In situ 13CO2 pulse labelling of field-grown eucalypt trees revealed the effects of potassium nutrition and throughfall exclusion on phloem transport of photosynthetic carbon

Starch depletion in the xylem and phloem ray parenchyma of grapevine stems under drought

While nonstructural carbohydrate (NSC) storage can support long-lived woody plants during abiotic stress, the timing and extent of their use are less understood, as are the thresholds for cell mortality as NSCs and water supplies are consumed. Here, we combine physiological and imaging tools to study the response of Vitis riparia to a 6-week experimental drought. We focused on the spatial and temporal dynamics of starch consumption and cell viability in the xylem and phloem of the stem. Starch dynamics were further corroborated with enzymatic starch digestion and X-ray microcomputed tomography imaging. Starch depletion in the stems of droughted plants was detected after 2 weeks and continued over time. We observed distinct differences in starch content and cell viability in the xylem and phloem. By the end of the drought, nearly all the starch was consumed in the phloem ray parenchyma (98 % decrease), and there were almost no metabolically active cells in the phloem. In contrast, less starch was consumed in the xylem ray parenchyma (30 % decrease), and metabolically active cells remained in the ray and vessel-associated parenchyma in the xylem. Our data suggest that the higher proportion of living cells in the phloem and cambium, combined with smaller potential NSC storage area, rapidly depleted starch, which led to cell death. In contrast, the larger cross-sectional area of the xylem ray parenchyma with higher NSC storage and lower metabolically active cell populations depleted starch at a slower pace. Why NSC source-sink relationships between xylem and phloem do not allow for a more uniform depletion of starch in ray parenchyma over time is unclear. Our data help to pinpoint the proximate and ultimate causes of plant death during prolonged drought exposure and highlight the need to consider the influence of within-organ starch dynamics and cell mortality on abiotic stress response.

Carbon allocation in cassava is affected by water deficit and potassium application - A 13 C-CO2 pulse labelling assessment

RATIONALE

Cassava production faces challenges in a changing climate. Pulse labelling cassava with ¹³ C-CO₂ has the potential to elucidate carbon allocation mechanisms of cassava under drought stress and with potassium application. Understanding these mechanisms could guide efforts to mitigate effects of drought in cassava cropping systems.

METHODS

Forty-eight cassava plants received a nutrient solution high or low in potassium. Water deficit was imposed on half of the plants at bulk root initiation stage, after which they were labelled for 8 h with ¹³ C-CO₂ in a 15 m³ growth chamber. Plants were harvested 8 h, 9 days and 24 days after labelling, and separated into leaves, stems and roots. δ¹³ C values of the different parts were measured using an isotope ratio mass spectrometer, from which ¹³ C excess was calculated.

RESULTS

Water deficit decreased transpiration (P < 0.001) and increased carbon respiration (P < 0.05). Potassium application increased assimilate distribution to the roots (P < 0.05) at 9 days after labelling, more strongly for plants under water deficit. The opposite was found at 24 days (P < 0.05) with the legacy of water deficit additionally increasing assimilate distribution to roots (P < 0.05). Youngest, fully expanded leaves contained up to 47% of initial ¹³ C excess at 24 days after labelling.

CONCLUSIONS

Pulse labelling proved to be successful in shedding light on carbon allocation in relation to water and potassium availability. This technique, once adapted to field conditions, could further be used to improve fertilizer recommendations or change agronomic practices to cope with plant stress.

Negative effects of low root temperatures on water and carbon relations in temperate tree seedlings assessed by dual isotopic labelling

