Height-related decreases in mesophyll conductance, leaf photosynthesis and compensating adjustments associated with leaf nitrogen concentrations in Pinus densiflora

Coordination of water use strategies and leaf economic traits in coexisting exotic and native woody species from evergreen and deciduous broadleaf forests

The leaf economics spectrum (LES) describes the covariation of traits relevant for carbon and nutrient economy in different plant species. However, much less is known about the correlation of LES with leaf water economy, not only because some woody species do not follow the rules, but also because they are rarely tested on the widespread, non-native, fast-growing trees. We hypothesized that fast-growing exotic species that spread on the fast side of the LES coordinate their water-use strategies (WUS) to maintain rapid growth, and that the pattern of coordination differs between evergreen and deciduous forests. Using 4 exotic and 4 native species from evergreen and deciduous broadleaf forests in China, we measured 17 traits of LES and WUS and analyzed their functional roles in different species groups. Our results suggest that LES plays a more important role in the coexistence of species within a community, while WUS contributes more to the distribution of species across different regions. The multidimensional coordination of LES and WUS could better explain the growth and distribution of different plant species and shed light on the coexistence of species from different forest types, especially fast-growing woody exotics.

Variations in water use efficiency and carbon and nitrogen concentrations in red heart Chinese fir

Temperature can significantly (P < 0.05) affect plant growth by modifying water use strategies, which are determined by intrinsic water use efficiency (WUE i). Red Heart Chinese Fir (Cunninghamia lanceolata) is one of the most important ecological and economic plantation species in China. However, the C. lanceolata water use strategy in response to increased temperatures and uneven temporal distribution of precipitation during the growing season is rarely reported. In a 7-year-old C. lanceolata plantation, differences in WUEi and C and N concentrations in different organs were analysed by anova, and the δ13C stable isotope, C, and N concentrations in stems determined at different tree heights. Stepwise regression and variance inflation factor were used to remove autocorrelated factors, and structural equation modelling was then used to explore relationships between WUEi and climate and biological factors. WUEi differed significantly between leaf and branch at different standardized precipitation evapotranspiration indices (SPEI). WUEi and N concentration decreased with age. The highest WUEi in branches and leaves were 92.7 and 88.4 μmol·mol-1 in 2020 (SPEI = 0.00), respectively. δ13C increased with relative tree height but N concentration and C/N ratio were not affected. Air temperatures has increased in between 2014 and 2020. WUEi and N concentration decreased with increasing branch and leaf age, but C concentration increased. SPEI significantly positively affected WUEi (P < 0.05), and WUE i was significantly negatively related to C concentration, which is consistent with the trade-off between C and water.

Photosynthetic responses of large old Zelkova serrata (Thunb.) Makino trees to different growth environments

Large old trees, which provide ecosystem services and serve as a historical and cultural heritage, are exposed to various environmental threats, such as habitat fragmentation and climate change, necessitating diagnosis of tangible and intangible stresses and their effects on tree growth for effective management. This study investigated the photosynthetic characteristics of 25 large old Zelkova serrata (Thunb.) Makino trees in Chungcheong Province, Korea, and identified the physical environmental factors affecting their physiological responses. Maximum assimilation rate (Amax) was the highest in July (summer), transpiration rate (E) and stomatal conductance (gs) increased from May (spring) to September (fall), and water use efficiency (WUE) was the highest in May (spring) and decreased until September (fall). Amax decreased as tree height increased. Ambient CO2 and vapor pressure deficit (VPD) were negatively correlated with photosynthetic parameters throughout the growth season and in July (summer) and September (fall), respectively. Physical environmental factors exhibited complex effect on physiological activities, which increased with wide growth space and decreased with deep soil covering and high impervious ground surface ratio. Physiological responses differed with surface types within the growth space, with bare land showing higher mean Amax, E, and gs than areas with mulching material or concrete. This study quantitatively determined the physiological activities of large old Z. serrata and proposes appropriate management measures for ensuring their healthy growth in abiotic stress environment.

