Woody tissue photosynthesis reduces stem CO2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

Partitioning seasonal stem carbon dioxide efflux into stem respiration, bark photosynthesis, and transport-related flux in Scots pine

Stem CO2 efflux is an important component of the carbon balance in forests. The efflux is considered to principally reflect the net result of two dominating and opposing processes: stem respiration and stem photosynthesis. In addition, transport of CO2 in xylem sap is thought to play an appreciable role in affecting the net flux. This work presents an approach to partition stem CO2 efflux among these processes using sap-flux data and CO2-exchange measurements from dark and transparent chambers placed on mature Scots pine (Pinus sylvestris) trees. Seasonal changes and monthly parameters describing the studied processes were determined. Respiration contributed most to stem net CO2 flux, reaching up to 79% (considering the sum of the absolute values of stem respiration, stem photosynthesis, and flux from CO2 transported in xylem sap to be 100%) in June, when stem growth was greatest. The contribution of photosynthesis accounted for up to 13% of the stem net CO2 flux, increasing over the monitoring period. CO2 transported axially with sap flow decreased towards the end of the growing season. At a reference temperature, respiration decreased starting around midsummer, while its temperature sensitivity increased during the summer. A decline was observed for photosynthetic quantum yield around midsummer together with a decrease in light-saturation point. The proposed approach facilitates modeling net stem CO2 flux at a range of time scales.

The benefits of woody plant stem photosynthesis extend to hydraulic function and drought survival in Parkinsonia florida

As climate change exacerbates drought stress in many parts of the world, understanding plant physiological mechanisms for drought survival is critical to predicting ecosystem responses. Stem net photosynthesis, which is common in arid environments, may be a drought survival trait, but whether the additional carbon fixed by stems contributes to plant hydraulic function and drought survival in arid land plants is untested. We conducted a stem light-exclusion experiment on saplings of a widespread North American desert tree species, Parkinsonia florida L., and after shading acclimation, we then subjected half of the plants to a drought treatment to test the interaction between light exclusion and water limitation on growth, leaf and stem photosynthetic gas exchange, xylem embolism assessed with micro-computed tomography and gravimetric techniques, and survival. Growth, stem photosynthetic gas exchange, hydraulic function and survival all showed expected reductions in response to light exclusion. However, stem photosynthesis mitigated the drought-induced reductions in gas exchange, xylem embolism (percent loss of conductivity, PLC) and mortality. The highest mortality was in the combined light exclusion and drought treatment, and was related to stem PLC and native sapwood-specific hydraulic conductivity. This research highlights the integration of carbon economy and water transport. Our results show that additional carbon income by photosynthetic stems has an important role in the growth and survival of a widespread desert tree species during drought. This shift in function under conditions of increasing stress underscores the importance of considering stem photosynthesis for predicting drought-induced mortality not only for the additional supply of carbon, but also for its extended benefits for hydraulic function.

The quandary of sources and sinks of CO2 efflux in tree stems-new insights and future directions

Stem respiration (RS) substantially contributes to the return of photo assimilated carbon to the atmosphere and, thus, to the tree and ecosystem carbon balance. Stem CO2 efflux (ECO2) is often used as a proxy for RS. However, this metric has often been challenged because of the uncertain origin of CO2 emitted from the stem due to post-respiratory processes. In this Insight, we (i) describe processes affecting the quantification of RS, (ii) review common methodological approaches to quantify and model RS and (iii) develop a research agenda to fill the most relevant knowledge gaps that we identified. Dissolution, transport and accumulation of respired CO2 away from its production site, reassimilation of respired CO2 via stem photosynthesis and the enzyme phosphoenolpyruvate carboxylase, axial CO2 diffusion in the gas phase, shifts in the respiratory substrate and non-respiratory oxygen (O2) consumption are the most relevant processes causing divergence between RS and measured stem gas exchange (ECO2 or O2 influx, IO2). Two common methodological approaches to estimate RS, namely the CO2 mass balance approach and the O2 consumption technique, circumvent some of these processes but have yielded inconsistent results regarding the fate of respired CO2. Stem respiration modelling has recently progressed at the organ and tree levels. However, its implementation in large-scale models, commonly operated from a source-driven perspective, is unlikely to reflect adequate mechanisms. Finally, we propose hypotheses and approaches to advance the knowledge of the stem carbon balance, the role of sap pH on RS, the reassimilation of respired CO2, RS upscaling procedures, large-scale RS modelling and shifts in respiratory metabolism during environmental stress.

