Bulk Leaf Research Articles

Linking stomatal size and density to water use efficiency and leaf carbon isotope ratio in juvenile and mature trees.

Water-use efficiency (WUE) is affected by multiple leaf traits, including stomatal morphology. However, the impact of stomatal morphology on WUE across different ontogenetic stages of tree species is not well-documented. Here, we investigated the relationship between stomatal morphology, intrinsic water-use efficiency (iWUE) and leaf carbon isotope ratio (δ13C). We sampled 190 individuals, including juvenile and mature trees belonging to 18 temperate broadleaved tree species and 9 genera. We measured guard cell length (GCL), stomatal density (SD), specific leaf area (SLA), iWUE and bulk leaf δ13C as a proxy for long-term WUE. Leaf δ13C correlated positively with iWUE across species in both juvenile and mature trees, while GCL showed a negative and SD a positive effect on iWUE and leaf δ13C. Within species, however, only GCL was significantly associated with iWUE and leaf δ13C. SLA had a minor negative influence on iWUE and leaf δ13C, but this effect was inconsistent between juvenile and mature trees. We conclude that GCL and SD can be considered functional morphological traits related to the iWUE and leaf δ13C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.

Do leaf lignin content or leaf mass-to-area bias the estimation of intrinsic water use efficiency from leaf bulk δ13C? A test with seedlings from five oak species

Key MessageLeaves of seedlings from five oak species (Quercus robur L.; Q. pubescens L.; Q. suber L.; Q. afares Pomel; Q. ilex L.) displayed large, mainly inter-specific, differences in leaf mass-to-area ratio (LMA) and lignin content, as well as in the 13C composition of bulk leaf biomass. The variation in leaf lignin content and LMA did not impact the offset between the 13C composition measured in bulk leaf material versus soluble sugars. This observation, as well as the similar correlations between intrinsic leaf water use efficiency and the 13C compositions of bulk material or soluble sugars extracted from leaves, confirms their reliable use as a proxy for the former even when there is a large variation in LMA or lignin among samples.ContextCarbon isotope composition (δ13C) of bulk leaf biomass is frequently used as a proxy for intrinsic water use efficiency (iWUE) in large-scale intra- and inter-specific comparisons. However, post-photosynthetic 13C discrimination during the synthesis of lignin combined with differences in leaf mass-to-area ratio (LMA) may bias the relationship between δ13C of bulk leaf matter and iWUE and thus its use as a proxy of iWUE.AimsTo quantify the impact of differences in lignin content and LMA on the relationship between δ13C of bulk leaf biomass and iWUE over a large gradient of lignin contents across five oak species (deciduous: Quercus robur, Q. pubescens, Q. afares and evergreen: Q. ilex and Q. suber).MethodsWe measured lignin content, LMA, and δ13C of bulk leaf biomass and of soluble sugars extracted from the leaves, as well as intrinsic water use efficiency (derived from leaf gas exchange) in seedlings of the five oak species grown under common conditions in a greenhouse and measured in a climate chamber.ResultsThere was a large range (mainly across species) in lignin content (4 to 33%) and LMA (60–180 g m−2). δ13C of bulk leaf biomass and soluble sugars were tightly correlated, showing a significant mean offset of − 0.4‰. This offset was stable across species and not correlated to the lignin content of the leaves. A very loose correlation was found between the offset and LMA, mainly due to one species.ConclusionOur results are a demonstration that potential variations in leaf lignin content or LMA have no or only a little effect on the δ13C of bulk leaf biomass. They are unlikely to cause a bias when using bulk leaf δ13C as a proxy for variations in intrinsic water use efficiency among Mediterranean and temperate broad-leaf forest tree species.