The role of height-driven constraints and compensations on tree vulnerability to drought

Frequent observations of higher mortality in larger trees than in smaller ones during droughts have sparked an increasing interest in size-dependent drought-induced mortality. However, the underlying physiological mechanisms are not well understood, with height-associated hydraulic constraints often being implied as the potential mechanism driving increased drought vulnerability. We performed a quantitative synthesis on how key traits that drive plant water and carbon economy change with tree height within species and assessed the implications that the different constraints and compensations may have on the interacting mechanisms (hydraulic failure, carbon starvation and/or biotic-agent attacks) affecting tree vulnerability to drought. While xylem tension increases with tree height, taller trees present a range of structural and functional adjustments, including more efficient water use and transport and greater water uptake and storage capacity, that mitigate the path-length-associated drop in water potential. These adaptations allow taller trees to withstand episodic water stress. Conclusive evidence for height-dependent increased vulnerability to hydraulic failure and carbon starvation, and their coupling to defence mechanisms and pest and pathogen dynamics, is still lacking. Further research is needed, particularly at the intraspecific level, to ascertain the specific conditions and thresholds above which height hinders tree survival under drought.

Leaf-level coordination principles propagate to the ecosystem scale

Fundamental axes of variation in plant traits result from trade-offs between costs and benefits of resource-use strategies at the leaf scale. However, it is unclear whether similar trade-offs propagate to the ecosystem level. Here, we test whether trait correlation patterns predicted by three well-known leaf- and plant-level coordination theories - the leaf economics spectrum, the global spectrum of plant form and function, and the least-cost hypothesis - are also observed between community mean traits and ecosystem processes. We combined ecosystem functional properties from FLUXNET sites, vegetation properties, and community mean plant traits into three corresponding principal component analyses. We find that the leaf economics spectrum (90 sites), the global spectrum of plant form and function (89 sites), and the least-cost hypothesis (82 sites) all propagate at the ecosystem level. However, we also find evidence of additional scale-emergent properties. Evaluating the coordination of ecosystem functional properties may aid the development of more realistic global dynamic vegetation models with critical empirical data, reducing the uncertainty of climate change projections.

Contrasting physiological traits of shade tolerance in Pinus and Podocarpaceae native to a tropical Vietnamese forest: insight from an aberrant flat-leaved pine

The absence of pines from tropical forests is a puzzling biogeographical oddity potentially explained by traits of shade intolerance. Pinus krempfii (Lecomte), a flat-leaved pine endemic to the Central Highlands of Vietnam, provides a notable exception as it seems to compete successfully with shade-tolerant tropical species. Here, we test the hypothesis that successful conifer performance at the juvenile stage depends on physiological traits of shade tolerance by comparing the physiological characteristics of P. krempfii to coexisting species from two taxa: the genus Pinus, and a relatively abundant and shade-tolerant conifer family found in pantropical forests, the Podocarpaceae. We examined leaf photosynthetic, respiratory and biochemical traits. Additionally, we compiled attainable maximum photosynthesis, maximum RuBP carboxylation (Vcmax) and maximum electron transport (Jmax) values for Pinus and Podocarpaceae species from the literature. In our literature compilation, P. krempfii was intermediate between Pinus and Podocarpaceae in its maximum photosynthesis and its Vcmax. Pinus exhibited a higher Vcmax than Podocarpaceae, resulting in a less steep slope in the linear relationship between Jmax and Vcmax. These results suggest that Pinus may be more shade intolerant than Podocarpaceae, with P. krempfii falling between the two taxa. However, in contrast, Vietnamese conifers' leaf mass per areas and biochemical traits did not highlight the same intermediate nature of P. krempfii. Furthermore, regardless of leaf morphology or family assignation, all species demonstrated a common and extremely high carbon gain efficiency. Overall, our findings highlight the importance of shade-tolerant photosynthetic traits for conifer survival in tropical forests. However, they also demonstrate a diversity of shade tolerance strategies, all of which lead to the persistence of Vietnamese juvenile conifers in low-light tropical understories.

Determination of the most effective design for the measurement of photosynthetic light-response curves for planted Larix olgensis trees

