Plant responses to the projected future levels of CO2 were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO2 were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO2 Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO2 on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO2 under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO2 stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO2 improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO2 stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO2 does not directly stimulate C4 photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO2 in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE

Changes in temperature, CO 2 , and precipitation under the scenarios of climate change for the next 30 yr present a challenge to crop production. This review focuses on the impact of temperature, CO 2 , and ozone on agronomic crops and the implications for crop production. Understanding these implications for agricultural crops is critical for developing cropping systems resilient to stresses induced by climate change. There is variation among crops in their response to CO 2 , temperature, and precipitation changes and, with the regional differences in predicted climate, a situation is created in which the responses will be further complicated. For example, the temperature effects on soybean [Glycine max (L.) Merr.] could potentially cause yield reductions of 2.4% in the South but an increase of 1.7% in the Midwest. The frequency of years when temperatures exceed thresholds for damage during critical growth stages is likely to increase for some crops and regions. The increase in CO 2 contributes significantly to enhanced plant growth and improved water use efficiency (WUE); however, there may be a downscaling of these positive impacts due to higher temperatures plants will experience during their growth cycle. A challenge is to understand the interactions of the changing climatic parameters because of the interactions among temperature, CO 2 , and precipitation on plant growth and development and also on the biotic stresses of weeds, insects, and diseases. Agronomists will have to consider the variations in temperature and precipitation as part of the production system if they are to ensure the food security required by an ever increasing population.

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Climate Impacts on Agriculture: Implications for Crop Production

Terrestrial plants remove CO2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves. The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon. Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange. We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO2 fertilization effect. The results suggest a partial closure of stomata-small pores on the leaf surface that regulate gas exchange-to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation-climate models.

/pdf/increase-in-forest-water-use-efficiency-as-atmospheric-3jmikmh5e3.pdf

Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise

Rice is arguably the most important food source on the planet and is consumed by over half of the world’s population. Considerable increases in yield are required over this century to continue feeding the world’s growing population. This meta-analysis synthesizes the research to date on rice responses to two elements of global change, rising atmospheric carbon dioxide concentration ([CO2]) and rising tropospheric ozone concentration ([O3]). On an average, elevated [CO2] (627 ppm) increased rice yields by 23%. Modest increases in grain mass and larger increases in panicle and grain number contributed to this response. The response of rice to elevated [CO2] varied with fumigation technique. The more closely the fumigation conditions mimicked field conditions, the smaller was the stimulation of yield by elevated [CO2]. Free air concentration enrichment (FACE) experiments showed only a 12% increase in rice yield. The rise in atmospheric [CO2] will be accompanied by increases in tropospheric O3 and temperature. When compared with rice grown in charcoal-filtered air, rice exposed to 62 ppb O3 showed a 14% decrease in yield. Many determinants of yield, including photosynthesis, biomass, leaf area index, grain number and grain mass, were reduced by elevated [O3]. While there have been too few studies of the interaction of CO2 and O3 for meta-analysis, the interaction of temperature and CO2 has been studied more widely. Elevated temperature treatments negated any enhancement in rice yield at elevated [CO2], which suggests that identifying high temperature tolerant germplasm will be key to realizing yield benefits in the future.

Rice production in a changing climate: a meta-analysis of responses to elevated carbon dioxide and elevated ozone concentration

Water use efficiency (WUE) is defined as the amount of carbon assimilated as biomass or grain produced per unit of water used by the crop. One of the primary questions being asked is how plants will respond to a changing climate with changes in temperature, precipitation, and carbon dioxide (CO2) that affect their WUE At the leaf level, increasing CO2 increases WUE until the leaf is exposed to temperatures exceeded the optimum for growth (i.e., heat stress) and then WUE begins to decline. Leaves subjected to water deficits (i.e., drought stress) show varying responses in WUE. The response of WUE at the leaf level is directly related to the physiological processes controlling the gradients of CO2 and H2O, e.g., leaf:air vapor pressure deficits, between the leaf and air surrounding the leaf. There a variety of methods available to screen genetic material for enhanced WUE under scenarios of climate change. When we extend from the leaf to the canopy, then the dynamics of crop water use and biomass accumulation have to consider soil water evaporation rate, transpiration from the leaves, and the growth pattern of the crop. Enhancing WUE at the canopy level can be achieved by adopting practices that reduce the soil water evaporation component and divert more water into transpiration which can be through crop residue management, mulching, row spacing, and irrigation. Climate change will affect plant growth, but we have opportunities to enhance WUE through crop selection and cultural practices to offset the impact of a changing climate.

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Water-Use Efficiency: Advances and Challenges in a Changing Climate

Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentration of CO2 ([CO2]) have been shown to decrease stomatal conductance (gs) for a wide range of species under numerous conditions. Less well understood, however, is the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET). Since many changes at the soil, plant, and canopy microclimate levels may feed back on ET, it is not certain that a decrease in gs will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO2] on gs at the leaf to ecosystem ET, soybean (Glycine max) was grown in field conditions under control (approximately 375 μmol CO2 mol−1 air) and elevated [CO2] (approximately 550 μmol mol−1) using free air CO2 enrichment. ET was determined from the time of canopy closure to crop senescence using a residual energy balance approach over four growing seasons. Elevated [CO2] caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield. Ecosystem ET was linked with gs of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in gs results in a 8.6% decrease in ET; this relationship was not altered by growth at elevated [CO2]. The findings are consistent with model and historical analyses that suggest that, despite system feedbacks, decreased gs of upper canopy leaves at elevated [CO2] results in decreased transfer of water vapor to the atmosphere.

/pdf/decreases-in-stomatal-conductance-of-soybean-under-open-air-2jbzcyngec.pdf

Decreases in Stomatal Conductance of Soybean under Open-Air Elevation of [CO2] Are Closely Coupled with Decreases in Ecosystem Evapotranspiration

Decreases in Stomatal Conductance of Soybean under Open-Air Elevation of (CO 2 ) Are Closely Coupled with Decreases in Ecosystem Evapotranspiration 1,2(W)(OA)

Devin R. Quarles

Papers

Decreases in Stomatal Conductance of Soybean under Open-Air Elevation of [CO2] Are Closely Coupled with Decreases in Ecosystem Evapotranspiration

Decreases in Stomatal Conductance of Soybean under Open-Air Elevation of (CO 2 ) Are Closely Coupled with Decreases in Ecosystem Evapotranspiration 1,2(W)(OA)