Low root zone temperatures restrict water and carbon (C) uptake and transport in plants and may contribute to the low temperature limits of tree growth. Here, we quantified the effects of low root temperatures on xylem conductance, photosynthetic C assimilation and phloem C transport in seedlings of four temperate tree species (two broad-leaved and two conifer species) by applying a simultaneous stable isotope labelling of 2H-enriched source water and 13C-enriched atmospheric CO2. Six days before the pulse labelling, the seedlings were transferred to hydroponic tubes and exposed to three different root temperatures (2, 7 and 15 °C), while all seedlings received the same, warm air temperatures (between 18 and 24 °C). Root cooling led to drought-like symptoms with reduced growth, leaf water potentials and stomatal conductance, indicating increasingly adverse conditions for water uptake and transport with decreasing root temperatures. Averaged across all four species, water transport to leaves was reduced by 40% at 7 °C and by 70% at 2 °C root temperature relative to the 15 °C treatment, while photosynthesis was reduced by 20 and 40% at 7 and 2 °C, respectively. The most severe effects were found on the phloem C transport to roots, which was reduced by 60% at 7 °C and almost ceased at 2 °C in comparison with the 15 °C root temperature treatment. This extreme effect on C transport was likely due to a combination of simultaneous reductions of phloem loading, phloem mass flow and root growth. Overall, the dual stable isotope labelling proved to be a useful method to quantify water and C relations in cold-stressed trees and highlighted the potentially important role of hydraulic constraints induced by low soil temperatures as a contributing factor for the climatic distribution limits of temperate tree species.

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.

High resilience of carbon transport in long-term drought-stressed mature Norway spruce trees within 2 weeks after drought release

Under ongoing global climate change, drought periods are predicted to increase in frequency and intensity in the future. Under these circumstances, it is crucial for tree's survival to recover their restricted functionalities quickly after drought release. To elucidate the recovery of carbon (C) transport rates in c. 70-year-old Norway spruce (Picea abies [L.] KARST.) after 5 years of recurrent summer droughts, we conducted a continuous whole-tree ¹³ C labeling experiment in parallel with watering. We determined the arrival time of current photoassimilates in major C sinks by tracing the ¹³ C label in stem and soil CO₂ efflux, and tips of living fine roots. In the first week after watering, aboveground C transport rates (CTR) from crown to trunk base were still 50% lower in previously drought-stressed trees (0.16 ± 0.01 m h^-1 ) compared to controls (0.30 ± 0.06 m h^-1 ). Conversely, CTR below ground, that is, from the trunk base to soil CO₂ efflux were already similar between treatments (c. 0.03 m h^-1 ). Two weeks after watering, aboveground C transport of previously drought-stressed trees recovered to the level of the controls. Furthermore, regrowth of water-absorbing fine roots upon watering was supported by faster incorporation of ¹³ C label in previously drought-stressed (within 12 ± 10 h upon arrival at trunk base) compared to control trees (73 ± 10 h). Thus, the whole-tree C transport system from the crown to soil CO₂ efflux fully recovered within 2 weeks after drought release, and hence showed high resilience to recurrent summer droughts in mature Norway spruce forests. This high resilience of the C transport system is an important prerequisite for the recovery of other tree functionalities and productivity.

Potassium supply modulates Eucalyptus leaf water-status under PEG-induced osmotic stress: integrating leaf gas exchange, carbon and nitrogen isotopic composition and plant growth

The objective of this study was to quantify the effect of potassium (K) supply on osmotic adjustment and drought avoidance mechanisms of Eucalyptus seedlings growing under short-term water stress. The effects of K supply on plant growth, nutritional status, leaf gas exchange parameters, leaf water potential (Ψw), leaf area (LA), stomatal density (SD), leaf carbon (C) and nitrogen (N) isotopic compositions (δ13C and δ15N ‰) and leaf C/N ratio under polyethylene glycol (PEG)-induced water deficit were measured. Under both control (non-PEG) and osmotic stress (+PEG) conditions, K supply increased plant growth, boosting dry matter yield with decreased C/N leaf ratio and δ15N ‰ values. The +PEG significantly reduced LA, plant growth, dry matter yield, Ψw, number of stomata per plant and leaf gas exchange, relative to non-PEG condition. Potassium supply alleviated osmotic-induced alterations in Eucalyptus seedlings by better regulating leaf development as well as SD, thus improving the rate of leaf gas exchange parameters, mesophyll conductance to CO2 (lower δ13C ‰ values) and water use efficiency (WUE). Consequently, K-supplied plants under drought better acclimated to osmotic stress than K-deficient plants, which in turn induced lower CO2 assimilation and dry matter yield, as well as higher leaf δ13C ‰ and δ15N ‰ values. In conclusion, management practices should seek to optimize K-nutrition to improve WUE, photosynthesis-related parameters and plant growth under water deficit conditions.