Exploring the feasibility of a single-protoplast proteomic analysis

BackgroundRecent advances in high-resolution mass spectrometry have now enabled the study of proteomes at the single-cell level, offering the potential to unveil novel aspects of cellular processes. Remarkably, there has been no prior attempt to investigate single-plant cell proteomes. In this study, we aimed to explore the feasibility of conducting a proteomic analysis on individual protoplasts.FindingsAs a result, our analysis identified 978 proteins from the 180 protoplasts, aligning with well-known biological processes in plant leaves, such as photosynthetic electron transport in photosystem II. Employing the SCP package in the SCoPE2 workflow revealed a notable batch effect and extensive missing values in the data. Following correction, we observed the heterogeneity in single-protoplast proteome expression. Comparing the results of single-protoplast proteomics with those of bulk leaf proteomics, we noted that only a small fraction of bulk data was detected in the single-protoplast proteomics data, highlighting a technical limitation of the current single-cell proteomics method.ConclusionsIn summary, we demonstrated the feasibility of conducting a single-protoplast proteomic experiment, revealing heterogeneity in plant cellular proteome expression. This underscores the importance of analyzing a substantial number of plant cells to discern statistically significant changes in plant cell proteomes upon perturbation such as abscisic acid treatment in future studies. We anticipate that our study will contribute to advancing single-protoplast proteomics in the near future.

Leaf n-alkane δ13C improves upon conventional bulk leaf δ13C for assessing drought sensitivity of winter wheat cultivars

Efficiently assessing drought resistance of different cultivars is crucial for genotypic selection. The existing indicators, however, do not adequately cover both aspects of drought resistance: grain yield differences among cultivars under drought conditions, and drought sensitivity of a specific cultivar in response to water condition changes from wet to dry. Carbon isotopes have shown promise in this regard, but the commonly used bulk leaf carbon isotope (δ13Cbulk) is not consistently correlated with grain yield, resulting in an inability to consistently predict drought resistance. This study explored the effectiveness of carbon isotopes, particularly the n-alkane carbon isotope (δ13Calk), in assessing drought resistance of different cultivars in both aspects of drought resistance and provide insights into the potential mechanisms behind the inconsistent relationships between δ13Cbulk and grain yield. We conducted field experiments involving multiple cultivars and irrigation treatments on winter wheat (Triticum aestivum L.) and measured the δ13Cbulk and δ13Calk on various growth stages and organs. Our study found that neither leaf δ13Cbulk nor δ13Calk could consistently predict the grain yield difference among cultivars under the same water treatment. This inconsistency is due to the influence of cultivar characteristics and post-photosynthetic carbon isotope fractionations. However, leaf δ13Calk demonstrated a more robust and consistent correlation than leaf δ13Cbulk as a surrogate for the drought sensitivity of different cultivars to reduced water conditions. The accuracy of evaluation can be influenced by the sampling periods and organs, with the optimal strategy being the collection of flag leaf at early grain filling stage. The key findings of our study highlight the advantages of leaf δ13Calk in assessing the crop drought sensitivity and evaluating drought-resistant cultivars.

Differences in photosynthetic and water use characteristics of four co-existing tree species at the treeline of the Japanese Alps

ABSTRACT Physiological characteristics of trees in the alpine treeline are critical for assessing the vulnerability of alpine forest ecosystems, but differences in water relations between co-existing deciduous and evergreen trees remain poorly understood. In this study, we analyzed the photosynthetic and water use characteristics of two deciduous broad-leaved (Sorbus matsumurana and Betula ermanii) and two evergreen coniferous (Abies mariesii and Pinus pumila) tree species at the treeline – i.e. the line connecting the highest patches of forest composed of trees at least three meters high – of Mt. Norikura in central Japan, to explore the physiological adaptations of trees to alpine environments. We monitored diurnal changes in leaf gas exchange rates and leaf and soil water potentials in each tree species during the growing season, as well as bulk leaf water relations based on pressure – volume curves and whole-tree hydraulic structures. The four species under study relied on different physiological characteristics at both leaf and plant levels for adaptation to their natural habitat. For maintaining water in leaves, S. matsumurana showed higher water use efficiency (WUE; the ratio of photosynthesis to transpiration) and osmotic adjustment. In contrast, B. ermanii showed lower WUE, while maintaining higher soil-to-leaf hydraulic conductivity with less stomatal control. In turn, A. mariesii showed higher WUE and leaf succulence. Meanwhile, P. pumila showed lower WUE but compensated for water loss through higher leaf hydraulic capacitance and tissue elasticity. Our observations demonstrate that tree species rely on specific physiological characteristics to adapt to the alpine treeline environment.

