Fate of xylem-transported 11C- and 13C-labeled CO2 in leaves of poplar

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

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

Guide to Plant-PET Imaging Using 11CO2

Due to its high sensitivity and specificity for tumor detection, positron emission tomography (PET) has become a standard and widely used molecular imaging technique. Given the popularity of PET, both clinically and preclinically, its use has been extended to study plants. However, only a limited number of research groups worldwide report PET-based studies, while we believe that this technique has much more potential and could contribute extensively to plant science. The limited application of PET may be related to the complexity of putting together methodological developments from multiple disciplines, such as radio-pharmacology, physics, mathematics and engineering, which may form an obstacle for some research groups. By means of this manuscript, we want to encourage researchers to study plants using PET. The main goal is to provide a clear description on how to design and execute PET scans, process the resulting data and fully explore its potential by quantification via compartmental modeling. The different steps that need to be taken will be discussed as well as the related challenges. Hereby, the main focus will be on, although not limited to, tracing 11CO2 to study plant carbon dynamics.

What is the fate of xylem-transported CO2 in Kranz-type C4 plants?

High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C₃ plants exit leaves in the dark. In the light, C₃ species use a small portion of xylem-transported CO₂ for leaf photosynthesis. However, it is not known if xylem-transported CO₂ will exit leaves in the dark or be used for photosynthesis in the light in Kranz-type C₄ plants. Cut leaves of Amaranthus hypochondriacus were placed in one of three solutions of [NaH¹³ CO₃ ] dissolved in KCl water to measure the efflux of xylem-transported CO₂ exiting the leaf in the dark or rates of assimilation of xylem-transported CO₂ * in the light, in real-time, using a tunable diode laser absorption spectroscope. In the dark, the efflux of xylem-transported CO₂ increased with increasing rates of transpiration and [¹³ CO₂ *]; however, rates of ¹³ C_efflux in A. hypochondriacus were lower compared to C₃ species. In the light, A. hypochondriacus fixed nearly 75% of the xylem-transported CO₂ supplied to the leaf. Kranz anatomy and biochemistry likely influence the efflux of xylem-transported CO₂ out of cut leaves of A. hypochondriacus in the dark, as well as the use of xylem-transported CO₂ * for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz-type C₄ species over C₃ species.

Contribution and consequences of xylem-transported CO2 assimilation for C3 plants

Traditionally, leaves were thought to be supplied with CO₂ for photosynthesis by the atmosphere and respiration. Recent studies, however, have shown that the xylem also transports a significant amount of inorganic carbon into leaves through the bulk flow of water. However, little is known about the dynamics and proportion of xylem-transported CO₂ that is assimilated, vs simply lost to transpiration. Cut leaves of Populus deltoides and Brassica napus were placed in either KCl or one of three [NaH¹³ CO₃ ] solutions dissolved in water to simultaneously measure the assimilation and the efflux of xylem-transported CO₂ exiting the leaf across light and CO₂ response curves in real-time using a tunable diode laser absorption spectroscope. The rates of assimilation and efflux of xylem-transported CO₂ increased with increasing xylem [¹³ CO₂ *] and transpiration. Under saturating irradiance, rates of assimilation using xylem-transported CO₂ accounted for c. 2.5% of the total assimilation in both species in the highest [¹³ CO₂ *]. The majority of xylem-transported CO₂ is assimilated, and efflux is small compared to respiration. Assimilation of xylem-transported CO₂ comprises a small portion of total photosynthesis, but may be more important when CO₂ is limiting.

Isotope ratio laser spectroscopy to disentangle xylem-transported from locally respired CO2 in stem CO2 efflux

Respired CO2 in woody tissues radially diffuses to the atmosphere or it is transported upward with the transpiration stream, making the origin of CO2 in stem CO2 efflux (EA) uncertain, which may confound stem respiration (RS) estimates. An aqueous 13C-enriched solution was infused into stems of Populus tremula L. trees, and real-time measurements of 13C-CO2 and 12C-CO2 in EA were performed via Cavity Ring Down Laser Spectroscopy (CRDS). The contribution of locally respired CO2 (LCO2) and xylem-transported CO2 (TCO2) to EA was estimated from their different isotopic composition. Mean daily values of TCO2/EA ranged from 13% to 38%, evidencing the notable role that xylem CO2 transport plays in the assessment of stem respiration. Mean daily TCO2/EA did not differ between treatments of drought stress and light exclusion of woody tissues, but they showed different TCO2/EA dynamics on a sub-daily time scale. Sub-daily CO2 diffusion patterns were explained by a light-induced axial CO2 gradient ascribed to woody tissue photosynthesis, and the resistance to radial CO2 diffusion determined by bark water content. Here, we demonstrate the outstanding potential of CRDS paired with 13C-CO2 labelling to advance in the understanding of CO2 movement at the plant-atmosphere interface and the respiratory physiology in woody tissues.

