1a. Objectives (from AD-416)
Objective 1: Define the key regulatory elements controlling photosynthate partitioning and nitrate assimilation and their interactions; develop and begin to test strategies to modify those processes for agricultural purposes. Objective 2: Determine the mechanistic basis for limitations on photosynthetic performance including those imposed by agriculturally significant stresses. Objective 3: Establish the major features controlling the response of photosynthetic productivity in soybean and corn to elevated atmospheric CO2, tropospheric ozone, and their interactions with drought and temperature, explore the bases for genetic variability in responses, and test potential transgenic amelioration strategies. Objective 4: Determine the environmental impacts of land cover change associated with alternative bioenergy crops.
1b. Approach (from AD-416)
Investigate isoform specificity for nitrate reductase (NR) posttranslational modification in vivo, and elucidate the impact of 14-3-3 binding on NR protein degradation. Localize the membrane binding site(s) on sucrose synthase and identify factors that may control the interaction. Use high-resolution spatial and temporal analysis of leaf growth to identify specific areas where leaf growth is occurring. Determine the biochemical factors responsible for the lower activation state of Rubisco, at high temperatures and test potential transgenic amelioration strategies. Further elucidate the role of Rubisco activase in thermal sensitivity/tolerance. Determine the biochemical basis for the "Green Seed Problem" of canola. Perform metabolite analysis of growing leaves under elevated CO2 and O3 to identify key components that may be involved in controlling growth. Determine the factors that lower the activation state of Rubisco under sink- and/or N-limited conditions, which are often encountered when plants are grown under high CO2. Explore the interaction of elevated CO2 with drought on soybean performance. Explore the interaction of elevated CO2 with temperature on soybean and corn performance. Determine if growth at elevated CO2 enhances or ameliorates oxidative stress. Determine the impact of land cover change from row crops to perennial grasses on hydrological cylce and carbon biosequestration.
3. Progress Report
Experiments are underway to investigate the effect of the predicted increase in the frequency and severity of heat waves on both corn and soybean production. Three meter diameter plots of soybean were warmed by 6°C for three consecutive days during three different developmental stages for each crop to test the dual hypothesis that 1) warming during reproductive stages of corn and soybean development will have the largest negative effects but that 2) heat waves during reproductive developmental phases will have much greater yield impact because the much longer flowering interval in soybean will allow for yield compensation by later flowering. A full suite of physiological and agronomic measurements are being conducted just prior to, during and after the heat wave treatments. End of season yield measurements will be taken.
1. Accelerating yield potential gain in soybean: Potential targets for biotechnological improvement. The historical basis for yield potential gains in soybean over the past 90 years were examined by ARS scientists in Urbana, IL together with potential metabolic targets for achieving further yield potential gains. The potential targets for the biotechnological improvement of soybean include photosynthetic efficiency, optimizing the delivery and utilization of carbon, more efficient nitrogen fixation and altering flower initiation and abortion. For example, it was shown that the increase in a single photosynthetic enzyme, sedoheptulose bisphosphatase, increased the photosynthetic performance at atmospheric carbon dioxide concentrations expected for the middle of this century. The impact of this research is that it identifies multiple strategies for improving soybean yields and boosting production both in today’s environment as well as in the environment of the near future that will be warmer and have higher concentrations of both carbon dioxide and ozone in atmosphere.
2. Variation in the ozone tolerance of five agriculturally important nitrogen fixing crop plants. The responses of soybean, pea, French bean, chickpea and alfalfa – all commercially important crops and important for global food security, were compared in research conducted by ARS scientists in Urbana, IL. When grown in elevated ozone levels, measurements of leaf water vapor exchange with the atmosphere and of photosynthesis revealed a broad range of ozone sensitive physiological processes. The most sensitive species, French bean and alfalfa each showed strongly reduced leaf water vapor exchange and photosynthesis whereas the most tolerant crop, alfalfa did not. The impact of this research is that it demonstrates that the underlying mechanisms for ozone tolerance and sensitivity differ across plants even within a single crop type, in this case nitrogen fixing crops.
