. Here, we use the meta-analysis of previous studies in crops to test these hypotheses. Nutrients acquired mostly by mass flow were decreased significantly more by elevated [CO2] than nutrients acquired by diffusion to the roots through the soil, supporting the first hypothesis. Similarly, Mg showed large concentration declines in leaves and wheat stems, but smaller decreases in other tissues. Because chlorophyll requires a large fraction of total plant Mg, and chlorophyll concentration is reduced by growth in elevated [CO2], this supports the second hypothesis. Understanding these mechanisms may guide efforts to improve nutrient content, and allow modeling of nutrient changes and health impacts under future climate change scenarios.

Photosynthetic and respiratory exchanges of CO2 by plants with the atmosphere are significantly larger than anthropogenic CO2 emissions, and these fluxes will change as growing conditions are altered by climate change. Understanding feedbacks in CO2 exchange is important to predicting future atmospheric [CO2] and climate change. At the tissue and plant scale, respiration is a key determinant of growth and yield. Although the stimulation of C 3 photosynthesis by growth at elevated [CO2] can be predicted with confidence, the nature of changes in respiration is less certain. This is largely because the mechanism of the respiratory response is insufficiently understood. Molecular, biochemical and physiological changes in the carbon metabolism of soybean in a free-air CO 2 enrichment experiment were investigated over 2 growing seasons. climate change ͉ elevated CO2 ͉ free air CO2 enrichment ͉ metabolic ͉ soybean T he rate at which atmospheric CO 2 concentration ([CO 2 ]) is rising and driving climate change is the net consequence of anthropogenic carbon emissions plus ecosystem processes that release or remove carbon from the atmosphere. Carbon emission to the atmosphere from fossil fuel burning, cement production and land use change has risen to Ϸ10 PgCy Ϫ1 (1). Dark respiration from plants in terrestrial ecosystems is a much larger flux, emitting 50-60 PgCy Ϫ1 (2). The change in plant respiration that will occur by the middle to end of this century in direct response to rising [CO 2 ] has long been of interest and uncertainty (3, 4). Changes in respiration will combine with the well characterized stimulation of C 3 photosynthesis by elevated [CO 2 ] to impact the net primary productivity of ecosystems and their capacity to act as sources or sinks of carbon. Key synthesis papers have variously concluded that elevated [CO 2 ] will cause plant respiration to increase as much as 11%, decrease as much as 18%, or not change (5-8). This uncertainty corresponds to an increase or decrease in carbon release to the atmosphere similar in size to current anthropogenic carbon emissions. The primary reason for uncertainty is that the mechanisms of plant respiratory responses to elevated [CO 2 ] have not been resolved (3,(5)(6)(7)(8). This knowledge gap also restricts our understanding at the tissue and whole-plant scales of how elevated [CO 2 ] impacts growth and crop yield. Our research tested the hypothesis that plants respond to the greater carbon supply resulting from long-term growth at elevated [CO 2 ] through acclimation for increased metabolic capacity and greater respiratory flux. Results and DiscussionThe mechanisms by which field-grown plants respond to growth at elevated [CO 2 ] were investigated in this study by combining genomic analysis with biochemical and physiological phenotyping of soybean in a free-air CO 2 enrichment (FACE) experiment. Soybean was grown over its entire lifecycle in 4 plots at ambient [CO 2 ] (Ϸ380 mol⅐mol Ϫ1 ) and 4 plots at elevated [CO 2 ] (Ϸ550 mol⅐mol Ϫ1 ). This model system featured: (i...

Experimental elevation of [CO 2 ] around C 3 crops in the field has been shown to increase yields by suppressing the Rubisco oxygenase reaction and, in turn, photorespiration. Bioengineering a cyanobacterial carbon-concentrating mechanism (CCM) into C 3 crop species provides a potential means of elevating [CO 2 ] at Rubisco, thereby decreasing photorespiration and increasing photosynthetic efficiency and yield. The cyanobacterial CCM is an attractive alternative relative to other CCMs, because its features do not require anatomical changes to leaf tissue. However, the potential benefits of engineering the entire CCM into a C 3 leaf are unexamined. Here, a CO 2 and HCO 3 2 diffusion-reaction model is developed to examine how components of the cyanobacterial CCM affect leaf light-saturated CO 2 uptake (A sat ) and to determine whether a different Rubisco isoform would perform better in a leaf with a cyanobacterial CCM. The results show that the addition of carboxysomes without other CCM components substantially decreases A sat and that the best first step is the addition of HCO 3 2 transporters, as a single HCO 3 2 transporter increased modeled A sat by 9%. Addition of all major CCM components increased A sat from 24 to 38 mmol m 22 s 21 . Several Rubisco isoforms were compared in the model, and increasing ribulose bisphosphate regeneration rate will allow for further improvements by using a Rubisco isoform adapted to high [CO 2 ]. Results from field studies that artificially raise [CO 2 ] suggest that this 60% increase in A sat could result in a 36% to 60% increase in yield.

It is anticipated that enrichment of the atmosphere with CO 2 will increase photosynthetic carbon assimilation in C3 plants. Analysis