A photosynthetic light-response (PLR) curve is a mathematical description of a single biochemical process and has been widely applied in many eco-physiological models. To date, many PLR measurement designs have been suggested, although their differences have rarely been explored, and the most effective design has not been determined. In this study, we measured three types of PLR curves (High, Middle and Low) from planted Larix olgensis trees by setting 31 photosynthetically active radiation (PAR) gradients. More than 530 million designs with different combinations of PAR gradients from 5 to 30 measured points were conducted to fit each of the three types of PLR curves. The influence of different PLR measurement designs on the goodness of fit of the PLR curves and the accuracy of the estimated photosynthetic indicators were analysed, and the optimal design was determined. The results showed that the measurement designs with fewer PAR gradients generally resulted in worse predicted accuracy for the photosynthetic indicators. However, the accuracy increased and remained stable when more than ten measurement points were used for the PAR gradients. The mean percent error (M%E) of the estimated maximum net photosynthetic rate (Pmax) and dark respiratory rate (Rd) for the designs with less than ten measurement points were, on average, 16.4 times and 20.1 times greater than those for the designs with more than ten measurement points. For a single tree, a unique PLR curve design generally reduced the accuracy of the predicted photosynthetic indicators. Thus, three optimal measurement designs were provided for the three PLR curve types, in which the root mean square error (RMSE) values reduced by an average of 8.3% and the coefficient of determination (R2) values increased by 0.3%. The optimal design for the High PLR curve type should shift more towards high-intensity PAR values, which is in contrast to the optimal design for the Low PLR curve type, which should shift more towards low-intensity PAR values.

Photosynthesis and photosynthetic efficiencies along the terrestrial plant's phylogeny: lessons for improving crop photosynthesis

Photosynthesis is the basis of all life on Earth. Surprisingly, until very recently, data on photosynthesis, photosynthetic efficiencies, and photosynthesis limitations in terrestrial land plants other than spermatophytes were very scarce. Here we provide an updated data compilation showing that maximum photosynthesis rates (expressed either on an area or dry mass basis) progressively scale along the land plant's phylogeny, from lowest values in bryophytes to largest in angiosperms. Unexpectedly, both photosynthetic water (WUE) and nitrogen (PNUE) use efficiencies also scale positively through the phylogeny, for which it has been commonly reported that these two efficiencies tend to trade-off between them when comparing different genotypes or a single species subject to different environmental conditions. After providing experimental evidence that these observed trends are mostly due to an increased mesophyll conductance to CO₂ - associated with specific anatomical changes - along the phylogeny, we discuss how these findings on a large phylogenetic scale can provide useful information to address potential photosynthetic improvements in crops in the near future.

Diurnal variation in mesophyll conductance and its influence on modelled water-use efficiency in a mature boreal Pinus sylvestris stand

Mesophyll conductance (gm) is a critical variable for the use of stable carbon isotopes to infer photosynthetic water-use efficiency (WUE). Although gm is similar in magnitude to stomatal conductance (gs), it has been measured less often, especially under field conditions and at high temporal resolution. We mounted an isotopic CO2 analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic 13C discrimination (Δ13C) on mature Pinus sylvestris trees. This allowed the calculation of gm, gs, net photosynthesis (Anet), and WUE. These measurements highlighted the asynchronous diurnal behaviour of gm and gs. While gs declined from around 10:00, Anet declined first after 12:00, and gm remained near its maximum until 16:00. We suggest that high gm played a role in supporting an extended Anet peak despite stomatal closure. Comparing three models to estimate WUE from ∆13C, we found that a simple model, assuming constant net fractionation during carboxylation (27‰), predicted WUE well, but only for about 75% of the day. A more comprehensive model, accounting explicitly for gm and the effects of daytime respiration and photorespiration, gave reliable estimates of WUE, even in the early morning hours when WUE was more variable. Considering constant, finite gm or gm/gs yielded similar WUE estimates on the diurnal scale, while assuming infinite gm led to overestimation of WUE. These results highlight the potential of high-resolution gm measurements to improve modelling of Anet and WUE and demonstrate that such gm data can be acquired, even under field conditions.

Dynamic Simulation of the Crown Net Photosynthetic Rate for Young Larix olgensis Henry Trees