Diurnal variations in the thickness of the inner bark of tree trunks in relation to xylem water potential and phloem turgor

The inner bark plays important roles in tree stems, including radial exchange of water with the xylem and translocation of carbohydrates. Both processes affect the water content and the thickness of the inner bark on a diurnal basis. For the first time, we simultaneously measured the diurnal variations in the inner bark thickness of hinoki cypress (Chamaecyparis obtusa) by using point dendrometers and those of local xylem potential by using stem psychrometers located next to the dendrometers to determine how these variations were related to each other, to phloem turgor and carbohydrate transport. We also estimated the axial hydrostatic pressure gradient by measuring the osmolality of the sap extracted from the inner bark. The inner bark shrunk during the day and swelled during the night with an amplitude related to day-to-day and seasonal variations in climate. The relationship between changes in xylem water potential and inner bark thickness exhibited a hysteresis loop during the day with a median lag of 2 h. A phloem turgor-related signal can be retrieved from the diurnal variations in the inner bark thickness, which was higher at the upper than at the lower position along the trunk. However, a downward hydrostatic pressure gradient was only observed at dawn, suggesting diurnal variations in the phloem sap flow velocity.

Combating Dual Challenges in Maize Under High Planting Density: Stem Lodging and Kernel Abortion

High plant density is considered a proficient approach to increase maize production in countries with limited agricultural land; however, this creates a high risk of stem lodging and kernel abortion by reducing the ratio of biomass to the development of the stem and ear. Stem lodging and kernel abortion are major constraints in maize yield production for high plant density cropping; therefore, it is very important to overcome stem lodging and kernel abortion in maize. In this review, we discuss various morphophysiological and genetic characteristics of maize that may reduce the risk of stem lodging and kernel abortion, with a focus on carbohydrate metabolism and partitioning in maize. These characteristics illustrate a strong relationship between stem lodging resistance and kernel abortion. Previous studies have focused on targeting lignin and cellulose accumulation to improve lodging resistance. Nonetheless, a critical analysis of the literature showed that considering sugar metabolism and examining its effects on lodging resistance and kernel abortion in maize may provide considerable results to improve maize productivity. A constructive summary of management approaches that could be used to efficiently control the effects of stem lodging and kernel abortion is also included. The preferred management choice is based on the genotype of maize; nevertheless, various genetic and physiological approaches can control stem lodging and kernel abortion. However, plant growth regulators and nutrient application can also help reduce the risk for stem lodging and kernel abortion in maize.

134Cs Uptake and Growth at Various Cs+ and K+ Levels in Arabidopsis AtKUP7 Mutants

Radiocaesium is a pollutant with a high risk for the environment, agricultural production, and human health. It is mobile in ecosystems and can be taken up by plants via potassium transporters. In this study, we focused on the role of potassium transporter AtKUP7 of the KT/HAK/KUP family in Cs+ and K+ uptake by plants and in plant tolerance to caesium toxicity. We detected that Arabidopsiskup7 mutant accumulates significantly lower amounts of 134Cs in the root (86%) and in the shoot (69%) compared to the wild-type. On the other hand ability of the mutant to grow on media with toxic (100 and 200 µM) concentrations of Cs+ was not changed; moreover its growth was not impaired on low K+. We further investigated another mutant line in AtKUP7 and found that the growth phenotype of the kup7 mutants in K+ deficient conditions is much milder than previously published. Also, their accumulation of K+ in shoots is hindered only by severe potassium shortage.

Potassium deficiency reconfigures sugar export and induces catecholamine accumulation in oil palm leaves

Metabolic effects of potassium (K) deficiency have been described for nearly 70 years but specific effects of low K availability on sugar composition, sugar export rate and its relationship with other leaf metabolites are not very well documented. Having such pieces of information is nevertheless essential to identify metabolic signatures to monitor K fertilization. This is particularly true in oil-producing crop species such as oil palm (Elaeis guineensis), which is strongly K-demanding and involves high sugar dependence for fruit formation because of low carbon use efficiency in lipid synthesis. Here, we used metabolic analyses, measured sugar export rates with ¹³C isotopic labeling and examined the effects of K availability on both leaflet and rachis sugar metabolism in oil palm seedlings. We show that low K leads to a modification of sugar composition mostly in rachis and decreased sucrose and hexose export rates from leaflets. As a result, leaflets contained more starch and induced alternative pathways such as raffinose synthesis, although metabolites of the raffinose pathway remained quantitatively minor. The alteration of glycolysis by low K was compensated for by an increase in alternative sugar phosphate utilization by tyrosine metabolism, resulting in considerable amounts of tyramine and dopamine.

