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Rhizospheric microorganisms can alter plant physiology and morphology in many different ways including through the emission of volatile organic compounds (VOCs). Here we demonstrate that VOCs from beneficial root endophytic Serendipita spp. are able to improve the performance of in vitro grown Arabidopsis seedlings, with an up to 9.3-fold increase in plant biomass. Additional changes in VOC-exposed plants comprised petiole elongation, epidermal cell and leaf area expansion, extension of the lateral root system, enhanced maximum quantum efficiency of photosystem II (Fv/Fm), and accumulation of high levels of anthocyanin. Notwithstanding that the magnitude of the effects was highly dependent on the test system and cultivation medium, the volatile blends of each of the examined strains, including the references S. indica and S. williamsii , exhibited comparable plant growth-promoting activities. By combining different approaches, we provide strong evidence that not only fungal respiratory CO2 accumulating in the headspace, but also other volatile compounds contribute to the observed plant responses. Volatile profiling identified methyl benzoate as the most abundant fungal VOC, released especially by Serendipita cultures that elicit plant growth promotion. However, under our experimental conditions, application of methyl benzoate as a sole volatile did not affect plant performance, suggesting that other compounds are involved or that the mixture of VOCs, rather than single molecules, accounts for the strong plant responses. Using Arabidopsis mutant and reporter lines in some of the major plant hormone signal transduction pathways further revealed the involvement of auxin and cytokinin signaling in Serendipita VOC-induced plant growth modulation. Although we are still far from translating the current knowledge into the implementation of Serendipita VOCs as biofertilizers and phytostimulants, volatile production is a novel mechanism by which sebacinoid fungi can trigger and control biological processes in plants, which might offer opportunities to address agricultural and environmental problems in the future. [ABSTRACT FROM AUTHOR]

Respired CO2 in woody tissues can build up in the xylem and dissolve in the sap solution to be transported through the plant. From the sap, a fraction of the CO2 can either be radially diffuse to the atmosphere or be assimilated in chloroplasts present in woody tissues. These processes occur simultaneously in stems and branches, making it difficult to study their specific dynamics. Therefore, an 11C-enriched aqueous solution was administered to young branches of Populus tremula L. which were subsequently imaged by positron emission tomography (PET). This approach allows in vivo visualization of the internal movement of CO2 inside branches at high spatial and temporal resolution, and enables direct measurement of the transport speed of xylem-transported CO2 (v CO2). Through compartmental modeling of the dynamic data obtained from the PET images, we (i) quantified v CO2 and (ii) proposed a new method to assess the fate of xylem-transported 11CO2 within the branches. It was found that a fraction of 0.49 min−1 of CO2 present in the xylem was transported upwards. A fraction of 0.38 min−1 diffused radially from the sap to the surrounding parenchyma and apoplastic spaces (CO2,PA) to be assimilated by woody tissue photosynthesis. Another 0.12 min−1 of the xylem-transported CO2 diffused to the atmosphere via efflux. The remaining CO2 (i.e. 0.01 min−1) was stored as CO2,PA, representing the build-up within parenchyma and apoplastic spaces to be assimilated or directed to the atmosphere. Here, we demonstrate the outstanding potential of 11CO2-based plant-PET in combination with compartmental modeling to advance our understanding of internal CO2 movement and the respiratory physiology within woody tissues. [ABSTRACT FROM AUTHOR]

A substantial portion of locally respired CO2 in stems can be assimilated by chloroplast‐containing tissues. Woody tissue photosynthesis (Pwt) therefore plays a major role in the stem carbon balance. To study the impact of Pwt on stem carbon cycling along a gradient of water availability, stem CO2 efflux (EA), xylem CO2 concentration ([CO2]), and xylem water potential (Ψxylem) were measured in 4‐year‐old Populus tremula L. trees exposed to drought stress and different regimes of light exclusion of woody tissues. Under well‐watered conditions, local Pwt decreased EA up to 30%. Axial CO2 diffusion (Dax) induced by distant Pwt caused an additional decrease in EA of up to 25% and limited xylem [CO2] build‐up. Under drought stress, absolute decreases in EA driven by Pwt remained stable, denoting that Pwt was not affected by drought. At the end of the dry period, when transpiration was low, local Pwt and Dax offset 20% and 10% of stem respiration on a daily basis, respectively. These results highlight (a) the importance of Pwt for an adequate interpretation of EA measurements and (b) homeostatic Pwt along a drought stress gradient, which might play a crucial role to fuel stem metabolism when leaf carbon uptake and phloem transport are limited. Woody tissue photosynthesis remains constant during drought, highlighting its importance as reliable carbon source when leaf assimilation is compromised. In addition, stem CO2 efflux was substantially lowered by local woody tissue photosynthesis, altering its temperature sensitivity. [ABSTRACT FROM AUTHOR]

Summary: Mechanistic models of plant respiration remain poorly developed, especially in stems and woody tissues where measurements of CO2 efflux do not necessarily reflect local respiratory activity.We built a process‐based model of stem respiration that couples water and carbon fluxes at the organ level (TReSpire). To this end, sap flow, stem diameter variations, xylem and soil water potential, stem temperature, stem CO2 efflux and nonstructural carbohydrates were measured in a maple tree, while xylem CO2 concentration and additional stem and xylem diameter variations were monitored in an ancillary tree for model validation.TReSpire realistically described: (1) turgor pressure to differentiate growing from nongrowing metabolism; (2) maintenance expenditures in xylem and outer tissues based on Arrhenius kinetics and nitrogen content; and (3) radial CO2 diffusivity and CO2 solubility and transport in the sap solution. Collinearity issues with phloem unloading rates and sugar–starch interconversion rates suggest parallel submodelling to close the stem carbon balance.TReSpire brings a breakthrough in the modelling of stem water and carbon fluxes at a detailed (hourly) temporal resolution. TReSpire is calibrated from a sink‐driven perspective, and has potential to advance our understanding on stem growth dynamics, CO2 fluxes and underlying respiratory physiology across different species and phenological stages. See also the Commentary on this article by Meir et al., 225: 1824–1827. [ABSTRACT FROM AUTHOR]