Numerical integration of the instantaneous net photosynthetic rate (An) is a common method for calculating the long-term CO2 uptake of trees, and accurate dynamic simulation of the crown An has been receiving substantial attention. Tree characteristics are challenging to assess given their aerodynamically coarse crown properties, spatiotemporal variation in leaf functional traits and microenvironments. Therefore, the variables associated with the dynamic variations in the crown An must be identified. The relationships of leaf temperature (Tleaf), the vapor pressure deficit (VPD), leaf mass per area (LMA) and the relative depth into the crown (RDINC) with the parameters of the photosynthetic light-response (PLR) model of Larix olgensis Henry were analyzed. The LMA, RDINC and VPD were highly correlated with the maximum net photosynthetic rate (Amax). The VPD was the key variable that mainly determined the variation in the apparent quantum yield (AQY). Tleaf exhibited a significant exponential correlation with the dark respiration rate (Rd). According to the above correlations, the crown PLR model of L. olgensis trees was constructed by linking VPD, LMA and RDINC to the original PLR equation. The model performed well, with a high coefficient of determination (R2) value (0.883) and low root mean square error (RMSE) value (1.440 μmol m−2 s−1). The extinction coefficient (k) of different pseudowhorls within a crown was calculated by the Beer–Lambert equation based on the observed photosynthetically active radiation (PAR) distribution. The results showed that k was not a constant value but varied with the RDINC, solar elevation angle (ψ) and cumulative leaf area of the whole crown (CLA). Thus, we constructed a k model by reparameterizing the power function of RDINC with the ψ and CLA, and the PAR distribution within a crown was therefore well estimated (R2 = 0.698 and RMSE = 174.4 μmol m−2 s−1). Dynamic simulation of the crown An for L. olgensis trees was achieved by combining the crown PLR model and dynamic PAR distribution model. Although the models showed some weakened physiological biochemical processes during photosynthesis, they enabled the estimation of long-term CO2 uptake for an L. olgensis plantation, and the results could be easily fitted to gas-exchange measurements.

Coastal water quality assessment and groundwater transport in a subtropical mangrove swamp in Daya Bay, China

Coastal water quality assessment is challenging due to the complex hydrological environment in mangrove swamps. Such assessment requires a good understanding of swamp hydrology and potential solute reactions. In this study, we investigated the concentration variations of a suite of major elements (Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄^2-, HCO₃^- and Cl^-), nutrients such as dissolved inorganic phosphorous (DIP) and silicate (DSi), and heavy metals (Cu, Zn, As, Hg, Cd, Pb and Cr) along a typical mangrove transect in Daya Bay, China. The transect comprises a tidal creek, a mudflat and two mangrove zones. The major elements exhibited various degrees of dilution and loss, such as losses of K⁺ due to uptake by mangrove trees and of SO₄^2- due to microbial sulfate reduction. Numerical simulations of groundwater flow showed that the inland fresh groundwater can continuously discharge through the landward mangrove zone, where high concentrations of DIP and DSi occurred. However, the middle mangrove zone served as a sink of DIP and DSi due to the weak hydrodynamic environment. The spatial distribution of heavy metal concentrations showed that only Cu pollution occurred in the creek zone. The Metal Pollution Index (MPI) was developed to compare the enrichment capability of heavy metals at the different sampling sites. Results showed that the intertidal groundwater has the strongest heavy metal enrichment capability, followed by sea water, inland groundwater and river water. Within the intertidal zone, the creek zone has the highest MPI, followed by the mangrove zone and the mudflat zone. The local water circulation occurred around the creek should contribute the high MPI in the creek zone.

Spatial and Seasonal Variations of Standardized Photosynthetic Parameters under Different Environmental Conditions for Young Planted Larix olgensis Henry Trees

Understanding the spatial and seasonal variations in leaf physiology is critical for accurately modeling the carbon uptake, physiological processes and growth of entire canopies and stands. For a 17-year-old Larix olgensis Henry plantation, vertical whorl-by-whorl sampling and analyses of seasonally repeated measurements of major photosynthetic parameters were conducted, and the correlations between photosynthetic parameters and environmental conditions, leaf morphological traits and spatial position within the crown were analyzed. According to the correlations, the photosynthetic parameters were standardized based on the environmental conditions to avoid the influence of the changing environment on the patterns of spatial and seasonal variations of photosynthetic parameters. The results showed that the standardized light-saturated net photosynthetic rate (SPmax), standardized dark respiration (SRd) and standardized stomatal conductance under saturated light (Sgs-sat) were all negatively related to the relative depth into the crown (RDINC) throughout the growing season. However, their vertical patterns were different during the development of the phenological phase. In addition, different gradients of environmental conditions also influenced the values and the range of the vertical variation in photosynthesis. High temperature and low humidity usually resulted in smaller values and weaker vertical variations of SPmax and Sgs-sat, but larger values and more obvious vertical variations in SRd. SPmax and Sgs-sat usually exhibited a parabolic seasonal pattern in different vertical positions within the crown; however, SRd generally followed a concave pattern. These seasonal patterns were all weaker with increasing RDINC. Different environments also exhibited a significant influence on the seasonal patterns of photosynthesis. We suggested that standardization is necessary before analyzing spatial and seasonal variations. A single environmental condition could not represent the spatial and seasonal patterns under all gradients of the environment. Spatial and seasonal variations should be simultaneously analyzed because they are related to each other.