δ15 N values in plants are determined by both nitrate assimilation and circulation

Nitrogen (N) assimilation is associated with ¹⁴ N/¹⁵ N fractionation such that plant tissues are generally ¹⁵ N-depleted compared to source nitrate. In addition to nitrate concentration, the δ¹⁵ N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of δ¹⁵ N values in nitrate is limited by the relatively small number of compound-specific data. We extensively measured δ¹⁵ N in nitrate at different time points, in sunflower and oil palm grown at fixed nitrate concentration, with nitrate circulation being varied using potassium (K) conditions and waterlogging. There were strong interorgan δ¹⁵ N differences for contrasting situations between the two species, and a high ¹⁵ N-enrichment in root nitrate. Modelling shows that this ¹⁵ N-enrichment can be explained by nitrate circulation and compartmentalisation whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a c. 30‰ difference between leaves and roots. Accordingly, waterlogging and low K conditions, which down-regulate sap circulation, cause a decrease in the leaf-to-root isotopic difference. Our study thus suggests that plant δ¹⁵ N can be used as a natural tracer of N fluxes between organs and highlights the potential importance of δ¹⁵ N of circulating phloem nitrate.

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.

Water relations of two Sicilian grapevine cultivars in response to potassium availability and drought stress

We investigated the response of two Sicilian grapevine cultivars, Catarratto and Nero d'Avola, to potassium deficiency and drought stress. Two-year-old plants grafted on 1103 Paulsen were grown in agriperlite, with or without potassium in the fertigation solution for six weeks, and subjected to moderate drought stress by suspending irrigation for one week. Potassium content of leaves, roots and xylem sap were measured with an ion-selective electrode. Changes in stomatal conductance, stem and leaf water potential and hydraulic conductance were compared between genotypes and treatments. Potassium deficiency led to significant decreases in leaf potassium content in both cultivars and under both well-watered and drought stress conditions. Potassium content in xylem sap showed no significant differences between cultivars and was correlated with stem hydraulic conductance, particularly in the drought stress treatments. Under drought stress conditions, potassium availability led to an increase in stomatal conductance, particularly in Nero d'Avola. Both cultivars showed a rather isohydric behavior under these experimental conditions, and the level of isohydry varied with potassium availability. These results can be useful for the development of optimal fertigation practices and the selection of drought tolerant varieties.

Carbon isotopic tracing of sugars throughout whole-trees exposed to climate warming

Trees allocate C from sources to sinks by way of a series of processes involving carbohydrate transport and utilization. Yet these dynamics are not well characterized in trees, and it is unclear how these dynamics will respond to a warmer world. Here, we conducted a warming and pulse-chase experiment on Eucalyptus parramattensis growing in a whole-tree chamber system to test whether warming impacts carbon allocation by increasing the speed of carbohydrate dynamics. We pulse-labelled large (6-m tall) trees with ¹³ C-CO₂ to follow recently fixed C through different organs by using compound-specific isotope analysis of sugars. We then compared concentrations and mean residence times of individual sugars between ambient and warmed (+3°C) treatments. Trees dynamically allocated ¹³ C-labelled sugars throughout the aboveground-belowground continuum. We did not, however, find a significant treatment effect on C dynamics, as sugar concentrations and mean residence times were not altered by warming. From the canopy to the root system, ¹³ C enrichment of sugars decreased, and mean residence times increased, reflecting dilution and mixing of recent photoassimilates with older reserves along the transport pathway. Our results suggest that a locally endemic eucalypt was seemingly able to adjust its physiology to warming representative of future temperature predictions for Australia.

Potassium in Root Growth and Development

Potassium is an essential macronutrient that has been partly overshadowed in root science by nitrogen and phosphorus. The current boom in potassium-related studies coincides with an emerging awareness of its importance in plant growth, metabolic functions, stress tolerance, and efficient agriculture. In this review, we summarized recent progress in understanding the role of K+ in root growth, development of root system architecture, cellular functions, and specific plant responses to K+ shortage. K+ transport is crucial for its physiological role. A wide range of K+ transport proteins has developed during evolution and acquired specific functions in plants. There is evidence linking K+ transport with cell expansion, membrane trafficking, auxin homeostasis, cell signaling, and phloem transport. This places K+ among important general regulatory factors of root growth. K+ is a rather mobile element in soil, so the absence of systemic and localized root growth response has been accepted. However, recent research confirms both systemic and localized growth response in Arabidopsis thaliana and highlights K+ uptake as a crucial mechanism for plant stress response. K+-related regulatory mechanisms, K+ transporters, K+ acquisition efficiency, and phenotyping for selection of K+ efficient plants/cultivars are highlighted in this review.

