Seasonal variations drive short-term dynamics and partitioning of recently assimilated carbon in the foliage of adult beech and pine

Sources of carbon supporting the fast growth of developing immature moso bamboo (Phyllostachys edulis) culms: inference from carbon isotopes and anatomy

Phyllostachys edulis is a spectacularly fast-growing species that completes its height growth within 2 months after the shoot emerges without producing leaves (fast-growing period, FGP). This phase was considered heterotrophic, with the carbon necessary for the growth being transferred from the mature culms via the rhizomes, although previous studies observed key enzymes and anatomical features related to C₄-carbon fixation in developing culms. We tested whether C₄-photosynthesis or dark-CO₂ fixation through anaplerotic reactions significantly contributes to the FGP, resulting in differences in the natural abundance of δ¹³C in bulk organic matter and organic compounds. Further, pulse-¹³CO₂-labelling was performed on developing culms, either from the surface or from the internal hollow, to ascertain whether significant CO₂ fixation occurs in developing culms. δ¹³C of young shoots and developing culms were higher (-26.3 to -26.9 ‰) compared to all organs of mature bamboos (-28.4 to -30.1 ‰). Developing culms contained chlorophylls, most observed in the skin tissues. After pulse-¹³CO₂-labelling, the polar fraction extracted from the skin tissues was slightly enriched in ¹³C, and only a weak ¹³C enrichment was observed in inner tissues. Main carbon source sustaining the FGP was not assimilated by the developing culm, while a limited anaplerotic fixation of respired CO₂ cannot be excluded and is more likely than C₄-photosynthetic carbon fixation.

Estimating intra-seasonal photosynthetic discrimination and water use efficiency using δ13C of leaf sucrose in Scots pine

Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (δ13C) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow δ13C signals within trees. To study how the choice of organic material may bias the interpretation of δ13C records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time δ13C of different leaf carbon pools with δ13C of assimilates estimated by a chamber-Picarro system (δ13CA_Picarro), and a photosynthetic discrimination model (δ13CA_model). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of δ13CA_Picarro and δ13CA_model. Consequently, in comparison with the other carbon pools, sucrose δ13C was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from δ13C of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose δ13C to understand plant physiological responses in depth.

Tree organ growth and carbon allocation dynamics impact the magnitude and δ13C signal of stem and soil CO2 fluxes

Incomplete knowledge of carbon (C) allocation dynamics in trees hinders accurate modeling and future predictions of tree growth. We studied C allocation dynamics in a mature Pinus sylvestris L. dominated forest with a novel analytical approach, allowing the first comparison of: (i) magnitude and δ13C of shoot, stem and soil CO2 fluxes (Ashoot, Rstem and Rsoil), (ii) concentration and δ13C of compound-specific and/or bulk non-structural carbohydrates (NSCs) in phloem and roots and (iii) growth of stem and fine roots. Results showed a significant effect of phloem NSC concentrations on tracheid growth, and both variables significantly impacted Rstem. Also, concentrations of root NSCs, especially starch, had a significant effect on fine root growth, although no effect of root NSC concentrations or root growth was detected on Rsoil. Time series analysis between δ13C of Ashoot and δ13C of Rstem or δ13C of Rsoil revealed strengthened C allocation to stem or roots under high C demands. Furthermore, we detected a significant correlation between δ13C of Rstem and δ13C of phloem sucrose and glucose, but not for starch or water-soluble carbohydrates. Our results indicate the need to include C allocation dynamics into tree growth models. We recommend using compound-specific concentration and δ13C analysis to reveal C allocation processes that may not be detected by the conventional approach that utilizes bulk organic matter.

Explicitly accounting for needle sugar pool size crucial for predicting intra-seasonal dynamics of needle carbohydrates δ18 O and δ13 C

We explore needle sugar isotopic compositions (δ¹⁸ O and δ¹³ C) in boreal Scots pine (Pinus sylvestris) over two growing seasons. A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict δ¹⁸ O and δ¹³ C of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool. Model results agreed well with observed needle water δ¹⁸ O, δ¹⁸ O and δ¹³ C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ¹³ C (R²  = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO₂ concentration ratio (C_i /C_a ) were the dominant drivers of δ¹⁸ O and δ¹³ C variability, respectively. However, the variability of needle sugar δ¹⁸ O and δ¹³ C was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and C_i /C_a , due to the large needle sugar pool, which caused the signal formation period to vary seasonally from 2 d to more than 5 d. Furthermore, accounting for a temperature-sensitive biochemical ¹⁸ O-fractionation factor and mesophyll resistance in ¹³ C-discrimination were critical. Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.

