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1. Photosynthetic responses of switchgrass to light and CO₂ under different precipitation treatments

   Kieffer, Christina; Kaur, Navneet; Li, Jianwei; Matamala, Roser; Fay, Philip; Hui, Dafeng. (May 2024, GCB bioenergy)

   Long, Steve. (Ed.)

   Switchgrass (Panicum virgatum L.) is a prominent bioenergy crop with robust resilience to environmental stresses. However, our knowledge regarding how precipitation changes affect switchgrass photosynthesis and its responses to light and CO2 remains limited. To address this knowledge gap, we conducted a field precipitation experiment with five different treatments, including -50%, -33%, 0%, +33%, and +50% of ambient precipitation. To determine the responses of leaf photosynthesis to CO2 concentration and light, we measured leaf net photosynthesis of switchgrass under different CO2 concentrations and light levels in 2020 and 2021 for each of the five precipitation treatments. We first evaluated four light and CO2 response models (i.e., rectangular hyperbola model, nonrectangular hyperbola model, exponential model, and the modified rectangular hyperbola model) using the measurements in the ambient precipitation treatment. Based on the fitting criteria, we selected the nonrectangular hyperbola model as the optimal model and applied it to all precipitation treatments, and estimated model parameters. Overall, the model fit field measurements well for the light and CO2 response curves. Precipitation change did not influence the maximum net photosynthetic rate (Pmax) but influenced other model parameters including quantum yield (α), convexity (θ), dark respiration (Rd), light compensation point (LCP), and saturated light point (LSP). Specifically, the mean Pmax of five precipitation treatments was 17.6 μmol CO2 m-2s-1, and the ambient treatment tended to have a higher Pmax. The +33% treatment had the highest α, and the ambient treatment had lower θ and LCP, higher Rd, and relatively lower LSP. Furthermore, precipitation significantly influenced all model parameters of CO2 response. The ambient treatment had the highest Pmax, largest α, and lowest θ, Rd, and CO2 compensation point LCP. Overall, this study improved our understanding of how switchgrass leaf photosynthesis responds to diverse environmental factors, providing valuable insights for accurately modeling switchgrass ecophysiology and productivity. 

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2. What is the fate of xylem‐transported CO ₂ in Kranz‐type C ₄ plants?

   https://doi.org/10.1111/nph.15908  

   Stutz, Samantha S.; Hanson, David T. (June 2019, New Phytologist)

   Summary High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C₃plants exit leaves in the dark. In the light, C₃species use a small portion of xylem‐transported CO₂for leaf photosynthesis. However, it is not known if xylem‐transported CO₂will exit leaves in the dark or be used for photosynthesis in the light in Kranz‐type C₄plants.Cut leaves ofAmaranthus hypochondriacuswere placed in one of three solutions of [NaH¹³CO₃] dissolved in KCl water to measure the efflux of xylem‐transported CO₂exiting the leaf in the dark or rates of assimilation of xylem‐transported CO₂* in the light, in real‐time, using a tunable diode laser absorption spectroscope.In the dark, the efflux of xylem‐transported CO₂increased with increasing rates of transpiration and [¹³CO₂*]; however, rates of¹³C_effluxinA. hypochondriacuswere lower compared to C₃species. In the light,A. hypochondriacusfixed nearly 75% of the xylem‐transported CO₂supplied to the leaf.Kranz anatomy and biochemistry likely influence the efflux of xylem‐transported CO₂out of cut leaves ofA. hypochondriacusin the dark, as well as the use of xylem‐transported CO₂* for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz‐type C₄species over C₃species. 

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3. Albedo of crops as a nature-based climate solution to global warming

   https://doi.org/10.1088/1748-9326/ad5fa2  

   Lei, Cheyenne; Chen, Jiquan; Ibáñez, Inés; Sciusco, Pietro; Shirkey, Gabriela; Lei, Ming; Reich, Peter; Robertson, G_Philip (July 2024, Environmental Research Letters)