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.

Metabolic responses to potassium availability and waterlogging reshape respiration and carbon use efficiency in oil palm

Oil palm is by far the major oil-producing crop on the global scale, with c. 62 Mt oil produced each year. This species is a strong potassium (K)-demanding species cultivated in regions where soil K availability is generally low and waterlogging due to tropical heavy rains can limit further nutrient absorption. However, the metabolic effects of K and waterlogging have never been assessed precisely. Here, we examined the metabolic response of oil palm saplings in the glasshouse under controlled conditions (nutrient composition with low or high K availability, with or without waterlogging), using gas exchange, metabolomics and proteomics analyses. Our results showed that both low K and waterlogging have a detrimental effect on photosynthesis but stimulate leaf respiration, with differential accumulation of typical metabolic intermediates and enzymes of Krebs cycle and alternative catabolic pathways. In addition, we found a strong relationship between metabolic composition, the rate of leaf dark respiration, and cumulated respiratory loss. Advert environmental conditions (here, low K and waterlogging) therefore have an enormous effect on respiration in oil palm. Leaf metabolome and proteome appear to be good predictors of carbon balance, and open avenues for cultivation biomonitoring using functional genomics technologies.

Methods for Assessing the Role of Phloem Transport in Plant Stress Responses

Delivery of carbohydrates to tissues that need them under stress is important for plant defenses and survival. Yet, little is known on how phloem function is altered under stress, and how that influences plant responses to stress. This is because phloem is a challenging tissue to study. It consists of cells of various types with soft cell walls, and the cells show strong wounding reactions to protect their integrity, making both imaging and functional studies challenging. This chapter summarizes theories on how phloem transport is affected by stress and presents methods that have been used to gain the current knowledge. These techniques range from tracer studies and imaging to carbon balance and anatomical analyses. Advances in these techniques in the recent years have considerably increased our ability to investigate phloem function, and application of the new methods on plant stress studies will help provide a more comprehensive picture of phloem function and its limitations under stress.

Microsite Effects on Physiological Performance of Betula ermanii at and Beyond an Alpine Treeline Site on Changbai Mountain in Northeast China

The alpine treeline demarcates the temperature-limited upper elevational boundary of the tree life form. However, this treeline does not always occur exclusively as a sharp “line”, outposts of tree groups (OTG) with a height of at least 3 m are often observed in microsites up to several hundred meters beyond the line of continuous forest on some mountains. This suggests that other factors such as microenvironment may play a significant role in compensating for the alpine tree facing growth-limiting low temperature conditions. To test the microenvironment effects, this study compared the differences in growing conditions (climate and soil properties) and ecophysiological performance of Erman’s birch (Betula ermanii Cham.) trees growing in a continuous treeline site (CTL, ~1950 m above sea level, a.s.l.) and OTGs (~2050 m a.s.l.) on Changbai Mountain in northeastern China. The results show the average 2-m air temperature for OTG was slightly lower in the non-growing season than which at the CTL (−10.2 °C < −8.4 °C), there was no difference in growing season air temperature and soil temperature at 10 cm depth between CTL and OTG. The contents of focal soil nutrients in CTL and OTG were similar. Difference in K and Mn contents between sites were detected in leaves, difference in K, Mn, and Zn in shoots. However, comparing similarity of ecophysiological performances at an individual level, trees at CTL and OTG show no significant difference. Our study reveals that mature trees at the CTL and OTG experience generally similar environmental conditions (climate and soil properties) and exhibit similar overall ecophysiological performance (reflected in carbon reserves and nutrients). This might provide insight into how mature trees might be able to survive in areas higher than the continuous treeline, as well as the importance of microclimatic amelioration provided by protective microsites and the trees themselves.