Water deficit and potassium affect carbon isotope composition in cassava bulk leaf material and extracted carbohydrates.

Cassava (Manihot esculenta Crantz) is an important root crop, which despite its drought tolerance suffers considerable yield losses under water deficit. One strategy to increase crop yields under water deficit is improving the crop's transpiration efficiency, which could be achieved by variety selection and potassium application. We assessed carbon isotope composition in bulk leaf material and extracted carbohydrates (soluble sugar, starch, and cellulose) of selected leaves one month after inducing water deficit to estimate transpiration efficiency and storage root biomass under varying conditions in a greenhouse experiment. A local and improved variety were grown in sand, supplied with nutrient solution with two potassium levels (1.44 vs. 0.04 mM K+) and were subjected to water deficit five months after planting. Potassium application and selection of the improved variety both increased transpiration efficiency of the roots with 58% and 85% respectively. Only in the improved variety were 13C ratios affected by potassium application (up to - 1.8‰ in δ13C of soluble sugar) and water deficit (up to + 0.6‰ in δ13C of starch and soluble sugar). These data revealed a shift in substrate away from transitory starch for cellulose synthesis in young leaves of the improved variety under potassium deficit. Bulk δ13C of leaves that had fully developed prior to water deficit were the best proxies for storage root biomass (r = - 0.62, r = - 0.70) and transpiration efficiency (r = - 0.68, r = - 0.58) for the local and improved variety respectively, making laborious extractions redundant. Results obtained from the youngest fully developed leaf, commonly used as a diagnostic leaf, were complicated by remobilized assimilates in the improved variety, making them less suitable for carbon isotope analysis. This study highlights the potential of carbon isotope composition to assess transpiration efficiency and yield, depending on the chosen sampling strategy as well as to unravel carbon allocation processes.

18 O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C3 grass-atmospheric drivers and physiological relations.

The 18 O enrichment (Δ18 O) of leaf water affects the Δ18 O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ18 O of bulk leaf water (Δ18 OLW ) and leaf sucrose (Δ18 OSucrose ). We grew Lolium perenne (a C3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 μmol mol-1 ) as factors, and determined Δ18 OLW , Δ18 OSucrose and morphophysiological leaf parameters, including transpiration (Eleaf ), stomatal conductance (gs ) and mesophyll conductance to CO2 (gm ). The Δ18 O of photosynthetic medium water (Δ18 OSSW ) was estimated from Δ18 OSucrose and the equilibrium fractionation between water and carbonyl groups (εbio ). Δ18 OSSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ18 Oe ) with adjustments that correlated with gas exchange parameters (gs or total conductance to CO2 ). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ18 OLW was a poor proxy for Δ18 OSucrose , mainly due to opposite Δ18 O responses of nonphotosynthetic tissue water (Δ18 Onon-SSW ) relative to Δ18 OSSW , driven by atmospheric conditions.

Creating a virtual leaf.

When microscopy meets modelling the exciting concept of a 'virtual leaf' is born. The goal of a 'virtual leaf' is to capture complex physiology in a virtual environment, resulting in the capacity to run experiments computationally. One example of a 'virtual leaf' application is capturing 3D anatomy from volume microscopy data and estimating where water evaporates in the leaf and the proportions of apoplastic, symplastic and gas phase water transport. The same 3D anatomy could then be used to improve established 3D reaction-diffusion models, providing a better understanding of the transport of CO2 across the stomata, through the airspace and across the mesophyll cell wall. This viewpoint discusses recent progress that has been made in transitioning from a bulk leaf approach to a 3D understanding of leaf physiology, in particular, the movement of CO2 and H2O within the leaf.