Bicarbonate stimulates non-structural carbohydrate pools of Camptotheca acuminata

The role of root-derived dissolved inorganic carbon (DIC) has been emphasized lately, as it can provide an alternative source of carbon for photosynthesis. The fate of newly fixed DIC and its effect on non-structural carbohydrate (NSC) pools has not been thoroughly elucidated to date. To this end, we used ¹³ C (NaHCO₃ ) as a substrate tracer to investigate the incorporation of newly fixed bicarbonate into the plant organs and NSC compounds of Camptotheca acuminata seedlings for 24 and 72 h. NSC levels across the organs were all markedly increased within 24 h of labeling treatment and afterward only decreased in stems at 72 h. The variation range of NSC concentrations in roots was considerably smaller than in the stem and leaves. As time passed, the δ¹³ C in NSC compounds was significantly affected by ¹³ C labeling and was more positive in the roots than in the stem and leaves. Starch was more ¹³ C-enriched than was soluble carbohydrate, and the δ¹³ C of root starch was as high as -4.70‰. Bicarbonate incorporation into newly formed NSC compounds contributed up to 0.24% of the root starch within 72 h. These data provided strong evidence that bicarbonate not only acted as a C source that contributed slightly to the NSC pools but also stimulated the increase in NSC pools. The present study expands our understanding of the rapid change of NSC pools across the organs in response to bicarbonate.

Inside out: efflux of carbon dioxide from leaves represents more than leaf metabolism

High concentrations of inorganic carbon in the xylem, produced from root, stem, and branch respiration, travel via the transpiration stream and eventually exit the plant through distant tissues as CO2. Unlike previous studies that focused on the efflux of CO2 from roots and woody tissues, we focus on efflux from leaves and the potential effect on leaf respiration measurements. We labeled transported inorganic carbon, spanning reported xylem concentrations, with 13C and then manipulated transpiration rates in the dark in order to vary the rates of inorganic carbon supply to cut leaves from Brassica napus and Populus deltoides. We used tunable diode laser absorbance spectroscopy to directly measure the rate of gross 13CO2 efflux, derived from inorganic carbon supplied from outside of the leaf, relative to gross 12CO2 efflux generated from leaf cells. These experiemnts showed that 13CO2 efflux was dependent upon the rate of inorganic carbon supply to the leaf and the rate of transpiration. Our data show that the gross leaf efflux of xylem-transported CO2 is likely small in the dark when rates of transpiration are low. However, gross leaf efflux of xylem-transported CO2 could approach half the rate of leaf respiration in the light when transpiration rates and branch inorganic carbon concentrations are high, irrespective of the grossly different petiole morphologies in our experiment.

Why small fluxes matter: the case and approaches for improving measurements of photosynthesis and (photo)respiration

Since its inception, the Farquhar et al. (1980) model of photosynthesis has been a mainstay for relating biochemistry to environmental conditions from chloroplast to global levels in terrestrial plants. Many variables could be assigned from basic enzyme kinetics, but the model also required measurements of maximum rates of photosynthetic electron transport (J max ), carbon assimilation (Vcmax ), conductance of CO2 into (g s ) and through (g m ) the leaf, and the rate of respiration during the day (R d ). This review focuses on improving the accuracy of these measurements, especially fluxes from photorespiratory CO2, CO2 in the transpiration stream, and through the leaf epidermis and cuticle. These fluxes, though small, affect the accuracy of all methods of estimating mesophyll conductance and several other photosynthetic parameters because they all require knowledge of CO2 concentrations in the intercellular spaces. This review highlights modified methods that may help to reduce some of the uncertainties. The approaches are increasingly important when leaves are stressed or when fluxes are inferred at scales larger than the leaf.

Plant-PET Scans: In Vivo Mapping of Xylem and Phloem Functioning

Medical imaging techniques are rapidly expanding in the field of plant sciences. Positron emission tomography (PET) is advancing as a powerful functional imaging technique to decipher in vivo the function of xylem water flow (with (15)O or (18)F), phloem sugar flow (with (11)C or (18)F), and the importance of their strong coupling. However, much remains to be learned about how water flow and sugar distribution are coordinated in intact plants, both under present and future climate regimes. We propose to use PET analysis of plants (plant-PET) to visualize and generate these missing data about integrated xylem and phloem transport. These insights are crucial to understanding how a given environment will affect plant physiological processes and growth.