Physiological and structural tradeoffs underlying the leaf economics spectrum

The leaf economics spectrum (LES) represents a suite of intercorrelated leaf traits concerning construction costs per unit leaf area, nutrient concentrations, and rates of carbon fixation and tissue turnover. Although broad trade-offs among leaf structural and physiological traits have been demonstrated, we still do not have a comprehensive view of the fundamental constraints underlying the LES trade-offs. Here, we investigated physiological and structural mechanisms underpinning the LES by analysing a novel data compilation incorporating rarely considered traits such as the dry mass fraction in cell walls, nitrogen allocation, mesophyll CO₂ diffusion and associated anatomical traits for hundreds of species covering major growth forms. The analysis demonstrates that cell wall constituents are major components of leaf dry mass (18-70%), especially in leaves with high leaf mass per unit area (LMA) and long lifespan. A greater fraction of leaf mass in cell walls is typically associated with a lower fraction of leaf nitrogen (N) invested in photosynthetic proteins; and lower within-leaf CO₂ diffusion rates, as a result of thicker mesophyll cell walls. The costs associated with greater investments in cell walls underpin the LES: long leaf lifespans are achieved via higher LMA and in turn by higher cell wall mass fraction, but this inevitably reduces the efficiency of photosynthesis.

Increased Needle Nitrogen Contents Did Not Improve Shoot Photosynthetic Performance of Mature Nitrogen-Poor Scots Pine Trees

Numerous studies have shown that temperate and boreal forests are limited by nitrogen (N) availability. However, few studies have provided a detailed account of how carbon (C) acquisition of such forests reacts to increasing N supply. We combined measurements of needle-scale biochemical photosynthetic capacities and continuous observations of shoot-scale photosynthetic performance from several canopy positions with simple mechanistic modeling to evaluate the photosynthetic responses of mature N-poor boreal Pinus sylvestris to N fertilization. The measurements were carried out in August 2013 on 90-year-old pine trees growing at Rosinedalsheden research site in northern Sweden. In spite of a nearly doubling of needle N content in response to the fertilization, no effect on the long-term shoot-scale C uptake was recorded. This lack of N-effect was due to strong light limitation of photosynthesis in all investigated canopy positions. The effect of greater N availability on needle photosynthetic capacities was also constrained by development of foliar phosphorus (P) deficiency following N addition. Thus, P deficiency and accumulation of N in arginine appeared to contribute toward lower shoot-scale nitrogen-use efficiency in the fertilized trees, thereby additionally constraining tree-scale responses to increasing N availability. On the whole our study suggests that the C uptake response of the studied N-poor boreal P. sylvestris stand to enhanced N availability is constrained by the efficiency with which the additional N is utilized. This efficiency, in turn, depends on the ability of the trees to use the greater N availability for additional light capture. For stands that have not reached canopy closure, increase in leaf area following N fertilization would be the most effective way for improving light capture and C uptake while for mature stands an increased leaf area may have a rather limited effect on light capture owing to increased self-shading. This raises the question if N limitation in boreal forests acts primarily by constraining growth of young stands while the commonly recorded increase in stem growth of mature stands following N addition is primarily the result of altered allocation and only to a limited extent the result of increased stand C-capture.

Seasonal and within-canopy variation in shoot-scale resource-use efficiency trade-offs in a Norway spruce stand

Previous leaf-scale studies of carbon assimilation describe short-term resource-use efficiency (RUE) trade-offs where high use efficiency of one resource requires low RUE of another. However, varying resource availabilities may cause long-term RUE trade-offs to differ from the short-term patterns. This may have important implications for understanding canopy-scale resource use and allocation. We used continuous gas exchange measurements collected at five levels within a Norway spruce, Picea abies (L.) karst., canopy over 3 years to assess seasonal differences in the interactions between shoot-scale resource availability (light, water and nitrogen), net photosynthesis (An ) and the use efficiencies of light (LUE), water (WUE) and nitrogen (NUE) for carbon assimilation. The continuous data set was used to develop and evaluate multiple regression models for predicting monthly shoot-scale An . These models showed that shoot-scale An was strongly dependent on light availability and was generally well described with simple one- or two-parameter models. WUE peaked in spring, NUE in summer and LUE in autumn. However, the relative importance of LUE for carbon assimilation increased with canopy depth at all times. Our results suggest that accounting for seasonal and within-canopy trade-offs may be important for RUE-based modelling of canopy carbon uptake.

