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1. 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)

   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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2. Climate warming and tree carbon use efficiency in a whole‐tree 13 CO 2 tracer study

   https://doi.org/10.1111/nph.15721  

   Drake, John E. ; Furze, Morgan E. ; Tjoelker, Mark G. ; Carrillo, Yolima ; Barton, Craig V. M. ; Pendall, Elise ( March 2019 , New Phytologist)

   Autotrophic respiration is a major driver of the global C cycle and may contribute a positive climate warming feedback through increased atmospheric concentrations ofCO₂. The extent of this feedback depends on plants' ability to acclimate respiration to maintain a constant carbon use efficiency (CUE).

   We quantified respiratory partitioning of gross primary production (GPP) andCUEof field‐grown trees in a long‐term warming experiment (+3°C). We delivered a¹³C–CO₂pulse to whole tree crowns and chased that pulse in the respiration of leaves, whole crowns, roots, and soil. We also measured the isotopic composition of soil microbial biomass and the respiration rates of leaves and whole crowns.

   We documented homeostatic respiratory acclimation of foliar and whole‐crown respiration rates; the trees adjusted to experimental warming such that leaf‐level respiration rates were not increased. Experimental warming had no detectable impact on respiratory partitioning or mean residence times. Of the¹³C label acquired by the trees, aboveground respiration consumed 10%, belowground respiration consumed 40%, and the remaining 50% was retained.

   Experimental warming of +3°C did not alter respiratory partitioning at the scale of entire trees, suggesting that complete acclimation of respiration to warming is likely to dampen a positive climate warming feedback.

    

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3. Using multirate rapid A / C i curves as a tool to explore new questions in the photosynthetic physiology of plants

   https://doi.org/10.1111/nph.15657  

   Stinziano, Joseph R. ; Adamson, Rachael K. ; Hanson, David T. ( January 2019 , New Phytologist)

   Steady‐state photosyntheticCO₂responses (A/C_icurves) are used to assess environmental responses of photosynthetic traits and to predict future vegetative carbon uptake through modeling. The recent development of rapidA/C_icurves (RACiRs) permits faster assessment of these traits by continuously changing [CO₂] around the leaf, and may reveal additional photosynthetic properties beyond what is practical or possible with steady‐state methods.

   Gas exchange necessarily incorporates photosynthesis and (photo)respiration. Each process was expected to respond on different timescales due to differences in metabolite compartmentation, biochemistry and diffusive pathways. We hypothesized that metabolic lags in photorespiration relative to photosynthesis/respiration andCO₂diffusional limitations can be detected by varying the rate of change in [CO₂] duringRACiR assays. We tested these hypotheses through modeling and experiments at ambient and 2% oxygen.

   Our data show that photorespiratory delays cause offsets in predictedCO₂compensation points that are dependent on the rate of change in [CO₂]. Diffusional limitations may reduce the rate of change in chloroplastic [CO₂], causing a reduction in apparentRACiR slopes under highCO₂ramp rates.

   MultirateRACiRs may prove useful in assessing diffusional limitations to gas exchange and photorespiratory rates.

    

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4. Spring photosynthetic onset and net CO 2 uptake in Alaska triggered by landscape thawing

   https://doi.org/10.1111/gcb.14283  

   Parazoo, Nicholas C. ; Arneth, Almut ; Pugh, Thomas A. M. ; Smith, Ben ; Steiner, Nicholas ; Luus, Kristina ; Commane, Roisin ; Benmergui, Josh ; Stofferahn, Eric ; Liu, Junjie ; et al ( May 2018 , Global Change Biology)

   The springtime transition to regional‐scale onset of photosynthesis and net ecosystem carbon uptake in boreal and tundra ecosystems are linked to the soil freeze–thaw state. We present evidence from diagnostic and inversion models constrained by satellite fluorescence and airborneCO₂from 2012 to 2014 indicating the timing and magnitude of spring carbon uptake in Alaska correlates with landscape thaw and ecoregion. Landscape thaw in boreal forests typically occurs in late April (DOY111 ± 7) with a 29 ± 6 day lag until photosynthetic onset. North Slope tundra thaws 3 weeks later (DOY133 ± 5) but experiences only a 20 ± 5 day lag until photosynthetic onset. These time lag differences reflect efficient cold season adaptation in tundra shrub and the longer dehardening period for boreal evergreens. Despite the short transition from thaw to photosynthetic onset in tundra, synchrony of tundra respiration with snow melt and landscape thaw delays the transition from net carbon loss (at photosynthetic onset) to net uptake by 13 ± 7 days, thus reducing the tundra net carbon uptake period. Two globalCO₂inversions using aCASA‐GFEDmodel prior estimate earlier northern high latitude net carbon uptake compared to our regional inversion, which we attribute to (i) early photosynthetic‐onset model prior bias, (ii) inverse method (scaling factor + optimization window), and (iii) sparsity of available AlaskanCO₂observations. Another global inversion with zero prior estimates the same timing for net carbon uptake as the regional model but smaller seasonal amplitude. The analysis of Alaskan eddy covariance observations confirms regional scale findings for tundra, but indicates that photosynthesis and net carbon uptake occur up to 1 month earlier in evergreens than captured by models orCO₂inversions, with better correlation to above‐freezing air temperature than date of primary thaw. Further collection and analysis of boreal evergreen species over multiple years and at additional subarctic flux towers are critically needed.

    

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5. Mitogen‐activated protein kinases MPK 4 and MPK 12 are key components mediating CO 2 ‐induced stomatal movements

   https://doi.org/10.1111/tpj.14087  

   Tõldsepp, Kadri ; Zhang, Jingbo ; Takahashi, Yohei ; Sindarovska, Yana ; Hõrak, Hanna ; Ceciliato, Paulo H. O. ; Koolmeister, Kaspar ; Wang, Yuh‐Shuh ; Vaahtera, Lauri ; Jakobson, Liina ; et al ( October 2018 , The Plant Journal)

   Respiration in leaves and the continued elevation in the atmosphericCO₂concentration causeCO₂‐mediated reduction in stomatal pore apertures. Several mutants have been isolated for which stomatal responses to both abscisic acid (ABA) andCO₂are simultaneously defective. However, there are only few mutations that impair the stomatal response to elevatedCO₂, but not toABA. Such mutants are invaluable in unraveling the molecular mechanisms of earlyCO₂signal transduction in guard cells. Recently, mutations in the mitogen‐activated protein (MAP) kinase,MPK12, have been shown to partially impairCO₂‐induced stomatal closure. Here, we show thatmpk12plants, in whichMPK4is stably silenced specifically in guard cells (mpk12 mpk4GChomozygous double‐mutants), completely lackCO₂‐induced stomatal responses and have impaired activation of guard cell S‐type anion channels in response to elevatedCO₂/bicarbonate. However,ABA‐induced stomatal closure, S‐type anion channel activation andABA‐induced marker gene expression remain intact in thempk12 mpk4GCdouble‐mutants. These findings suggest thatMPK12 andMPK4 act very early inCO₂signaling, upstream of, or parallel to the convergence ofCO₂andABAsignal transduction. The activities ofMPK4 andMPK12 protein kinases were not directly modulated byCO₂/bicarbonatein vitro, suggesting that they are not directCO₂/bicarbonate sensors. Further data indicate thatMPK4 andMPK12 have distinguishable roles in Arabidopsis and that the previously suggested role ofRHC1 in stomatalCO₂signaling is minor, whereasMPK4 andMPK12 act as key components of early stomatalCO₂signal transduction.

    

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