A comparison of methods for excluding light from stems to evaluate stem photosynthesis

Premise

A comparison of methods using different materials to exclude light from stems to prevent stem CO2 exchange (i.e., photosynthesis), without affecting stem conductance to water vapor, surface temperature, and relative humidity, was conducted on stems of avocado trees in California.

Methods and Results

The experiment featured three materials: aluminum foil, paper-based wrap, and mineral-based paint. We examined stem CO2 exchange with and without the light exclusion treatments. We also examined stem surface temperature, relative humidity, and photosynthetic active radiation (PAR) under the cover materials. All materials reduced PAR and stem CO2 exchange. However, aluminum foil reduced stem surface temperature and increased relative humidity.

Conclusions

Methods used to study stem CO2 exchange through light exclusion have historically relied on methods that may induce experimental artifacts. Among the methods tested here, mineral-based paint effectively reduced PAR without affecting stem surface temperature and relative humidity around the stem.

Carbon isotope composition of respired CO2 in woody stems and leafy shoots of three tree species along the growing season: physiological drivers for respiratory fractionation

The carbon isotope composition of respired CO2 (δ13CR) and bulk organic matter (δ13CB) of various plant compartments informs about the isotopic fractionation and substrate of respiratory processes, which are crucial to advance the understanding of carbon allocation in plants. Nevertheless, the variation across organs, species and seasons remains poorly understood. Cavity Ring-Down Laser Spectroscopy was applied to measure δ13CR in leafy shoots and woody stems of maple (Acer platanoides L.), oak (Quercus robur L.) and cedar (Thuja occidentalis L.) trees during spring and late summer. Photosynthesis, respiration, growth and non-structural carbohydrates were measured in parallel to evaluate potential drivers for respiratory fractionation. The CO2 respired by maple and oak shoots was 13C-enriched relative to δ13CB during spring, but not late summer or in the stem. In cedar, δ13CR did not vary significantly throughout organs and seasons, with respired CO2 being 13C-depleted relative to δ13CB. Shoot δ13CR was positively related to leaf starch concentration in maple, while stem δ13CR was inversely related to stem growth. These relations were not significant for oak or cedar. The variability in δ13CR suggests (i) different contributions of respiratory pathways between organs and (ii) seasonality in the respiratory substrate and constitutive compounds for wood formation in deciduous species, less apparent in evergreen cedar, whose respiratory metabolism might be less variable.

Structure and function of bark and wood chloroplasts in a drought-tolerant tree (Fraxinus ornus L.)

Leaves are the most important photosynthetic organs in most woody plants, but chloroplasts are also found in organs optimized for other functions. However, the actual photosynthetic efficiency of these chloroplasts is still unclear. We analyzed bark and wood chloroplasts of Fraxinus ornus L. saplings. Optical and spectroscopic methods were applied to stem samples and compared with leaves. A sharp light gradient was detected along the stem radial direction, with blue light mainly absorbed by the outer bark, and far-red-enriched light reaching the underlying xylem and pith. Chlorophylls were evident in the xylem rays and the pith and showed an increasing concentration gradient toward the bark. The stem photosynthetic apparatus showed features typical of acclimation to a low-light environment, such as larger grana stacks, lower chlorophyll a/b and photosystem I/II ratios compared with leaves. Despite likely receiving very few photons, wood chloroplasts were photosynthetically active and fully capable of generating a light-dependent electron transport. Our data provide a comprehensive scenario of the functional features of bark and wood chloroplasts in a woody species and suggest that stem photosynthesis is coherently optimized to the prevailing micro-environmental conditions at the bark and wood level.