Stocking method effects on forage accumulation, canopy attributes, and nutritive value of grazed Mulato II brachiariagrass pastures

AbstractRotational stocking (RS) is generally associated with the intensification of pasture‐based animal production systems, although many studies have shown little advantage over continuous stocking (CS). The objective of this research was to describe and explain the effects of two average canopy heights (20 and 30 cm) and three stocking methods (CS; rotational stocking with lenient defoliation, RSL; and rotational stocking with moderate defoliation, RSM) on forage accumulation (FA), vertical distribution of plant‐part components, and nutritive value of ‘Mulato II’ hybrid brachiariagrass (Brachiaria spp. syn. Urochloa spp.) during two summer rainy seasons in Piracicaba, São Paulo, Brazil. Pastures were maintained at average canopy height of 20 and 30 cm under CS, and treatments under RSL and RSM were imposed by variations of ±20 and 30% of the average canopy heights, representing defoliation intensities of 33 and 47% of the pre‐graze heights, respectively. The FA was not affected by CS, RSL, RSM stocking methods, averaging 8090 kg DM ha−1 year−1. Canopy bulk density and distribution of plant‐part components in the canopy profile were better at the average canopy height of 20 cm, accompanied by the greater nutritive value. The leaf bulk density was generally greater in RSM compared to CS. Pastures under CS had greater crude protein and in vitro digestible matter, and lesser neutral detergent fibre concentrations. Mulato II brachiariagrass should be managed at an average height of 20 cm under CS.

Unravelling δ13C signal in Scots pine trees for climate change and tree physiology studies

Stable carbon isotope composition (δ13C) recorded in trees responds sensitively to the changing environmental conditions and thus provides a powerful tool for paleoenvironmental reconstructions. The retrospective interpretation of tree δ13C signal depends on comprehensive understanding of how environmental and physiological signals are recorded in δ13C during photosynthesis and how the δ13C signal is modified after photosynthesis. This thesis aims to improve the understanding of photosynthetic and post-photosynthetic isotope fractionation processes, and to examine the suitability of tree ring δ13C for intra-seasonal reconstructions of intrinsic water use efficiency (iWUE). The former goal was studied by combining compound-specific isotope analysis of organic matter with isotope discrimination models and online δ13C measurements of leaf CO2 fluxes for field-grown mature Scots pine (Pinus sylvestris L.). The latter was achieved via comparing 18-year-long intra-seasonal iWUE chronologies estimated from laser ablation derived tree ring δ13C, gas exchange and eddy covariance data. Mesophyll conductance and time-integral effect of leaf assimilates had a clear impact on the intra-seasonal dynamics of leaf sugar δ13C. No significant use of reserves was observed for biomass growth of needles, stem or roots of Scots pine. Unlike sucrose, leaf bulk matters had a significant δ13C offset from new assimilates, leading to a distorted environmental signal documented in their δ13C. The reliability of tree ring δ13C data for intra-seasonal iWUE reconstructions was supported by an agreement of intra-seasonal patterns across the iWUE estimation methods. These results broaden our knowledge of the less well-known photosynthetic and post-photosynthetic isotopic fractionation processes, demonstrate the benefits of analysing sucrose δ13C for understanding plant physiological responses, and show that the tree ring δ13C based iWUE reconstructions can be extended to intra-seasonal scale. This information not only helps to better unravel δ13C signal in trees, but also improves reliable reconstructions of environmental and physiological signals from tree ring δ13C.