3. The increasing amount of ozone in the atmosphere will reduce the total amount of water used by soybean crops but the water use efficiency will also decline. ARS scientists at Urbana, IL examined over five growing seasons the impact of increasing ozone levels predicted for the middle of this century in the US Corn Belt on water use by soybean. It was shown ozone reduced water use but also reduced the amount of yield that was produced for each unit of water used; that is, water use efficiency of soybean declined with increasing ozone levels. The impact of this research is two-fold: 1) unless soybean can be adapted, both water use efficiency and yield of soybean will continue decline as ozone levels in soybean growing areas of the US increase over the next half century; 2) reduced water use results in an increase in soybean canopy temperatures and ground level heating that is not included in current general circulation models for predicting future environments and thus may be underestimating warming that is not the result of greenhouse gases.
4. The new generation of bioenergy crops remove carbon from the atmosphere and sequester a portion of it in soil thereby reducing the green house carbon footprint for bioenergy. The primary goal of bioenergy crops is to offset the use of fossil fuels and thereby reduce the amount of carbon and other pollutants that are emitted in energy production. An important concern in converting land from a current purpose to the new purpose of growing bioenergy crops is that this land use change could result in carbon emissions from the soils into the atmosphere and negate the short-term gains of growing biofuels to replace fossil fuels. Work conducted by ARS scientists at Urbana, IL showed that when converting from traditional corn and soybean row crop agriculture to growing perennial grasses species such as Miscanthus and switch grass, the improvement of the carbon foot print is immediate whereby perennial grasses sequester carbon even during the first few years of establishment. The impact of this accomplishment is that it removes significant uncertainty about the carbon balance of growing second-generation bioenergy crops in the US.
5. Deactivation of receptors; a new mechanism in understanding how crop plants sense and respond to changing environments. Plants have a large family of "receptors" that receive signals from the environment, which are instrumental in a plant's ability to respond appropriately to a particular change in its environment; for example to sense and respond to a pathogen or to sense and respond to change in water availability. Many of these receptors recognize specific plant hormones that are produced in response to specific changes in the environment. Protein receptor kinases are an important group of receptors that recognize plant hormones. Some receptor kinases are turned on when they come in contact with the appropriate hormone and initiated a programmed signaling cascade that eventually results in the physiological response of the plant to the environmental change. While much is known in detail about how these protein kinase receptors are turned on; rather little is known about how they are turned off which is just as important in mounting an appropriate physiological response. Work conducted by ARS scientists at Urbana, IL showed that the modification of a specific amino acid on some receptor kinases by phosphorylation turns off the response. The impact of this research is that it for the first time provides a potential mechanism by which we can manipulate the signaling cascade to fine tune the response to, for example, enhance pathogen resistance.
6. Greater than expected yield stimulation of cassava (Manihot esculenta) by future carbon dioxide [CO2] levels. The potential for tuber crops such as cassava, yams and potatoes to enhance food security in the future is underestimated. In tuber crops there is the potential for a much higher ratio of edible to non-edible components than in above ground grain and bean crops such as rice, wheat, maize or soybean. These tubers due to their large capacities to store carbohydrates are inherently strong photosynthate “sinks” implying that tuber crops should be better adapted to respond to the stimulatory effect of increasing atmospheric [CO2]. It follows that, as global atmospheric [CO2] continues to rise, so will the yield of tuber crops. People within the poorest populations depend disproportionately on cassava and other tropical tubers for food and as a cash crop. Thus, in the future, enhanced productivity of tuber crops should work to benefit those who need it most. ARS scientists in Urbana, IL discovered that Cassava fresh yields increased more than 100% when grown under field conditions in which the carbon dioxide concentration had been increased to the level that will be present in atmosphere in the middle of this century. The impact of this research is that, as the most important food crop for many undernourished populations, stimulation of cassava yields in the future elevated carbon dioxide world illustrates that at least this single element of global change will have a larger than expected positive impact on this crop and therefore on food security in the world’s most food insecure regions.Ainsworth, E.A., Bush, D.R. 2011. Carbohydrate export from the leaf - a highly regulated process and target to enhance photosynthesis and productivity. Plant Physiology. 155(1):64-69.