It's only natural: Plant respiration in unmanaged systems

Respiration plays a key role in the terrestrial carbon cycle and is a fundamental metabolic process in all plant tissues and cells. We review respiration from the perspective of plants that grow in their natural habitat and how it is influenced by wide-ranging elements at different scales, from metabolic substrate availability to shifts in climate. Decades of field-based measurements have honed our understanding of the biological and environmental controls on leaf, root, stem, and whole-organism respiration. Despite this effort, there remain gaps in our knowledge within and across species and ecosystems, especially in more challenging-to-measure tissues like roots. Recent databases of respiration rates and associated leaf traits from species representing diverse biomes, plant functional types, and regional climates have allowed for a wider-lens view at modeling this important CO2 flux. We also re-analyze published data sets to show that maximum leaf respiration rates (Rmax) in species from around the globe are related both to leaf economic traits and environmental variables (precipitation and air temperature), but that root respiration does not follow the same latitudinal trends previously published for leaf data. We encourage the ecophysiological community to continue to expand their study of plant respiration in tissues that are difficult to measure and at the whole plant and ecosystem levels to address outstanding questions in the field.

Comparison between the stem and leaf photosynthetic productivity in Eucalyptus urophylla plantations with different age

We developed a more realistic modeling framework by integrating stem photosynthesis into the canopy carbon assimilation model to compare the photosynthetic productivity between the stem and leaf of Eucalyptus urophylla plantations. Stems of Eucalyptus species with smooth outer bark have photosynthetic green tissue that can recycle internal stem CO₂. However, the potential contribution of stem photosynthesis to forest productivity has not previously been adequately quantified, and we also do not know how it compares to leaf photosynthetic productivity. To assist in addressing this knowledge gap, we conducted field surveys in Eucalyptus urophylla plantations of different ages and developed a more realistic modeling framework by integrating stem photosynthesis into the existing canopy carbon assimilation model. We calculated the proportion of tree stems shaded by neighboring tree trunks based on Poisson spatial point process. Under the stand density of 2000 trees per hectare, the light absorption area of tree trunks of 2-year-old and 7-year-old E. urophylla plantations were 0.11 (± 0.15) and 0.35 (± 0.12) m² stem m^-2 land, the stem photosynthetic productivity (GPP_stem) was 0.72 (± 0.45) and 1.81 (± 1.12) mol C m^-2 month^-1, and the ratios of GPP_stem to leaf photosynthetic productivity (GPP_leaf) were 5.10 and 8.17% for 2- and 7-year-old plantations, respectively. Overall, this study presents the feasibility of incorporating stem photosynthesis into the productivity prediction of E. urophylla plantations by developing the stem light absorption model.

PtoMPO1, a negative mediator, functions in poplar drought tolerance

Drought, as one of the most severe abiotic stresses in nature, adversely affects plant growth and development. Poplar is a woody plant which is prone to water-deficit sensitivity. Therefore, it is important to improve our understanding of how poplar responds to drought stress. Here, we cloned a gene from Populus tomentosa, namely PtoMPO1. PtoMPO1 encodes a DUF962 domain protein that is a homolog of yeast dioxygenase Mpo1 and Arabidopsis MHP1. The transcripts of PtoMPO1 were repressed by drought stress and ABA. Atmhp1-1 was a T-DNA insertion mutant lacking AtMHP1, and heteroexpression of PtoMPO1 in Atmhp1-1 significantly alleviated the sensitivity of Atmhp1-1 to ABA and NaCl, implying the functional replacement of PtoMPO1 to AtMHP1. PtoMPO1 overexpression decreased but PtoMPO1 mutation enhanced poplar drought tolerance. Furthermore, the expression of drought-related gene PtoRD26 is markedly lower in PtoMPO1-overexpressing plants and notably higher in Ptompo1 mutants compared to that in the wild type. Overall, these results suggested that PtoMPO1 functions as a novel negative mediator for drought tolerance in poplar.

MsTHI1 overexpression improves drought tolerance in transgenic alfalfa (Medicago sativa L.)

In recent years, drought stress caused by global warming has become a major constraint on agriculture. The thiamine thiazole synthase (THI1) is responsible for controlling thiamine production in plants displaying a response to various abiotic stresses. Nonetheless, most of the THI1 activities in plants remain largely unknown. In this study, we extracted MsTHI1 from alfalfa and demonstrated its beneficial impact on improving the resistance of plants to stress conditions. The highest levels of MsTHI1 expression were identified in alfalfa leaves, triggered by exposure to cold, drought, salt, or alkaline conditions. The upregulation of MsTHI1 in drought-stressed transgenic plants resulted in enhanced accumulation of vitamin B1 (VB1), chlorophyll a (Chl a), chlorophyll b (Chl b), soluble protein, higher soil and plant analyzer development (SPAD) value, and the activity of peroxidase (POD), maintained Fv/Fm, and decreased lipid peroxidation. Moreover, overexpression of MsTHI1 upregulated the transcription of THI4, TPK1, RbcX2, Cu/Zn-SOD, CPK13, and CPK32 and downregulated the transcription of TH1 and CPK17 in transgenic alfalfa under drought stress. These results suggested that MsTHI1 enhances drought tolerance by strengthening photosynthesis, regulating the antioxidant defense system, maintaining osmotic homeostasis, and mediating plant signal transduction.