Tissue-specific calcium and magnesium allocation to explain differences in bulk concentration in leaves of one-year-old seedlings of two olive (Olea europaea L.) cultivars

Olive tree (Olea europaea L.) leaves have recently been recognised as a valuable source in cosmetic and pharmaceutical industry as well as in preparation of health-supporting beverages. Little is known about the element composition of olive leaves and almost nothing about tissue-specific allocation of elements. Element composition and tissue-specific distribution were determined in leaves of two olive cultivars, Leccino and Istarska bjelica using micro-particle induced X-ray emission (micro-PIXE). In leaves of the Istarska bjelica cultivar larger bulk concentrations of potassium, sodium, molybdenum and boron, but smaller concentrations of calcium and magnesium were found than in leaves of the Leccino cultivar. Tissue-specific investigation revealed that larger concentration of calcium in epidermis and in leaf blade tissues (secondary veins, palisade and spongy mesophyll) contributed to the larger leaf bulk calcium concentration in the Leccino cultivar. For magnesium, all leaf tissues, except the bundle sheath cells and consequently the main vascular bundle, contributed to the larger bulk concentration in the Leccino cultivar. Potassium was not predominant in any of the leaf tissues examined, while sodium and molybdenum were below the limit of detection, and boron not detectable by micro-PIXE. The results indicate that sinks for calcium and magnesium are stronger in specific leaf tissues of the Leccino than of the Istarska bjelica cultivar. The new understanding of tissue-specific allocation of elements in leaves of olive will serve as a basis for detailed studies into the effects of foliar and/or soil fertilisers in olive.

Carbon isotope and soluble metabolites reflect physiological status among contrasting faba bean genotypes in response to water deficit.

Identification and validation of biomarkers and bioindicators to select genotypes with superior tolerance to water deficit (WD) under field conditions are paramount to plant breeding programs. However, the co-occurrence of different abiotic stresses such as WD, heat, and radiation makes it difficult to develop generalized protocols to monitor the physiological health of the plant system. The study assessed the most abundant carbohydrates and sugar alcohols in five faba bean (Vicia faba) genotypes under field conditions and the abundance of naturally occurring carbon isotopes in bulk leaf material to predict water use efficiency (WUE). Plant water status and biomass accumulation were also assessed. Among the accumulated sugars, inter-specific variation in glucose was most prevalent and was found at a higher concentration (8.52 mg g−1 leaf) in rainfed trial. myo-Inositol concentrations followed that of glucose accumulation in that the rainfed trial had higher amounts compared to the irrigated trial. WUE calculated from carbon isotope abundance was consistently offset with measured WUE from measurements of leaf gas exchange. All genotypes demonstrated significant relationships between predicted and measured WUE (p < 0.05) apart from control variety PBA Warda. Thus, bulk leaf-level carbon isotope abundance can be used to calculate WUE and used as an effective selection criterion for improving WUE in faba bean breeding programs under field conditions.

Divergent leaf and fine root "pressure-volume relationships" across habitats with varying water availability.

Fine roots and leaves, the direct interfaces of plants with their external environment along the soil-plant-atmosphere continuum, are at the front line to ensure plant adaptation to their growing habitat. This study aimed to compare the vulnerability to water deficit of fine roots and leaves of woody species from karst and mangrove forests-two water-stressed habitats-against that of timber and ornamental woody species grown in a well-watered common garden. Thus, pressure-volume curves in both organs of 37 species (about 12 species from each habitat) were constructed. Fine roots wilted at a less negative water potential than leaves in 32 species and before branch xylem lost 50% of its hydraulic conductivity in the 17 species with available data on branch xylem embolism resistance. Thus, turgor loss in fine roots can act as a hydraulic fuse mechanism against water stress. Mangroves had higher leaf resistance against wilting and lower leaf-specific area than the karst and common garden plants. Their fine roots had high specific root lengths (SRL) and high capacitance to buffer water stress. Karst species had high leaf bulk modulus, low leaf capacitance, and delayed fine root wilting. This study showed the general contribution of fine roots to the protection of the whole plant against underground water stress. Our findings highlight the importance of water storage in the leaves and fine roots of mangrove species and high tolerance to water deficit in the leaves of mangrove species and the fine roots of some karst species.