Climate warming and tree carbon use efficiency in a whole-tree 13 CO2 tracer study

Autotrophic respiration is a major driver of the global C cycle and may contribute a positive climate warming feedback through increased atmospheric concentrations of CO₂ . The extent of this feedback depends on plants' ability to acclimate respiration to maintain a constant carbon use efficiency (CUE). We quantified respiratory partitioning of gross primary production (GPP) and CUE of field-grown trees in a long-term warming experiment (+3°C). We delivered a ¹³ C-CO₂ pulse to whole tree crowns and chased that pulse in the respiration of leaves, whole crowns, roots, and soil. We also measured the isotopic composition of soil microbial biomass and the respiration rates of leaves and whole crowns. We documented homeostatic respiratory acclimation of foliar and whole-crown respiration rates; the trees adjusted to experimental warming such that leaf-level respiration rates were not increased. Experimental warming had no detectable impact on respiratory partitioning or mean residence times. Of the ¹³ C label acquired by the trees, aboveground respiration consumed 10%, belowground respiration consumed 40%, and the remaining 50% was retained. Experimental warming of +3°C did not alter respiratory partitioning at the scale of entire trees, suggesting that complete acclimation of respiration to warming is likely to dampen a positive climate warming feedback.

Limitations to using phloem sap to assess tree water and nutrient status

Rapid, reliable tools are needed to infer physiological and nutritional health for managing forest systems. Understanding the processes governing tree health is central to the development of these tools. Non-foliar approaches such as the collection of phloem sap reflect processes governing both the use and acquisition of plant water and nutrients at a wide range of temporal (diurnal to seasonal) and spatial (canopy) scales. Despite this, phloem sap is not commonly employed due to an incomplete understanding of transport and post-photosynthetic processes and their effects on chemical concentrations and carbon isotope discrimination. We highlight the need to characterize the influences of storage, remobilization and transport on the concentrations of metabolites to address the time and spatial decoupling of phloem contents to that of environmental stimuli. A conceptual framework is suggested to focus research on key phenomena regarding metabolite transport and highlight significant advantages, misconceptions and limitations to its application.

Repeated summer drought delays sugar export from the leaf and impairs phloem transport in mature beech

Phloem sustains maintenance and growth processes through transport of sugars from source to sink organs. Under low water availability, tree functioning is impaired, i.e., growth/photosynthesis decline and phloem transport may be hindered. In a 3-year throughfall exclusion (TE) experiment on mature European beech (Fagus sylvatica L.) we conducted 13CO2 branch labeling to investigate translocation of recently fixed photoassimilates under experimental drought over 2 years (2015 and 2016). We hypothesized (H1) that mean residence time of photoassimilates in leaves (MRT) increases, whereas (H2) phloem transport velocity (Vphloem) decreases under drought. Transport of carbohydrates in the phloem was assessed via δ13C of CO2 efflux measured at two branch positions following 13CO2 labeling. Pre-dawn water potential (ΨPD) and time-integrated soil water deficit (iSWD) were used to quantify drought stress. The MRT increased by 46% from 32.1 ± 5.4 h in control (CO) to 46.9 ± 12.3 h in TE trees, supporting H1, and positively correlated (P < 0.001) with iSWD. Confirming H2, Vphloem in 2016 decreased by 47% from 20.7 ± 5.8 cm h-1 in CO to 11.0 ± 2.9 cm h-1 in TE trees and positively correlated with ΨPD (P = 0.001). We suggest that the positive correlation between MRT and iSWD is a result of the accumulation of osmolytes maintaining cell turgor in the leaves under longer drought periods. Furthermore, we propose that the positive correlation between Vphloem and ΨPD is due to a lower water uptake of phloem conduits from surrounding tissues under increasing drought leading to a higher phloem sap viscosity and lower Vphloem. The two mechanisms increasing MRT and reducing Vphloem respond differently to low water availability and impair trees' carbon translocation under drought.

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.

Using 13C to Quantify Phloem Transport on Tall Plants in the Field

The difference in time lags between a labeling pulse of ¹³CO₂ of the foliage and the appearance of labeled C in the respiration at different locations along the stem of a tall plant is used to estimate at which velocities the isotope tracer, i.e., the labeled carbohydrates, are transported in the phloem sap. Here we describe a method for pulse-labeling tall plants in the field and subsequently tracing ¹³C in the respiratory efflux of CO₂.