Mechanistic drivers of stem respiration: A modelling exercise across species and seasons

Stem respiration (R_S ) plays a crucial role in plant carbon budgets. However, its poor understanding limits our ability to model woody tissue and whole-tree respiration. A biophysical model of stem water and carbon fluxes (TReSpire) was calibrated on cedar, maple and oak trees during spring and late summer. For this, stem sap flow, water potential, diameter variation, temperature, CO₂ efflux, allometry and biochemistry were monitored. Shoot photosynthesis (P_N ) and nonstructural carbohydrates (NSC) were additionally measured to evaluate source-sink relations. The highest R_S and stem growth was found in maple and oak during spring, both being seasonally decoupled from P_N and [NSC]. Temperature largely affected maintenance respiration (R_M ) in the short term, but temperature-normalized R_M was highly variable on a seasonal timescale. Overall, most of the respired CO₂ radially diffused to the atmosphere (>87%) while the remainder was transported upward with the transpiration stream. The modelling exercise highlights the sink-driven behaviour of R_S and the significance of overall metabolic activity on nitrogen (N) allocation patterns and N-normalized respiratory costs to capture R_S variability over the long term. These insights should be considered when modelling plant respiration, whose representation is currently biased towards a better understanding of leaf metabolism.

The Importance of Stem Photosynthesis for Two Desert Shrubs Across Different Groundwater Depths

Water availability could alter multiple ecophysiological processes such as water use strategy, photosynthesis, and respiration, thereby modifying plant water use and carbon gain. However, a lack of field observations hinders our understanding of how water availability affects stem photosynthesis at both organ and plant levels of desert shrubs. In this study, we measured gas exchange and oxygen stable isotopes to quantify water sources, stem recycling photosynthesis, and whole-plant carbon balance in two coexisting Haloxylon species (Haloxylon ammodendron and Haloxylon persicum) at different groundwater depths in the Gurbantonggut Desert. The overall aim of the study was to analyze and quantify the important role of stem recycling photosynthesis for desert shrubs (Haloxylon species) under different groundwater depths. The results showed that (1) regardless of changes in groundwater depth, H. ammodendron consistently used groundwater and H. persicum used deep soil water as their main water source, with greater than 75% of xylem water being derived from groundwater and deep soil water for the two species, respectively; (2) stem recycling photosynthesis refixed 72-81% of the stem dark respiration, and its contribution to whole-plant carbon assimilation was 10-21% for the two species; and (3) deepened groundwater increased stem water use efficiency and its contribution to whole-plant carbon assimilation in H. persicum but not in H. ammodendron. Our study provided observational evidence that deepened groundwater depth induced H. persicum to increase stem recycling photosynthetic capacity and a greater contribution to whole-plant carbon assimilation, but this did not occur on H. ammodendron. Our study indicates that stem recycling photosynthesis may play an important role in the survival of desert shrubs in drought conditions.

Bark Transpiration Rates Can Reach Needle Transpiration Rates Under Dry Conditions in a Semi-arid Forest