Peach water relations, gas exchange, growth and shoot mortality under water deficit in semi-arid weather conditions

In this study the sensitivity of peach tree (Prunus persica L.) to three water stress levels from mid-pit hardening until harvest was assessed. Seasonal patterns of shoot and fruit growth, gas exchange (leaf photosynthesis, stomatal conductance and transpiration) as well as carbon (C) storage/mobilization were evaluated in relation to plant water status. A simple C balance model was also developed to investigate sink-source relationship in relation to plant water status at the tree level. The C source was estimated through the leaf area dynamics and leaf photosynthesis rate along the season. The C sink was estimated for maintenance respiration and growth of shoots and fruits. Water stress significantly reduced gas exchange, and fruit, and shoot growth, but increased fruit dry matter concentration. Growth was more affected by water deficit than photosynthesis, and shoot growth was more sensitive to water deficit than fruit growth. Reduction of shoot growth was associated with a decrease of shoot elongation, emergence, and high shoot mortality. Water scarcity affected tree C assimilation due to two interacting factors: (i) reduction in leaf photosynthesis (-23% and -50% under moderate (MS) and severe (SS) water stress compared to low (LS) stress during growth season) and (ii) reduction in total leaf area (-57% and -79% under MS and SS compared to LS at harvest). Our field data analysis suggested a Ψ_stem threshold of -1.5 MPa below which daily net C gain became negative, i.e. C assimilation became lower than C needed for respiration and growth. Negative C balance under MS and SS associated with decline of trunk carbohydrate reserves – may have led to drought-induced vegetative mortality.

Seasonal variations in photosynthesis, intrinsic water-use efficiency and stable isotope composition of poplar leaves in a short-rotation plantation

Photosynthetic carbon assimilation and transpirational water loss play an important role in the yield and the carbon sequestration potential of bioenergy-devoted cultures of fast-growing trees. For six poplar (Populus) genotypes in a short-rotation plantation, we observed significant seasonal and genotypic variation in photosynthetic parameters, intrinsic water-use efficiency (WUEi) and leaf stable isotope composition (δ13C and δ18O). The poplars maintained high photosynthetic rates (between 17.8 and 26.9 μmol m(-2) s(-1) depending on genotypes) until late in the season, in line with their fast-growth habit. Seasonal fluctuations were mainly explained by variations in soil water availability and by stomatal limitation upon photosynthesis. Stomatal rather than biochemical limitation was confirmed by the constant intrinsic photosynthetic capacity (Vcmax) during the growing season, closely related to leaf nitrogen (N) content. Intrinsic water-use efficiency scaled negatively with carbon isotope discrimination (Δ13Cbl) and positively with the ratio between mesophyll diffusion conductance (gm) and stomatal conductance. The WUEi-Δ13Cbl relationship was partly influenced by gm. There was a trade-off between WUEi and photosynthetic N-use efficiency, but only when soil water availability was limiting. Our results suggest that seasonal fluctuations in relation to soil water availability should be accounted for in future modelling studies assessing the carbon sequestration potential and the water-use efficiency of woody energy crops.

The role of mesophyll conductance in the economics of nitrogen and water use in photosynthesis

A recent resurgence of interest in formal optimisation theory has begun to improve our understanding of how variations in stomatal conductance and photosynthetic capacity control the response of whole plant photosynthesis and growth to the environment. However, mesophyll conductance exhibits similar variation and has similar impact on photosynthesis as stomatal conductance; yet, the role of mesophyll conductance in the economics of photosynthetic resource use has not been thoroughly explored. In this article, we first briefly summarise the knowledge of how mesophyll conductance varies in relation to environmental factors that also affect stomatal conductance and photosynthetic capacity, and then we use a simple analytical approach to begin to explore how these important controls on photosynthesis should mutually co-vary in a plant canopy in the optimum. Our analysis predicts that when either stomatal or mesophyll conductance is limited by fundamental biophysical constraints in some areas of a canopy, e.g. reduced stomatal conductance in upper canopy leaves due to reduced water potential, the other of the two conductances should increase in those leaves, while photosynthetic capacity should decrease. Our analysis also predicts that if mesophyll conductance depends on nitrogen investment in one or more proteins, then nitrogen investment should shift away from Rubisco and towards mesophyll conductance if hydraulic or other constraints cause chloroplastic CO2 concentration to decline. Thorough exploration of these issues awaits better knowledge of whether and how mesophyll conductance is itself limited by nitrogen investment, and about how these determinants of photosynthetic CO2 supply and demand co-vary among leaves in real plant canopies.