Aerial and underground organs display specific metabolic strategies to cope with water stress under rising atmospheric CO2 in Fagus sylvatica L

Beech is known to be a moderately drought-sensitive tree species, and future increases in atmospheric concentrations of CO₂ ([CO₂ ]) could influence its ecological interactions, also with changes at the metabolic level. The metabolome of leaves and roots of drought-stressed beech seedlings grown under two different [CO₂ ] (400 (aCO₂ ) and 800 (eCO₂ ) ppm) was analyzed together with gas exchange parameters and water status. Water stress estimated from predawn leaf water potential (Ψ_pd ) was similar under both [CO₂ ], although eCO₂ had a positive impact on net photosynthesis and intrinsic water use efficiency. The aerial and underground organs showed different metabolomes. Leaves mainly stored C metabolites, while those of N and P accumulated differentially in roots. Drought triggered the proline and N-rich amino acids biosynthesis in roots through the activation of arginine and proline pathways. Besides the TCA cycle, polyols and soluble sugar biosynthesis were activated in roots, with no clear pattern seen in the leaves, prioritizing the root functioning as metabolites sink. eCO₂ slightly altered this metabolic acclimation to drought, reflecting mitigation of its effect. The leaves showed only minor changes, investing C surplus in secondary metabolites and malic acid. The TCA cycle metabolites and osmotically active substances increased in roots, but many other metabolites decreased as if the water stress was dampened. Above- and belowground plant metabolomes were differentially affected by two drivers of climate change, water scarcity and high [CO₂ ], showing different chemical responsiveness that could modulate the tree adaptation to future climatic scenarios.

Drought affects the fate of non-structural carbohydrates in hinoki cypress

Tree species that close stomata early in response to drought are likely to suffer from an imbalance between limited carbohydrate supply due to reduced photosynthesis and metabolic demand. Our objective was to clarify the dynamic responses of non-structural carbohydrates to drought in a water-saving species, the hinoki cypress (Chamaecyparis obtusa Sieb. et Zucc.). To this end, we pulse-labeled young trees with 13CO2 10 days after the beginning of the drought treatment. Trees were harvested 7 days later, early during drought progression, and 86 days later when they had suffered from a long and severe drought. The labeled carbon (C) was traced in phloem extract, in the organic matter and starch of all the organs, and in the soluble sugars (sucrose, glucose and fructose) of the most metabolically active organs (foliage, green branches and fine roots). No drought-related changes in labeled C partitioning between belowground and aboveground organs were observed. The C allocation between non-structural carbohydrates was altered early during drought progression: starch concentration was lower by half in the photosynthetic organs, while the concentration of almost all soluble sugars tended to increase. The preferential allocation of labeled C to glucose and fructose reflected an increased demand for soluble sugars for osmotic adjustment. After 3 months of a lethal drought, the concentrations of soluble sugars and starch were admittedly lower in drought-stressed trees than in the controls, but the pool of non-structural carbohydrates was far from completely depleted. However, the allocation to storage had been impaired by drought; photosynthesis and the sugar translocation rate had also been reduced by drought. Failure to maintain cell turgor through osmoregulation and to refill embolized xylem due to the depletion in soluble sugars in the roots could have resulted in tree mortality in hinoki cypress, though the total pool of carbohydrate was not completely depleted.