Drought can cause tree mortality through hydraulic failure and carbon starvation. To prevent excess water loss, plants typically close their stomata before massive embolism formation occurs. However, unregulated water loss through leaf cuticles and bark continues after stomatal closure. Here, we studied the diurnal and seasonal dynamics of bark transpiration and how it is affected by tree water availability. We measured continuously for six months water loss and CO₂ efflux from branch segments and needle-bearing shoots in Pinus halepensis growing in a control and an irrigation plot in a semi-arid forest in Israel. Our aim was to find out how much passive bark transpiration is affected by tree water status in comparison with shoot transpiration and bark CO₂ emission that involve active plant processes, and what is the role of bark transpiration in total tree water use during dry summer conditions. Maximum daily water loss rate per bark area was 0.03-0.14 mmol m^-2 s^-1, which was typically ~76% of the shoot transpiration rate (on leaf area basis) but could even surpass the shoot transpiration rate during the highest evaporative demand in the control plot. Irrigation did not affect bark transpiration rate. Bark transpiration was estimated to account for 64-78% of total water loss in drought-stressed trees, but only for 6-11% of the irrigated trees, due to differences in stomatal control between the treatments. Water uptake through bark was observed during most nights, but it was not high enough to replenish the lost water during the day. Unlike bark transpiration, branch CO₂ efflux decreased during drought due to decreased metabolic activity. Our results demonstrate that although bark transpiration represents a small fraction of the total water loss through transpiration from foliage in non-stressed trees, it may have a large impact during drought.

Efflux and assimilation of xylem-transported CO2 in stems and leaves of tree species with different wood anatomy

Determining the fate of CO₂ respired in woody tissues is necessary to understand plant respiratory physiology and to evaluate CO₂ recycling mechanisms. An aqueous ¹³ C-enriched CO₂ solution was infused into the stem of 3-4 m tall trees to estimate efflux and assimilation of xylem-transported CO₂ via cavity ring-down laser spectroscopy and isotope ratio mass spectrometry, respectively. Different tree locations (lower stem, upper stem and leafy shoots) and tissues (xylem, bark and leaves) were monitored in species with tracheid, diffuse- and ring-porous wood anatomy (cedar, maple and oak, respectively). Radial xylem CO₂ diffusivity and xylem [CO₂ ] were lower in cedar relative to maple and oak trees, thereby limiting label diffusion. Part of the labeled ¹³ CO₂ was assimilated in cedar (8.7%) and oak (20.6%) trees, mostly in xylem and bark tissues of the stem, while limited solution uptake in maple trees hindered the detection of label assimilation. Little label reached foliar tissues, suggesting substantial label loss along the stem-branch transition following reductions in the radial diffusive pathway. Differences in respiration rates and radial xylem CO₂ diffusivity (lower in conifer relative to angiosperm species) might reconcile discrepancies in efflux and assimilation of xylem-transported CO₂ so far observed between taxonomic clades.

Leaf and tree responses of young European aspen trees to elevated atmospheric CO2 concentration vary over the season

Elevated atmospheric CO2 concentration (eCO2) commonly stimulates net leaf assimilation, decreases stomatal conductance and has no clear effect on leaf respiration. However, effects of eCO2 on whole-tree functioning and its seasonal dynamics remain far more uncertain. To evaluate temporal and spatial variability in eCO2 effects, 1-year-old European aspen trees were grown in two treatment chambers under ambient (aCO2, 400 p.p.m.) and elevated (eCO2, 700 p.p.m.) CO2 concentrations during an early (spring 2019) and late (autumn 2018) seasonal experiment. Leaf (net carbon assimilation, stomatal conductance and leaf respiration) and whole-tree (stem growth, sap flow and stem CO2 efflux) responses to eCO2 were measured. Under eCO2, carbon assimilation was stimulated during the early (1.63-fold) and late (1.26-fold) seasonal experiments. Stimulation of carbon assimilation changed over time with largest increases observed in spring when stem volumetric growth was highest, followed by late season down-regulation, when stem volumetric growth ceased. The neutral eCO2 effect on stomatal conductance and leaf respiration measured at leaf level paralleled the unresponsive canopy conductance (derived from sap flow measurements) and stem CO2 efflux measured at tree level. Our results highlight that seasonality in carbon demand for tree growth substantially affects the magnitude of the response to eCO2 at both leaf and whole-tree level.

Low-cost chamber design for simultaneous CO2 and O2 flux measurements between tree stems and the atmosphere

Tree stem CO2 efflux is an important component of ecosystem carbon fluxes and has been the focus of many studies. While CO2 efflux can easily be measured, a growing number of studies have shown that it is not identical with actual in situ respiration. Complementing measurements of CO2 flux with simultaneous measurements of O2 flux provides an additional proxy for respiration, and the combination of both fluxes can potentially help getting closer to actual measures of respiratory fluxes. To date, however, the technical challenge to measure relatively small changes in O2 concentration against its high atmospheric background has prevented routine O2 measurements in field applications. Here, we present a new and low-cost field-tested device for autonomous real-time and quasi-continuous long-term measurements of stem respiration by combining CO2 (NDIR-based) and O2 (quenching-based) sensors in a tree stem chamber. Our device operates as a cyclic-closed system and measures changes in both CO2 and O2 concentration within the chamber over time. The device is battery powered with a >1-week power independence, and data acquisition is conveniently achieved by an internal logger. Results from both field and laboratory tests document that our sensors provide reproducible measurements of CO2 and O2 exchange fluxes under varying environmental conditions.