   Abstract Surface albedo can affect the energy budget and subsequently cause localized warming or cooling of the climate. When we convert a substantial portion of lands to agriculture, land surface properties are consequently altered, including albedo. Through crop selection and management, one can increase crop albedo to obtain higher levels of localized cooling effects to mitigate global warming. Still, there is little understanding about how distinctive features of a cropping system may be responsible for elevated albedo and consequently for the cooling potential of cultivated lands. To address this pressing issue, we conducted seasonal measurements of surface reflectivity during five growing seasons on annual crops of corn-soybean–winter wheat (Zea mays L.- Glycine max L.Merrill—Triticum aestivum L.; CSW) rotations at three agronomic intensities, a monoculture of perennial switchgrass, and perennial polycultures of early successional and restored prairie grasslands. We found that crop-species, agronomic intensity, seasonality, and plant phenology had significant effects on albedo. The mean ± SD of albedo was highest in perennial crops of switchgrass (Panicum virgatum; 0.179 ± 0.04), intermediate in early successional crops (0.170 ± 0.04), and lowest in a reduced input corn systems with cover crops (0.154 ± 0.02). Thestrongest cooling potentials were found in soybean (−0.450 kg CO₂e m⁻²yr⁻¹) and switchgrass (−0.367 kg CO₂e m⁻²yr⁻¹), with up to −0.265 kg CO₂e m⁻²yr⁻¹of localized climate cooling annually provided by different agroecosystems. We also demonstrated how diverse ecosystems, leaf canopy, and agronomic practices can affect surface reflectivity and provide another potential nature-based solution for reducing global warming at localized scales. 

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4. Seasonal decline in leaf photosynthesis in perennial switchgrass explained by sink limitations and water deficit

   https://doi.org/10.3389/fpls.2022.1023571  

   Tejera-Nieves, Mauricio; Abraha, Michael; Chen, Jiquan; Hamilton, Stephen K.; Robertson, G. Philip; Walker, Berkley James (January 2023, Frontiers in Plant Science)

   Leaf photosynthesis of perennial grasses usually decreases markedly from early to late summer, even when the canopy remains green and environmental conditions are favorable for photosynthesis. Understanding the physiological basis of this photosynthetic decline reveals the potential for yield improvement. We tested the association of seasonal photosynthetic decline in switchgrass ( Panicum virgatum L.) with water availability by comparing plants experiencing ambient rainfall with plants in a rainfall exclusion experiment in Michigan, USA. For switchgrass exposed to ambient rainfall, daily net CO 2 assimilation ( A n e t ' ) declined from 0.9 mol CO 2 m -2 day -1 in early summer to 0.43 mol CO 2 m -2 day -1 in late summer (53% reduction; P<0.0001). Under rainfall exclusion shelters, soil water content was 73% lower and A n e t ' was 12% and 26% lower in July and September, respectively, compared to those of the rainfed plants. Despite these differences, the seasonal photosynthetic decline was similar in the season-long rainfall exclusion compared to the rainfed plants; A n e t ' in switchgrass under the shelters declined from 0.85 mol CO 2 m -2 day -1 in early summer to 0.39 mol CO 2 m -2 day -1 (54% reduction; P<0.0001) in late summer. These results suggest that while water deficit limited A n e t ' late in the season, abundant late-season rainfalls were not enough to restore A n e t ' in the rainfed plants to early-summer values suggesting water deficit was not the sole driver of the decline. Alongside change in photosynthesis, starch in the rhizomes increased 4-fold (P<0.0001) and stabilized when leaf photosynthesis reached constant low values. Additionally, water limitation under shelters had no negative effects on the timing of rhizome starch accumulation, and rhizome starch content increased ~ 6-fold. These results showed that rhizomes also affect leaf photosynthesis during the growing season. Towards the end of the growing season, when vegetative growth is completed and rhizome reserves are filled, diminishing rhizome sink activity likely explained the observed photosynthetic declines in plants under both ambient and reduced water availability. 

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5. Boreal conifers maintain carbon uptake with warming despite failure to track optimal temperatures

   https://doi.org/10.1038/s41467-023-40248-3  

   Dusenge, Mirindi Eric; Warren, Jeffrey M; Reich, Peter B; Ward, Eric J; Murphy, Bridget K; Stefanski, Artur; Bermudez, Raimundo; Cruz, Marisol; McLennan, David A; King, Anthony W; et al (December 2023, Nature Communications)

   Abstract Warming shifts the thermal optimum of net photosynthesis (T_optA) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO₂) conditions. It is unclear whether shifts inT_optAof field-grown trees will keep pace with the temperatures predicted for the 21^stcentury under elevated atmospheric CO₂concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO₂levels, we show thatT_optAof mature boreal conifers increased with warming. However, shifts inT_optAdid not keep pace with warming asT_optAonly increased by 0.26–0.35 °C per 1 °C of warming. Net photosynthetic rates estimated at the mean growth temperature increased with warming in elevated CO₂spruce, while remaining constant in ambient CO₂spruce and in both ambient CO₂and elevated CO₂tamarack with warming. Although shifts inT_optAof these two species are insufficient to keep pace with warming, these boreal conifers can thermally acclimate photosynthesis to maintain carbon uptake in future air temperatures. 

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