C alifornia and the rest of the world are experiencing major changes in the availability of water and the concentration of atmospheric carbon dioxide (CO 2). Water and CO 2-along with energy from the sun-are the inputs to photosynthesis, the basis of plant growth and food production. Elevated CO 2 can influence the water use efficiency and yield of crop plants. A clearer understanding of the mechanisms involved in those responses may lead to the development of crop plant varieties better adapted to expected future water and CO 2 conditions. In recent years, the United States has experienced an increased frequency of heat waves and drought and related loss of crops (Lesk et al. 2016). In 2014, drought cost California an estimated $2.2 billion in lost agricultural production (Howitt et al. 2015). In California and elsewhere, the changing climate, population growth and mandates for sustainable management of surface water and groundwater are likely to make water for irrigation more scarce in the future. From 1950 to 2000, global atmospheric CO 2 concentration rose 33%. It continues to climb (fig. 1) and is a major cause of the continuing rise in global temperature (Victor et al. 2014). Average temperatures in California have risen 2.5°F since 1880, and this rate is expected to increase over the next century due to emissions of greenhouse gases (Pathak et al. 2018). Among other impacts, the rising temperature is expected to reduce the winter snowpack, effectively reducing California's water storage capacity and useful water supply (PPIC 2016). CO 2 and stomatal pore apertures Research in our laboratory investigates the mechanisms by which CO 2 elevation and drought cause closing of leaf stomata. Plant stomata are pores on leaf surfaces that enable the intake of CO 2 and the release of water vapor. Over 90% of the water lost by plants through evapotranspiration is released via stomata (Hetherington and Woodward 2003). Each stoma is surrounded by two specialized guard cells that open and close the central gas exchange pore (fig. 2). CO 2 closes stomatal pores, by entering the guard cells, where it is converted to bicarbonate. The increased concentration of bicarbonate is thought OUTLOOK Possible impacts of rising CO 2 on crop water use efficiency and food security Understanding the mechanisms involved in plants' response to rising CO 2 levels may lead to the development of crop plant varieties better adapted to future drought conditions.
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