A high stem to leaf ratio reduced rainfall use efficiency under altered rainfall patterns in a semi-arid grassland in northeast China

Rainfall use efficiency (RUE) is crucial for understanding the changes in grassland productivity due to variations in future rainfall patterns. Recently, numerous studies have been conducted on the relationship between RUE and the amount of rainfall, but there has been little research on the influence of rainfall distribution and the interactive effect of rainfall amounts and distribution on RUE. Here, a simulated rainfall experiment was conducted to evaluate the impacts of rainfall amount (average rainfall amount (R0), 334 mm; decreased (R-) and increased (R+) rainfall amounts, 233 mm and 434 mm, respectively) and dry intervals (comprising 6-day, 9-day, 12-day, 15-day, 18-day and 21-day intervals between rainfall) on productivity and RUE in Leymus chinensis (Trin.) Tzvel., a dominant grass of the Eastern Eurasian Steppe. Our results showed that (1) for biomass production and RUE, moderate extension of dry intervals was conducive to enhancing total biomass production and RUE. The peak values of total biomass and RUE appeared during the 15-day interval for R-, and the 18-day interval for R0 and R+. (2) For biomass allocation, extension of dry intervals decreased the stem to leaf ratio (S/L) and the root to shoot ratio (R/S). (3) Further, the S/L ratio was significantly negatively correlated with RUE. These results suggest that variations in rainfall patterns can alter the RUE by changing the S/L ratio, and finally influence biomass production in L. chinensis. These findings have important implications for understanding and predicting the effect of future climate change on productivity in semi-arid grassland.

Simultaneous tree stem and soil greenhouse gas (CO2 , CH4 , N2 O) flux measurements: a novel design for continuous monitoring towards improving flux estimates and temporal resolution

Tree stems and soils can act as sources and sinks for the greenhouse gases (GHG) carbon dioxide (CO₂ ), methane (CH₄ ), and nitrous oxide (N₂ O). Since both uptake and emission capacities can be large, especially in tropical rainforests, accurate assessments of the magnitudes and temporal variations of stem and soil GHG fluxes are required. We designed a new flexible stem chamber system for continuously measuring GHG fluxes in a French Guianese rainforest. Here, we describe this new system, which is connected to an automated soil GHG flux system, and discuss measurement uncertainty and potential error sources. In line with findings for soil GHG flux estimates, we demonstrated that lengthening the stem chamber closure time was required for accurate estimates of tree stem CH₄ and N₂ O flux but not tree stem CO₂ flux. The instrumented stem was a net source of CO₂ and CH₄ and a weak sink of N₂ O. Our experimental setup operated successfully in situ and provided continuous tree and soil GHG measurements at a high temporal resolution over an 11-month period. This automated system is a major step forward in the measurement of GHG fluxes in stems and the atmosphere concurrently with soil GHG fluxes in tropical forest ecosystems.

Xylem transport of root-derived CO2 caused a substantial underestimation of belowground respiration during a growing season

Previous research has indicated that a potentially large portion of root-respired CO₂ can move internally through tree xylem, but these reports are relatively scarce and have generally been limited to short observations. Our main objective was to provide a continuous estimate of the quantity and variability of root-respired CO₂ that moves either internally through the xylem (F_T ) or externally through the soil to the atmosphere (F_S ) over most of a growing season. Nine trees were measured in a Populus deltoides stand for 129 days from early June to mid-October. We calculated F_T as the product of sap flow and dissolved [CO₂ ] in the xylem (i.e., [CO₂ *]) and calculated F_S using the [CO₂ ] gradient method. During the study, stem and soil CO₂ concentrations, temperature, and sap flow were measured continuously. We determined that F_T accounted for 33% of daily total belowground CO₂ flux (i.e., F_S  + F_T ; F_B ) during our observation period that spanned most of a growing season. Cumulative daily F_T was lower than F_S 74% of the time, equivalent to F_S 26% of the time, and never exceeded F_S . One-third of the total CO₂ released by belowground respiration over most of the growing season in this forest stand followed the F_T pathway rather than diffusing into the soil. The magnitude of F_T indicates that measurements of F_S alone substantially underestimate total belowground respiration in some forest ecosystems by systematically underestimating belowground autotrophic respiration. The variability in F_T observed during the growing season demonstrated the importance of making long-term, high-frequency measurements of different flux pathways to better understand physiological and ecological processes and their implications to global change.