Linking canopy-scale mesophyll conductance and phloem sugar δ13 C using empirical and modelling approaches

Interpreting phloem carbohydrate or xylem tissue carbon isotopic composition as measures of water-use efficiency or past tree productivity requires in-depth knowledge of the factors altering the isotopic composition within the pathway from ambient air to phloem contents and tree ring. One of least understood of these factors is mesophyll conductance (g_m ). We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative g_m descriptions and a simple transport of sugars from foliage down the trunk. We parameterised the model for a boreal Scots pine stand and compared simulated g_m responses with weather variations. We further compared the simulated δ¹³ C of new photosynthates among the different g_m descriptions and against measured phloem sugar δ¹³ C. Simulated g_m estimates of the seven descriptions varied according to weather conditions, resulting in varying estimates of phloem δ¹³ C during cold/moist and warm/dry periods. The model succeeded in predicting a drought response and a postdrought release in phloem sugar δ¹³ C indicating suitability of the model for inverse prediction of leaf processes from phloem isotopic composition. We suggest short-interval phloem sampling during and after extreme weather conditions to distinguish between mesophyll conductance drivers for future model development.

Rhizosphere activity in an old-growth forest reacts rapidly to changes in soil moisture and shapes whole-tree carbon allocation

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought-induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests' resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied ¹³CO₂ pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in above- and belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

In situ 13CO2 labelling of rubber trees reveals a seasonal shift in the contribution of the carbon sources involved in latex regeneration

Rubber trees (Hevea brasiliensis) are the main source of natural rubber, extracted from latex, which exudes from the trunk after tapping. Tapped trees require large amounts of carbon (C) to regenerate the latex after its collection. Knowing the contribution of C sources involved in latex biosynthesis will help in understanding how rubber trees face this additional C demand. Whole crown 13CO2 pulse labelling was performed on 4-year-old rubber trees in June, when latex production was low, and in October, when it was high. 13C content was quantified in the foliage, phloem sap, wood, and latex. In both labelling periods, 13C was recovered in latex just after labelling, indicating that part of the carbohydrate was directly allocated to latex. However, significant amounts of 13C were still recovered in latex after 100 d and the peak was reached significantly later than in phloem sap, demonstrating the contribution of a reserve pool as a source of latex C. The contribution of new photosynthates to latex regeneration was faster and higher when latex metabolism was well established, in October, than in June. An improved understanding of C dynamics and the source-sink relationship in rubber tree is crucial to adapt tapping system practices and ensure sustainable latex production.

Physiological Mechanisms of Foliage Recovery after Spring or Fall Crown Scorch in Young Longleaf Pine (Pinus palustris Mill.)

We hypothesized that physiological and morphological responses to prescribed fire support the post-scorch foliage recovery and growth of young longleaf pine. Two studies conducted in central Louisiana identified three means of foliage regrowth after fire that included an increase in the gas exchange rate of surviving foliage for 3 to 4 months after fire. Saplings also exhibited crown developmental responses to repeated fire that reduced the risk of future crown scorch. Starch reserves were a source of carbon for post-scorch foliage regrowth when fire was applied in the early growing season. However, the annual dynamics of starch accumulation and mobilization restricted its effectiveness for foliage regrowth when fire was applied late in the growing season. As such, post-scorch foliage regrowth became increasingly dependent on photosynthesis as the growing season progressed. Additionally, the loss of foliage by fire late in the growing season interrupted annual starch dynamics and created a starch void between the time of late growing season fire and mid-summer of the next year. The occurrence of drought during both studies revealed barriers to foliage reestablishment and normal stem growth among large saplings. In study 1, spring water deficit at the time of May fire was associated with high crown scorch and poor foliage and stem growth among large saplings. We attribute this lag in stem growth to three factors: little surviving foliage mass, low fascicle gas exchange rates, and poor post-scorch foliage recovery. In study 2, May fire during a short window of favorable burning conditions in the tenth month of a 20-month drought also reduced stem growth among large saplings but this growth loss was not due to poor post-scorch foliage recovery. Application of this information to prescribed fire guidelines will benefit young longleaf pine responses to fire and advance efforts to restore longleaf pine ecosystems.

The 18 O-signal transfer from water vapour to leaf water and assimilates varies among plant species and growth forms

The ¹⁸ O signature of atmospheric water vapour (δ¹⁸ O_V ) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the ¹⁸ O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with ¹⁸ O-depleted water vapour (-106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the ¹⁸ O-signal transfer by calculating the mean residence time of O in leaf water (MRT_LW ) and sugars (MRT_Sugars ) and related it to leaf traits and physiological drivers. MRT_LW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRT_Sugars was shorter in C₃ and C₄ plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRT_Sugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ¹⁸ O_V variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ¹⁸ O_V variations, which is important for the interpretation of δ¹⁸ O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.