Gradients and dynamics of inner bark and needle osmotic potentials in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst)

Preconditions of phloem transport in conifers are relatively unknown. We studied the variation of needle and inner bark axial osmotic gradients and xylem water potential in Scots pine and Norway spruce by measuring needle and inner bark osmolality in saplings and mature trees over several periods within a growing season. The needle and inner bark osmolality was strongly related to xylem water potential in all studied trees. Sugar concentrations were measured in Scots pine, and they had similar dynamics to inner bark osmolality. The sucrose quantity remained fairly constant over time and position, whereas the other sugars exhibited a larger change with time and position. A small osmotic gradient existed from branch to stem base under pre-dawn conditions, and the osmotic gradient between upper stem and stem base was close to zero. The turgor in branches was significantly driven by xylem water potential, and the turgor loss point in branches was relatively close to daily minimum needle water potentials typically reported for Scots pine. Our results imply that xylem water potential considerably impacts the turgor pressure gradient driving phloem transport and that gravitation has a relatively large role in phloem transport in the stems of mature Scots pine trees.

Potassium in the Grape (Vitis vinifera L.) Berry: Transport and Function

K+ is the most abundant cation in the grape berry. Here we focus on the most recent information in the long distance transport and partitioning of K+ within the grapevine and postulate on the potential role of K+ in berry sugar accumulation, berry water relations, cellular growth, disease resistance, abiotic stress tolerance and mitigating senescence. By integrating information from several different plant systems we have been able to generate new hypotheses on the integral functions of this predominant cation and to improve our understanding of how these functions contribute to grape berry growth and ripening. Valuable contributions to the study of K+ in membrane stabilization, turgor maintenance and phloem transport have allowed us to propose a mechanistic model for the role of this cation in grape berry development.

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

¹³ CO₂ pulse-labelling experiments were performed in situ on adult beeches (Fagus sylvatica) and pines (Pinus pinaster) at different phenological stages to study seasonal and interspecific short-term dynamics and partitioning of recently assimilated carbon (C) in leaves. Polar fraction (PF, including soluble sugars, amino acids and organic acids) and starch were purified from foliage sampled during a 10-d chase period. C contents, isotopic compositions and ¹³ C dynamics parameters were determined in bulk foliage, PF and starch. Decrease in ¹³ C amount in bulk foliage followed a two-pool exponential model highlighting ¹³ C partitioning between 'mobile' and 'stable' pools, the relative proportion of the latter being maximal in beech leaves in May. Early in the growing season, new foliage acted as a strong C sink in both species, but although young leaves and needles were already photosynthesizing, the latter were still supplied with previous-year needle photosynthates 2 months after budburst. Mean ¹³ C residence times (MRT) were minimal in summer, indicating fast photosynthate export to supply perennial organ growth in both species. In late summer, MRT differed between senescing beech leaves and overwintering pine needles. Seasonal variations of ¹³ C partitioning and dynamics in field-grown tree foliage are closely linked to phenological differences between deciduous and evergreen trees.

Decoding Biosynthetic Pathways in Plants by Pulse-Chase Strategies Using (13)CO₂ as a Universal Tracer †

(13)CO₂ pulse-chase experiments monitored by high-resolution NMR spectroscopy and mass spectrometry can provide (13)C-isotopologue compositions in biosynthetic products. Experiments with a variety of plant species have documented that the isotopologue profiles generated with (13)CO₂ pulse-chase labeling are directly comparable to those that can be generated by the application of [U-(13)C₆]glucose to aseptically growing plants. However, the application of the (13)CO₂ labeling technology is not subject to the experimental limitations that one has to take into account for experiments with [U-(13)C₆]glucose and can be applied to plants growing under physiological conditions, even in the field. In practical terms, the results of biosynthetic studies with (13)CO₂ consist of the detection of pairs, triples and occasionally quadruples of (13)C atoms that have been jointly contributed to the target metabolite, at an abundance that is well above the stochastic occurrence of such multiples. Notably, the connectivities of jointly transferred (13)C multiples can have undergone modification by skeletal rearrangements that can be diagnosed from the isotopologue data. As shown by the examples presented in this review article, the approach turns out to be powerful in decoding the carbon topology of even complex biosynthetic pathways.