Stem metabolism under drought stress - a paradox of increasing respiratory substrates and decreasing respiratory rates

Metabolic changes underpinning drought-induced variations in stem respiration (R_s ) are unknown. We measured R_s rates and metabolite and gene expression profiles in Ulmus minor Mill. and Quercus ilex L. seedlings subjected to increasing levels of drought stress to better understand how carbon, nitrogen and energy metabolism interact during drought. In both species, only plants showing extreme stress symptoms - i.e. negligible rates of leaf stomatal conductance and photosynthesis, and high stem dehydration (30-50% of maximum water storage) and contraction (50-150 μm week^-1 ) - exhibited lower R_s rates than well-watered plants. Abundance of low-molecular weight sugars (e.g. glucose and fructose) and sugar alcohols (e.g. mannitol) increased with drought, at more moderate stress and to a higher extent in Q. ilex than U. minor. Abundance of amino acids increased at more severe stress, more abruptly, and to a higher extent in U. minor, coinciding with leaf senescence, which did not occur in Q. ilex. Organic acids changed less in response to drought: threonate and glycerate increased, and citrate decreased although slightly in both species. Transcripts of genes coding for enzymes of the Krebs cycle decreased in Q. ilex and increased in U. minor in conditions of extreme drought stress. The maintenance of R_s under severe growth and photosynthetic restrictions reveals the importance of stem mitochondrial activity in drought acclimation. The eventual decline in R_s diverts carbon substrates from entering the Krebs cycle that may help to cope with osmotic and oxidative stress during severe drought and to recover hydraulic functionality afterwards.

Heterologous expression of Arabidopsis thaliana rty gene in strawberry (Fragaria × ananassa Duch.) improves drought tolerance

BACKGROUND

Strawberry (Fragaria × ananassa Duch.) is an important fruit crop worldwide. It was particularly sensitive to drought stress because of their fibrous and shallow root systems. Mutant rty of Arabidopsis thaliana ROOTY (RTY) results in increased endogenous auxin levels, more roots, and shoot growth. It is still unclear whether the rty gene improves stress tolerance in strawberry.

RESULTS

rty gene was isolated from Arabidopsis thaliana and placed under the control of the cauliflower mosaic virus (CaMV) 35S promoter in the pBI121-rty binary vector carrying the selectable marker of neomycin phosphotransferase II (NPT II). Seven transgenic lines were confirmed by PCR and western blot analysis. Accumulations of IAA and ABA were significantly increased in the transgenic plants. The endogenous IAA contents were 46.5 ng g- 1 and 66.0 ng g- 1in control and transgenic plants respectively. The endogenous ABA contents in the control plant were 236.3 ng g- 1 and in transgenic plants were 543.8 ng g- 1. The production of adventitious roots and trichomes were enhanced in the transgenic plants. Furthermore, transcript levels of the genes including IAA and ABA biosynthetic, and stress-responsive genes, were higher in the transgenic plants than in the control plants under drought conditions. Water use efficiency and a reduced water loss rate were enhanced in the transgenic strawberry plants. Additionally, peroxidase and catalase activities were significantly higher in the transgenic plants than in the control plants. The experiment results revealed a novel function for rty related to ABA and drought responses.

CONCLUSIONS

The rty gene improved hormone-mediated drought tolerance in transgenic strawberry. The heterologous expression of rty in strawberry improved drought tolerance by promoting auxin and ABA accumulation. These phytohormones together brought about various physiological changes that improved drought tolerance via increased root production, trichome density, and stomatal closure. Our results suggested that a transgenic approach can be used to overcome the inherent trade-off between plant growth and drought tolerance by enhancing water use efficiency and reducing water loss rate under water shortage conditions.