Photosynthetic and anatomical responses of Eucalyptus grandis leaves to potassium and sodium supply in a field experiment

Although vast areas in tropical regions have weathered soils with low potassium (K) levels, little is known about the effects of K supply on the photosynthetic physiology of trees. This study assessed the effects of K and sodium (Na) supply on the diffusional and biochemical limitations to photosynthesis in Eucalyptus grandis leaves. A field experiment comparing treatments receiving K (+K) or Na (+Na) with a control treatment (C) was set up in a K-deficient soil. The net CO2 assimilation rates were twice as high in +K and 1.6 times higher in +Na than in the C as a result of lower stomatal and mesophyll resistance to CO2 diffusion and higher photosynthetic capacity. The starch content was higher and soluble sugar was lower in +K than in C and +Na, suggesting that K starvation disturbed carbon storage and transport. The specific leaf area, leaf thickness, parenchyma thickness, stomatal size and intercellular air spaces increased in +K and +Na compared to C. Nitrogen and chlorophyll concentrations were also higher in +K and +Na than in C. These results suggest a strong relationship between the K and Na supply to E. grandis trees and the functional and structural limitations to CO2 assimilation rates.

Variation in leaf and twig CO2 flux as a function of plant size: a comparison of seedlings, saplings and trees

Rates of tissue-level function have been hypothesized to decline as trees grow older and larger, but relevant evidence to assess such changes remains limited, especially across a wide range of sizes from saplings to large trees. We measured functional traits of leaves and twigs of three cold-temperate deciduous tree species in Minnesota, USA, to assess how these vary with tree height. Individuals ranging from 0.13 to 20 m in height were sampled in both relatively open and closed canopy environments to minimize light differences as a potential driver of size-related differences in leaf and twig properties. We hypothesized that (H1) gas-exchange rates, tissue N concentration and leaf mass per unit area (LMA) would vary with tree size in a pattern reflecting declining function in taller trees, yet maintaining (H2) bivariate trait relations, common among species as characterized by the leaf economics spectrum. Taking these two ideas together yielded a third, integrated hypothesis that (H3) nitrogen (N) content and gas-exchange rates should decrease monotonically with tree size and LMA should increase. We observed increasing LMA and decreasing leaf and twig Rd with increasing size, which matched predictions from H1 and H3. However, opposite to our predictions, leaf and twig N generally increased with size, and thus had inverse relations with respiration, rather than the predicted positive relations. Two exceptions were area-based leaf N of Prunus serotina Ehrh. in gaps and mass-based leaf N of Quercus ellipsoidalis E. J. Hill in gaps, both of which showed qualitatively hump-shaped patterns. Finally, we observed hump-shaped relationships between photosynthetic capacity and tree height, not mirroring any of the other traits, except in the two cases highlighted above. Bivariate trait relations were weak intra-specifically, but were generally significant and positive for area-based traits using the pooled dataset. Results suggest that different traits vary with tree size in different ways that are not consistent with a universal shift towards a lower 'return on investment' strategy. Instead, species traits vary with size in patterns that likely reflect complex variation in water, light, nitrogen and carbon availability, storage and use.

Effects of elevated CO₂ and temperature on photosynthesis and leaf traits of an understory dwarf bamboo in subalpine forest zone, China