Suitability Evaluation of Three Tropical Pasture Species (Mulato II, Gatton Panic, and Rhodes Grass) for Cultivation under a Subtropical Climate of Australia

Exploring improved tropical forages is considered to be an important approach in delivering quality and consistent feed options for dairy cattle in tropical and subtropical regions. The present study aimed to study the suitability of three improved tropical grasses, Chloris gayana ‘Rhodes grass cv. Reclaimer’ (RR), Megathyrsus maximus ‘Gatton Panic’ (GP), and Brachiaria ruziziensis x B. decumbens x B. brizantha ‘Brachiaria Mulato II’ (BM) evaluating their carbon assimilation, canopy structure, herbage plant–part accumulation and quality parameters under irrigated conditions. An experiment was conducted at Gatton Research Dairy (27°54′ S, 152°33′ E, 89 m asl) Queensland, Australia, which has a predominantly subtropical climate. Photosynthesis biochemistry, canopy structure, herbage accumulation, plant part composition, and nutritive value were evaluated. Photosynthesis biochemistry differed between pasture species. Efficiency of CO2 assimilation was highest for GP and quantum efficiency was highest for BM. Pasture canopy structure was significantly affected by an interaction between pasture species and harvest. Forage biomass accumulation was highest in GP, while BM produced more leaf and less stem compared to both GP and RR. A greater leafy stratum and lower stemmy stratum depth were observed in the vertical sward structure of BM. Brachiaria Mulato II showed greater carbon partitioning to leaves, leaf: stem ratio, canopy, and leaf bulk density. It also demonstrated greater nutritive value (Total digestible nutrients (TDN), acid detergent fibre (ADF), neutral detergent fibre (NDF), neutral detergent insoluble protein (NDICP), Starch, nonfibre carbohydrates (NFC), metabolisable energy (ME), mineral profile (Mg, P, K, Fe, Zn) and dietary cation–anion difference (DCAD) for leaf, stem, and the whole plant. Greater quantum efficiency, leaf accumulation, and nutritive value of BM observed in the present study suggest BM as an attractive forage option for dairying that warrants further research in pasture-based systems in tropical and subtropical climates.

Limiting resource and leaf functional traits jointly determine distribution patterns of leaf intrinsic water use efficiency along aridity gradients.

Intrinsic water use efficiency (iWUE) is a critical eco-physiological function allowing plants to adapt to water- and nutrient-limited habitats in arid and semi-arid regions. However, the distribution of iWUE in coexisting species along aridity gradients and its controlling factors are unknown. We established two transects along an aridity gradient in the grasslands of Losses Plateau (LP) and Inner Mongolia Plateau (MP) to elucidate the patterns and underlying mechanisms of iWUE distribution in coexisting species along aridity gradient. We determined leaf carbon (δ13C) and oxygen (δ18O) stable isotopes, functional traits related to carbon fixation, and limiting resources. Bulk leaf δ13C and δ18O were used as proxies for time-integrated iWUE and stomatal conductance (gs) during the growing season. Our results showed that variability in iWUE within transect was primarily controlled by species, sampling sites and an interactive effect between species and sampling sites. Mean values of iWUE (iWUEMean) increased and coefficient of variation (CV) in iWUE (iWUECV) decreased with an increase in aridity, demonstrating that increases in aridity lead to conservative and convergent water use strategies. Patterns of iWUEMean and iWUECV were controlled primarily by the ratio of soil organic carbon to total nitrogen in LP and soil moisture in MP. This revealed that the most limited resource drove the distribution patterns of iWUE along aridity gradients. Interspecific variation in iWUE within transect was positively correlated with Δ18O, indicating that interspecific variation in iWUE was primarily regulated by gs. Furthermore, relationship between iWUE and multi-dimensional functional trait spectrum indicated that species evolved species-specific strategies to adapt to a harsh habitat by partitioning limiting resources. Overall, these findings highlighted the interactive effects of limiting resources and leaf functional traits on plant adaptation strategies for iWUE, and emphasized the importance of considering biological processes in dissecting the underlying mechanisms of plant adaptation strategies at large regional scales.