Carbon-use strategies in stem radial growth of two oak species, one Temperate deciduous and one Mediterranean evergreen: what can be inferred from seasonal variations in the δ13C of the current year ring?

Tree ring synthesis is a key process in wood production; however, little is known of the origin and fate of the carbon involved. We used natural 13C abundance to investigate the carbon-use process for the ring development in a temperate deciduous (Quercus petraea (Matt.) Liebl.) and a Mediterranean evergreen (Quercus ilex L.) oak. The sapwood carbon reserves, phloem sucrose contents, stem respired CO2 efflux and their respective carbon isotope compositions (δ13C) were recorded over 1 year, in the native area of each species. The seasonal δ13C variation of the current year ring was determined in the total ring throughout the seasons, as well as in slices from the fully mature ring after the growth season (intra-ring pattern). Although the budburst dates of the two oaks were similar, the growth of Quercus ilex began 50 days later. Both species exhibited growth cessation during the hot and dry summer but only Q. ilex resumed in the autumn. In the deciduous oak, xylem starch storage showed clear variations during the radial growth. The intra-ring δ13C variations of the two species exhibited similar ranges, but contrasting patterns, with an early increase for Q. petraea. Comparison between δ13C of starch and total ring suggested that Q. petraea (but not Q. ilex) builds its rings using reserves during the first month of growth. Shifts in ring and soluble sugars δ13C suggested an interspecific difference in either the phloem unloading or the use of fresh assimilate inside the ring. A decrease in ring δ13C for both oaks between the end of the radial growth and the winter is attributed to a lignification of ring cell walls after stem increment. This study highlighted the differences in carbon-use during ring growth for evergreen and deciduous oaks, as well as the benefits of exploring the process using natural 13C abundance.

Differences in carbon isotope leaf-to-phloem fractionation and mixing patterns along a vertical gradient in mature European beech and Douglas fir

While photosynthetic isotope discrimination is well understood, the postphotosynthetic and transport-related fractionation mechanisms that influence phloem and subsequently tree ring δ¹³ C are less investigated and may vary among species. We studied the seasonal and diel courses of leaf-to-phloem δ¹³ C differences of water-soluble organic matter (WSOM) in vertical crown gradients and followed the assimilate transport via the branches to the trunk phloem at breast height in European beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii). δ¹³ C of individual sugars and cyclitols from a subsample was determined by compound-specific isotope analysis. In beech, leaf-to-phloem δ¹³ C differences in WSOM increased with height and were partly caused by biochemical isotope fractionation between leaf compounds. ¹³ C-Enrichment of phloem sugars relative to leaf sucrose implies an additional isotope fractionation mechanism related to leaf assimilate export. In Douglas fir, leaf-to-phloem δ¹³ C differences were much smaller and isotopically invariant pinitol strongly influenced leaf and phloem WSOM. Trunk phloem WSOM at breast height reflected canopy-integrated δ¹³ C in beech but not in Douglas fir. Our results demonstrate that leaf-to-phloem isotope fractionation and δ¹³ C mixing patterns along vertical gradients can differ between tree species. These effects have to be considered for functional interpretations of trunk phloem and tree ring δ¹³ C.

Drought impacts on tree phloem: from cell-level responses to ecological significance

On-going climate change is increasing the risk of drought stress across large areas worldwide. Such drought events decrease ecosystem productivity and have been increasingly linked to tree mortality. Understanding how trees respond to water shortage is key to predicting the future of ecosystem functions. Phloem is at the core of the tree functions, moving resources such as non-structural carbohydrates, nutrients, and defence and information molecules across the whole plant. Phloem function and ability to transport resources is tightly controlled by the balance of carbon and water fluxes within the tree. As such, drought is expected to impact phloem function by decreasing the amount of available water and new photoassimilates. Yet, the effect of drought on the phloem has received surprisingly little attention in the last decades. Here we review existing knowledge on drought impacts on phloem transport from loading and unloading processes at cellular level to possible effects on long-distance transport and consequences to ecosystems via ecophysiological feedbacks. We also point to new research frontiers that need to be explored to improve our understanding of phloem function under drought. In particular, we show how phloem transport is affected differently by increasing drought intensity, from no response to a slowdown, and explore how severe drought might actually disrupt the phloem transport enough to threaten tree survival. Because transport of resources affects other organisms interacting with the tree, we also review the ecological consequences of phloem response to drought and especially predatory, mutualistic and competitive relations. Finally, as phloem is the main path for carbon from sources to sink, we show how drought can affect biogeochemical cycles through changes in phloem transport. Overall, existing knowledge is consistent with the hypotheses that phloem response to drought matters for understanding tree and ecosystem function. However, future research on a large range of species and ecosystems is urgently needed to gain a comprehensive understanding of the question.