The dwarf bamboo (Fargesia rufa Yi), growing understory in subalpine dark coniferous forest, is one of the main foods for giant panda, and it influences the regeneration of subalpine coniferous forests in southwestern China. To investigate the effects of elevated CO₂, temperature and their combination, the dwarf bamboo plantlets were exposed to two CO₂ regimes (ambient and double ambient CO₂ concentration) and two temperatures (ambient and +2.2°C) in growth chambers. Gas exchange, leaf traits and carbohydrates concentration were measured after the 150-day experiment. Elevated CO₂ significantly increased the net photosynthetic rate (Anet ), intrinsic water-use efficiency (WUEi ) and carbon isotope composition (δ¹³C) and decreased stomatal conductance (g(s)) and total chlorophyll concentration based on mass (Chl(m)) and area (Chl(a)). On the other hand, elevated CO₂ decreased specific leaf area (SLA), which was increased by elevated temperature. Elevated CO₂ also increased foliar carbon concentration based on mass (C(m)) and area (C(a)), nitrogen concentration based on area (N(a)), carbohydrates concentration (i.e. sucrose, sugar, starch and non-structural carbohydrates) and the slope of the A(net)-N(a) relationship. However, elevated temperature decreased C(m), C(a) and N(a). The combination of elevated CO₂ and temperature hardly affected SLA, C(m), C(a), N(m), N(a), Chl(m) and Chl(a). Variables Anet and Na had positive linear relationships in all treatments. Our results showed that photosynthetic acclimation did not occur in dwarf bamboo at elevated CO₂ and it could adjust physiology and morphology to enable the capture of more light, to increase WUE and improve nutritional conditions.

On the complementary relationship between marginal nitrogen and water-use efficiencies among Pinus taeda leaves grown under ambient and CO2-enriched environments

BACKGROUND AND AIMS

Water and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration (E) can be decreased by reducing leaf stomatal conductance (g(s)) at the expense of lowering CO(2) uptake (A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when g(s) declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration (N) varies on time scales much longer than g(s), an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO(2) concentration (c(a)).

METHODS

Optimization theory is employed to quantify interactions between resource-use efficiencies under elevated c(a) and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, λ = (∂A/∂g(s))/(∂E/∂g(s)) and η = ∂A/∂N, respectively. The relationship between the two efficiencies and related variation in intercellular CO(2) concentration (c(i)) were examined using A/c(i) curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO(2) Enrichment experiment (North Carolina, USA).

KEY RESULTS

Optimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where η is complementary to the square-root of λ. The data support the model predictions that elevated c(a) increased η and λ, and at given c(a) and needle age-class, the two quantities varied among needles in an approximately complementary manner.

CONCLUSIONS

The derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of η, and assist in predicting how increasing c(a) influences ecosystem water use.

Mesophyll diffusion conductance to CO2: an unappreciated central player in photosynthesis

Mesophyll diffusion conductance to CO(2) is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g(m), and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance. Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.

Effects of potassium supply on limitations of photosynthesis by mesophyll diffusion conductance in Carya cathayensis

Potassium (K) influences the photosynthesis process in a number of ways; however, the mechanisms underlying the photosynthetic response to differences in K supply are not well understood. Concurrent measurements of gas exchange and chlorophyll fluorescence were made to investigate the effect of K nutrition on photosynthetic efficiency and mesophyll conductance (g(m)) in hickory seedlings (Carya cathayensis Sarg.) in a greenhouse. The results show that leaf K concentrations < 0.7-0.8% appeared to limit the leaf net CO2 assimilation rate (A), and that the relative limitation of photosynthesis due to g(m) and stomatal conductance (g(s)) decreased with increasing supplies of K. However, a sensitivity analysis indicated that A was most sensitive to the maximum carboxylation rate of Rubisco (V(c,max)) and the maximum rate of electron transport (J(max)). These results indicate that the photosynthetic rate is primarily limited by the biochemical processes of photosynthesis (V(c,max) and J(max)), rather than by g(m) and g(s) in K-deficient plants. Additionally, g(m) was closely correlated with g(s) and the leaf dry mass per unit area (M(A)) in hickory seedlings, which indicates that decreased g(m) and g(s) may be a consequence of leaf anatomical adaptation.

Canopy processes in a changing climate

Forest canopies exchange a large part of the mass and energy between the earth and the atmosphere. The processes that regulate these exchanges have been of interest to scientists from a diverse range of disciplines for a long time. The International Union of Forest Research Organizations (IUFRO) Canopy Processes Working Group provides a forum for these scientists to explore canopy processes at scales ranging from the leaf to the ecosystem. Given the changes in climate that are being experienced in response to rising [CO(2)], there is a need to understand how forest canopy processes respond to altered environments. Globally, native and managed forests represent the largest terrestrial biome and, in wood and soils, the largest terrestrial stores of carbon. Changing climates have significant implications for carbon storage in forests, as well as their water use, species diversity and management. In order to address these issues, the Canopy Processes Working Group held a travelling workshop in south-east Australia during October 2010 to examine the impact of changing climates on forest canopies, highlighting knowledge gaps and developing new research directions.