Relative growth rate for trees at the growth stage is coordinated with leaf bulk modulus of elasticity and osmotic potential in a subtropical forest of China

Current theoretical models for tree relative growth rate (RGR) have considered physiological processes based on carbon economy and nutrient-productivity relations, but have not taken water relations into account. According to the ecological energetics and Lockhart equation theories, if more energy is needed to maintain water balance in plants, less energy should be invested towards growth, resulting in a trade-off between growth and drought tolerance. Leaf pressure–volume (PV) curve parameters are the best recognised classical indicators of plant drought tolerance, however, the relationships between tree growth and leaf PV parameters have rarely been investigated in forest communities. In the study, we selected two evergreen (Lithocarpus glaber, Cyclobalanopsis oxyodon) and two deciduous species (Quercus serrata, Platycarya strobilacea) that dominate northern subtropical forests on Mt. Shennongjia, central China. For each species, we selected individuals at different developmental stages (growth vs. mature stage), and measured the relative growth rate of diameter at breast height (RGRDBH) and leaf PV curve parameters, including leaf saturated osmotic potential (Ψsat), maximum bulk elastic modulus (εmax), turgor loss osmotic potential (Ψtlp) and leaf water capacity (Cleaf). Correlation analysis and structural equation models (SEM) were applied to explore the relationships between RGRDBH and PV parameters. The results showed that: 1) at both stages, RGRDBH was positively correlated with Ψsat, but not with Ψtlp and Cleaf; 2) εmax was negatively related to RGRDBH at the growth stage; 3) the relationship between PV parameters and RGRDBH was determined mainly by inter-specific difference; 4) SEM showed that both Ψtlp and Ψsat were affected by εmax; and 5) due to the interaction between the PV parameters, Ψsat and εmax had a direct effect on RGRDBH only during the growth stage. Our results showed that the functional/structural coordination between PV parameters adjusted the RGRDBH of trees, and among different species, there exists a tradeoff between energy allocation for drought tolerance and growth. These findings can be applied to improve RGR models, and show how forests respond to warm-induced drought, which is a major threat to global forests in a future warmer climate.

Evapotranspiration partitioning based on field‐stable oxygen isotope observations for an urban locust forest land

AbstractThe stable water isotope method is widely applied to distinguish the evapotranspiration (ET) components across various vegetation‐covered surfaces. ET partitioning in the urban woodland area is useful for guiding precision irrigation, thereby promoting in urban water conservation. For the first time, this study partitions ET in urban locust forest areas based on stable water isotope observations for the period 2019–2020. The isotope composition of ET (δET) and soil evaporation (δE) were determined using the Keeling‐plot method and Craig–Gordon model, respectively. The steady‐state (SS) and the non‐steady‐state (NSS) assumptions were compared for estimating the isotope composition of vegetation transpiration (δT). The NSS outperformed SS in daily bulk leaf water isotopic component (δL,b) simulation and recommended to be used in δT determination for the urban forest land. Both methods resulted in similar estimates of δL,b and δT during the daytime; however, substantial difference was observed during the nighttime. The fraction of vegetation transpiration to evapotranspiration (FT) varies between 0.21 and 0.95 with an average value of 0.78 for the SS and varies between 0.22 and 0.97 with an average of 0.82 for the NSS. The FT in the urban forest land is higher than the natural forest land due to the urban heat island effect and higher planting densities. The seasonal FT variation is primarily controlled by the leaf area index (LAI) and soil moisture. The predictive uncertainty of δET and δT are much higher than δE, wherein the uncertainties of δT decreases as FT increases while the uncertainty of δET increases as FT increases. The uncertainty analysis highlights the importance of increasing the sampling frequency under low FT condition. This study revealed the seasonal change patterns of FT and its major governing factors in urban woodland areas, thus providing more insights on effective water management in the urban ecosystems.

Monitoring plant water status via static uniaxial compression of the leaf lamina.