Woody tissue photosynthesis increases radial stem growth of young poplar trees under ambient atmospheric CO2 but its contribution ceases under elevated CO2

Woody tissue photosynthesis (Pwt) contributes to the tree carbon (C) budget and generally stimulates radial stem growth under ambient atmospheric CO2 concentration (aCO2). Moreover, Pwt has potential to enhance tree survival under changing climates by delaying negative effects of drought stress on tree hydraulic functioning. However, the relevance of Pwt on tree performance under elevated atmospheric CO2 concentration (eCO2) remains unexplored. To fill this knowledge gap, 1-year-old Populus tremula L. seedlings were grown in two treatment chambers at aCO2 and eCO2 (400 and 660 ppm, respectively), and woody tissues of half of the seedlings in each treatment chamber were light-excluded to prevent Pwt. Radial stem growth, sap flow, leaf photosynthesis and stomatal and canopy conductance were measured throughout the growing season, and the concentration of non-structural carbohydrates (NSC) in stem tissues was determined at the end of the experiment. Fuelled by eCO2, an increase in stem growth of 18 and 50% was observed in control and light-excluded trees, respectively. Woody tissue photosynthesis increased radial stem growth by 39% under aCO2, while, surprisingly, no impact of Pwt on stem growth was observed under eCO2. By the end of the growing season, eCO2 and Pwt had little effect on stem growth, leaf photosynthesis acclimated to eCO2, but stomatal conductance did not, and homeostatic stem NSC pools were observed among combined treatments. Our results highlight that eCO2 potentially fulfils plant C requirements, limiting the contribution of Pwt to stem growth as atmospheric [CO2] rises, and that radial stem growth in young developing trees was C (source) limited during early phenological stages but transitioned towards sink-driven control at the end of the growing season.

Frost and drought: Effects of extreme weather events on stem carbon dynamics in a Mediterranean beech forest

The effects of short-term extreme events on tree functioning and physiology are still rather elusive. European beech is one of the most sensitive species to late frost and water shortage. We investigated the intra-annual C dynamics in stems under such conditions. Wood formation and stem CO₂ efflux were monitored in a Mediterranean beech forest for 3 years (2015-2017), including a late frost (2016) and a summer drought (2017). The late frost reduced radial growth and, consequently, the amount of carbon fixed in the stem biomass by 80%. Stem carbon dioxide efflux in 2016 was reduced by 25%, which can be attributed to the reduction of effluxes due to growth respiration. Counter to our expectations, we found no effects of the 2017 summer drought on radial growth and stem carbon efflux. The studied extreme weather events had various effects on tree growth. Even though late spring frost had a strong impact on beech radial growth in the current year, trees fully recovered in the following growing season, indicating high resilience of beech to this stressful event.

Woody tissue photosynthesis delays drought stress in Populus tremula trees and maintains starch reserves in branch xylem tissues

Photosynthesis in woody tissues (P_wt ) is less sensitive to water shortage than in leaves, hence, P_wt might be a crucial carbon source to alleviate drought stress. To evaluate the impact of P_wt on tree drought tolerance, woody tissues of 4-m-tall drought-stressed Populus tremula trees were subjected to a light-exclusion treatment across the entire plant to inhibit P_wt . Xylem water potential (Ψ_xylem ), sap flow ( FH2O ), leaf net photosynthesis (P_n,l ), stem diameter variations (ΔD), in vivo acoustic emissions in stems (AEs) and nonstructural carbohydrate concentrations ([NSC]) were monitored to comprehensively assess water and carbon relations at whole-tree level. Under well-watered conditions, P_wt kept Ψ_xylem at a higher level, lowered FH2O and had no effect on [NSC]. Under drought, Ψ_xylem , FH2O and P_n,l in light-excluded trees rapidly decreased in concert with reductions in branch xylem starch concentration. Moreover, sub-daily patterns of ΔD, FH2O and AEs were strongly related, suggesting that in vivo AEs may inform not only about embolism events, but also about capacitive release and replenishment of stem water pools. Results highlight the importance of P_wt in maintaining xylem hydraulic integrity under drought conditions and in sustaining NSC pools to potentially limit increases in xylem tension.