The impact of prolonged drought on phloem anatomy and phloem transport in young beech trees

Phloem failure has recently been recognized as one of the mechanisms causing tree mortality under drought, though direct evidence is still lacking. We combined 13C pulse-labelling of 8-year-old beech trees (Fagus sylvatica L.) growing outdoors in a nursery with an anatomical study of the phloem tissue in their stems to examine how drought alters carbon transport and phloem transport capacity. For the six trees under drought, predawn leaf water potential ranged from -0.7 to -2.4 MPa, compared with an average of -0.2 MPa in five control trees with no water stress. We also observed a longer residence time of excess 13C in the foliage and the phloem sap in trees under drought compared with controls. Compared with controls, excess 13C in trunk respiration peaked later in trees under moderate drought conditions and showed no decline even after 4 days under more severe drought conditions. We estimated higher phloem sap viscosity in trees under drought. We also observed much smaller sieve-tube radii in all drought-stressed trees, which led to lower sieve-tube conductivity and lower phloem conductance in the tree stem. We concluded that prolonged drought affected phloem transport capacity through a change in anatomy and that the slowdown of phloem transport under drought likely resulted from a reduced driving force due to lower hydrostatic pressure between the source and sink organs.

Metabolic response to elevated CO2 levels in Pinus pinaster Aiton needles in an ontogenetic and genotypic-dependent way

Global climate changes involve elevated atmospheric [CO₂], fostering the carbon allocation to tree sink tissues, partitioning it into metabolic pathways. We use metabolomics analysis in adult and juvenile needles of four Pinus pinaster genotypes exposed to two levels of growth [CO₂]: ambient (400 μmol mol^-1) and enriched (800 μmol mol^-1), to know if the metabolic responses are genotype-dependent and vary according to the stage of needle ontogeny. The eCO₂-induced changes in the needle metabolomes are more significant in secondary metabolism pathways and especially meaningful in juvenile needles. The heteroblasty has important consequences in the expression of the metabolome, and on the plasticity to CO₂, determining the level of specific metabolite accumulation, showing an interdependence between adult and juvenile needles. The P. pinaster needle metabolomes also show clear quantitative differences linked to genotype, as well as regarding the metabolic response to eCO₂, showing both, common and genotype-specific biochemical responses. Thus, the changes in flavonol levels are mainly genotype-independent, while those in terpenoid and free fatty acids are mainly genotype-dependent, ratifying the importance of genotype to determine the metabolic response to eCO₂. To understand the adaptation mechanisms that tree species can develop to cope with eCO₂ it is necessary to know the genetically distinct responses within a species to recognize the CO₂-induced changes from the divergent approaches, what can facilitate knowing also the possible interrelation of the physiological and metabolic responses. That could explain the controversial effects of eCO₂ on the carbon-based metabolite in conifers, at the inter- and intra-specific level.

Phloem transport in gymnosperms: a question of pressure and resistance

Even in the highest trees, carbon is efficiently distributed from leaves to heterotrophic tissues like fruit, flowers and roots. This long-distance transport happens in the highly specialized sieve elements of the phloem. In gymnosperms, sieve element anatomy appears to be less suited for mass flow of phloem sap than that of angiosperms. This review covers available data on gymnosperm phloem to evaluate if it functions differently from that of angiosperms. Although current evidence suggests that, despite a higher pathway resistance, a single source-to-sink turgor pressure gradient can drive mass flow, several questions remain unanswered. These include how endoplasmic reticulum-complexes in sieve elements influence flow, as well as what the effect of symplasmic coupling along the whole phloem pathway could be.