Turgor pressure is an essential, but difficult to measure indicator of plant water status. Turgor has been quantified by localized compression of cells or tissues, but a simple method to perform these measurements is lacking. We hypothesized that changes in leaf turgidity can be monitored by uniaxially compressing the leaf lamina and measuring the mechanical stress under a constrained thickness (stress relaxation)and that changes in leaf water content can be monitored by measuring the leaf thickness under constant mechanical stress. Using a simple, custom-built leaf squeeze-flow rheometer, we performed different compression tests on leaves from 13 plant species. The mechanical stress measured during stress relaxation was correlated with leaf bulk turgor pressure (R2 > 0.95) and thus with balancing pressure (R2 > 0.94); the leaf thickness measured under constant mechanical stress was correlated with relative water content (R2 > 0.74). The coefficients of these relationships were related to the leaf bulk osmotic pressure at the turgor-loss point. An idealized average-cell model suggests that, under isothermal conditions, the stationary bulk modulus during compression is largely determined by the bulk osmotic pressure. Our study presents an inexpensive, accessibleand automatable method to monitor plant water status noninvasively.

Divergence and conservative of stomatal conductance in coexisting species in response to climatic stress in Tibetan Plateau

Convergence of eco-physiological function in harsh habitats due to environmental filtering is well-known, however, some evidence of eco-physiological divergence in coexisting species also exists. Experimental data to verify whether divergence of eco-physiological function in coexisting species increases with increasing environmental stress remain limited. Here, we measured bulk leaf oxygen isotope and functional traits along a climatic stress gradient in a grassland in Tibetan Plateau where drought, excess radiation, and temperature increased from east to west. Enrichment (Δ18O) in bulk leaf oxygen isotope was used as proxy for time-integrated gs. We found that the leaf water use (Δ18O and leaf area [LA]) spectrum was independent of the leaf economic spectrum, indicating that coexisting species shifted from high nitrogen use efficiency (high ratio of leaf carbon to nitrogen) and water loss (high LA) in low-stress habitats to high photosynthetic capacity (high leaf nitrogen) and low water loss (low gs) in high-stress habitats. As the stress intensity of climate increases, higher mean and maximum and lower skewness (varying from positive to negative values) of Δ18O in coexisting species confirmed that the abundance of water-saving species increased to cope with climatic stress, while the greater standard deviation and kurtosis further indicated that climatic stress drove the divergence in gs in coexisting species. This study revealed that coexisting species adopt more diversified water use strategies and the abundance of water-saving species increases to cope with climatic stress, regardless of decreases in overall species abundance.

Heterogeneous isotope effects decouple conifer leaf and branch sugar δ18O and δ13C.

Isotope ratios of tree-ring cellulose are a prominent tool to reconstruct paleoclimate and plant responses to environmental variation. Current models for cellulose isotope ratios assume a transfer of the environmental signals recorded in bulk leaf water to carbohydrates and ultimately into stem cellulose. However, the isotopic signal of carbohydrates exported from leaf to branch may deviate from mean leaf values if spatial heterogeneity in isotope ratios exists in the leaf. We tested whether the isotopic heterogeneity previously observed along the length of a ponderosa pine (Pinus ponderosa) leaf water was preserved in photosynthetic products. We observed an increase in both sugar and bulk tissue δ18O values along the needle, but the increase in carbohydrate δ18O values was dampened relative to the trend observed in leaf water. In contrast, δ13C values of both sugar and bulk organic matter were invariant along the needle. Phloem-exported sugar measured in the branch below the needles did not match whole-needle values of δ18O or δ13C. Instead, there was a near-constant offset observed between the branch and needle sugar δ13C values, while branch δ18O values were most similar to δ18O values observed for sugar at the base of the needle. The observed offset between the branch and needle sugar δ18O values likely arises from partial isotope oxygen exchange between sugars and water during phloem loading and transport. An improved understanding of the conditions producing differential δ13C and δ18O isotope effects between branch phloem and needle sugars could improve tree-ring-based climate reconstructions.