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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 (CO 2 ) that affect their WUE At the leaf level, increasing CO 2 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 CO 2 and H 2 O, 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.

Novel satellite measurements of solar‐induced chlorophyll fluorescence (SIF) can improve our understanding of global photosynthesis; however, little is known about how to interpret the controls on its spectral variability. To address this, we disentangle simultaneous drivers of fluorescence spectra by coupling active and passive fluorescence measurements with photosynthesis. We show empirical and mechanistic evidence for where, why, and to what extent leaf fluorescence spectra change. Three distinct components explain more than 95% of the variance in leaf fluorescence spectra under both steady‐state and changing illumination conditions. A single spectral shape of fluorescence explains 84% of the variance across a wide range of species. The magnitude of this shape responds to absorbed light and photosynthetic up/down regulation; meanwhile, chlorophyll concentration and nonphotochemical quenching control 9% and 3% of the remaining spectral variance, respectively. The spectral shape of fluorescence is remarkably stable where most current satellite retrievals occur (“far‐red,” >740nm), and dynamic downregulation of photosynthesis reduces fluorescence magnitude similarly across the 670‐ to 850‐nm range. We conduct an exploratory analysis of hourly red and far‐red canopy SIF in soybean, which shows a subtle change in red:far‐red fluorescence coincident with photosynthetic downregulation but is overshadowed by longer‐term changes in canopy chlorophyll and structure. Based on our leaf and canopy analysis, caution should be taken when attributing large changes in the spectral shape of remotely sensed SIF to plant stress, particularly if data acquisition is temporally sparse. Ultimately, changes in SIF magnitude at wavelengths greater than 740 nm alone may prove sufficient for tracking photosynthetic dynamics.

The Midwest is one of the most important production areas for corn and soybean worldwide, but also comprises remnants of natural tallgrass prairie vegetation. Future predictions suggest that corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) production in the Midwest may be limited by precipitation and temperature due to climate change. Cross-biome long-term studies in situ are needed to understand carbon assimilation and impact of climate change on the entire region. In this study, we investigated the differences of gross primary production (GPP) and net ecosystem production (NEP) among typical (agro-) ecosystems of corn, soybean and tallgrass prairie from eddy flux stations from 2006 -2015 under contrasting weather conditions. Corn had the highest annual GPP and NEP with 1305 and 327 g C m -2 yr -1 , while soybean had significantly lower GPP and NEP with 630 and -34 g C m -2 yr -1 , excluding additional 2 carbon loss by yield. Corn and soybean NEP was linear related (p < 0.05) to leaf area index (LAI), height or phenological stage, confirming the strong link between plant growth and ecosystem carbon balance.Tallgrass prairie had average values of GPP and NEP of 916 and 61 g C m -2 yr -1 , excluding loss of carbon by annual burning. Thus, prairie GPP and NEP were significantly lower than corn, but significantly higher than soybean. Probably the long fallow period on cropland, which enhanced heterotrophic respiration, and the low carbon assimilation of soybean reduced its overall carbon balance. In total, the corn-soybean agroecosystem acted as a carbon source due to carbon loss by yield removal. Values for GPP and NEP were reflected in inherent water use efficiency (IWUE*) and light use efficiency (LUE) among the agroecosystems. In addition, IWUE*, LUE or GPP of crops and tallgrass prairie were linearly related (p < 0.05) to precipitation, volumetric soil water content (VWC) and maximum air temperature. Air temperature increased IWUE* in both, cropland and prairie vegetation. However, rainfall and VWC affected crops and prairie vegetation differently: while excessive rainfall and VWC reduced GPP or IWUE* in cropland, prairie vegetation GPP and LUE were adversely affected by reduced VWC or precipitation.Future measures of climate change adaption should consider the contrasting effects of precipitation and VWC among the different agro-ecosystems in the Midwestern USA.

Timely and accurate monitoring of crops is essential for food security. Here, we examine how well solar-induced chlorophyll fluorescence (SIF) can inform crop productivity across the United States. Based on tower-level observations and process-based modeling, we find highly linear gross primary production (GPP):SIF relationships for C4 crops, while C3 crops show some saturation of GPP at high light when SIF continues to increase. C4 crops yield higher GPP:SIF ratios (30-50%) primarily because SIF is most sensitive to the light reactions (does not account for photorespiration). Scaling to the satellite, we compare SIF from the TROPOspheric Monitoring Instrument (TROPOMI) against tower-derived GPP and county-level crop statistics. Temporally, TROPOMI SIF strongly agrees with GPP observations upscaled across a corn and soybean dominated cropland (R 2 = 0.89). Spatially, county-level TROPOMI SIF correlates with crop productivity (R 2 = 0.72; 0.86 when accounting for planted area and C3/C4 contributions), highlighting the potential of SIF for reliable crop monitoring.Plain Language Summary Crop monitoring is essential for ensuring food security, but reliable, instantaneous production estimates at the global scale are lacking. The monitoring of crop production in a changing climate is of paramount importance to sustainable food security. Accurate estimates of crop production are dependent on adequately quantifying crop photosynthesis. Our paper demonstrates that solar-induced chlorophyll fluorescence (SIF), an emission of red to far-red light from chlorophyll is highly correlated with crop photosynthesis. We show that a new high spatial resolution satellite SIF data set is highly correlated with crop productivity in the United States, which is benchmarked by the United States Department of Agriculture county-level crop statistics. These results will improve the understanding of crop production and carbon flux over agricultural lands, as well as provide an accurate, large-scale, and timely monitoring method for global crop production estimates.

Global climate is changing and will impact future production of all food and feed crops. Corn is no exception and to ensure a future supply we must begin to understand how climate impacts both the phenological development of corn and the productivity. Temperature and precipitation are the two climate factors that will have a major benefit on corn phenology and productivity. The warming climate will accelerate the phenological development because the number of thermal units required for leaf appearance is relatively constant in the vegetative stage. Productivity of corn is reduced when extreme temperature events occur during pollination and is further exaggerated when there are water deficits at pollination. During the grain-filling period, warm temperatures above the upper threshold cause a reduction in yield. Model estimates suggest that for every 1 C increase in temperature there is nearly a 10% yield reduction. To meet world demand, new adaptation practices are needed to provide water to the growing crop and avoid extreme temperature events during the growing season. Climate change will continue to affect corn production and understanding these effects will help determine where future production areas exist and innovative adaptation practices to benefit yield stability could be utilized.

Let's take a look at a possible future Germany that has reached its net-zero CO 2 emissions goal by 2050. What are the measures that have contributed to reaching this net-zero system? And what kind of implementation efforts are associated with this portfolio of measures?In this perspective, we outline how a carbon-neutral system for Germany in 2050 could look like, following three strategies of avoiding, reducing, and removing CO 2 emissions. We envision a net-zero-2050 Germany by combining analysis from an energy system model with insights into approaches that allow for a higher carbon circularity in the German system, and first results from assessments of national carbon dioxide removal (CDR) potentials.

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