Maintaining xylem water transport under drought is vital for plants, but xylem failure does occur when drought-induced embolisms form and progressively spread through the xylem. The hydraulic method is widely considered the gold standard to quantify drought-induced xylem embolism. The method determines hydraulic conductivity (K h) in cut branch samples, dehydrated to specific drought levels, by pushing water through them. The technique is widely considered for its reliable K h measurements, but there is some uncertainty in the literature over how to define stable K h and how that relates to the degree of xylem embolism formation. Therefore, the most common setup for this method was extended to measure four parameters: (i) inlet K h, (ii) outlet K h, (iii) radial flow from xylem to surrounding living tissue and (iv) the pressure difference across the sample. From a strictly theoretical viewpoint, hydraulic steady state, where inflow equals outflow and radial flow is zero, will result in stable K h. Application of the setup to Malus domestica Borkh. branches showed that achieving hydraulic steady state takes considerable time (up to 300 min) and that time to reach steady state increased with declining xylem water potentials. During each experimental run, K h and xylem water potentials dynamically increased, which was supported by X-ray computed microtomography visualizations of embolism refilling under both high- (8 kPa) and low-pressure (2 kPa) heads. Supplying pressurized water can hence cause artificial refilling of vessels, which makes it difficult to achieve a truly stable K h in partially embolized xylem. [ABSTRACT FROM AUTHOR]

Background and Aims Wood traits are increasingly being used to document tree performance. In the Congo Basin, however, weaker seasonality causes asynchrony of wood traits between trees. Here, we monitor growth and phenology data to date the formation of traits. Methods For two seasons, leaf and cambial phenology were monitored on four Terminalia superba trees (Mayombe) using cameras, cambial pinning and dendrometers. Subsequently, vessel lumen and parenchyma fractions as well as high-resolution isotopes (δ13C/δ18O) were quantified on the formed rings. All traits were dated and related to weather data. Key Results We observed between-tree differences in green-up of 45 d, with trees flushing before and after the rainy season. The lag between green-up and onset of xylem formation was 59 ± 21 d. The xylem growing season lasted 159 ± 17 d with between-tree differences of up to 53 d. Synchronized vessel, parenchyma and δ13C profiles were related to each other. Only parenchyma fraction and δ13C were correlated to weather variables, whereas the δ18O pattern showed no trend. Conclusions Asynchrony of leaf and cambial phenology complicates correct interpretation of environmental information recorded in wood. An integrated approach including high-resolution measurements of growth, stable isotopes and anatomical features allows exact dating of the formation of traits. This methodology offers a means to explore the asynchrony of growth in a rainforest and contribute to understanding this aspect of forest resilience. [ABSTRACT FROM AUTHOR]

To quantify stem respiration (RS) under elevated CO2 (eCO2), stem CO2 efflux (EA) and CO2 flux through the xylem (FT) should be accounted for, because part of respired CO2 is transported upwards with the sap solution. However, previous studies have used EA as a proxy of RS, which could lead to equivocal conclusions. Here, to test the effect of eCO2 on RS, both EA and FT were measured in a free‐air CO2 enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced EA and RS, which were unaffected by eCO2, likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO2 concentration measured near the stem base was higher under eCO2, and decreased along the stem resulting in a negative contribution of FT to RS, whereas the contribution of FT to RS under ambient CO2 was positive. Negative FT indicates net efflux of CO2 respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of RS on eCO2 and suggest stimulated root respiration under eCO2 that may shift vertical gradients in xylem [CO2] confounding the interpretation of EA measurements. In phosphorus‐limited sites, stem respiratory metabolism will remain unaffected by elevated CO2 as long as stem growth does not respond to it. Stimulated carbon allocation belowground and greater root respiration under elevated CO2 may shift vertical gradients in xylem [CO2] confounding the interpretation of stem CO2 efflux measurements. [ABSTRACT FROM AUTHOR]

To quantify stem respiration (RS) under elevated CO2 (eCO2), stem CO2 efflux (EA) and CO2 flux through the xylem (FT) should be accounted for, because part of respired CO2 is transported upwards with the sap solution. However, previous studies have used EA as a proxy of RS, which could lead to equivocal conclusions. Here, to test the effect of eCO2 on RS, both EA and FT were measured in a free‐air CO2 enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced EA and RS, which were unaffected by eCO2, likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO2 concentration measured near the stem base was higher under eCO2, and decreased along the stem resulting in a negative contribution of FT to RS, whereas the contribution of FT to RS under ambient CO2 was positive. Negative FT indicates net efflux of CO2 respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of RS on eCO2 and suggest stimulated root respiration under eCO2 that may shift vertical gradients in xylem [CO2] confounding the interpretation of EA measurements. In phosphorus‐limited sites, stem respiratory metabolism will remain unaffected by elevated CO2 as long as stem growth does not respond to it. Stimulated carbon allocation belowground and greater root respiration under elevated CO2 may shift vertical gradients in xylem [CO2] confounding the interpretation of stem CO2 efflux measurements. [ABSTRACT FROM AUTHOR]

Background and Aims Currently, functional–structural plant models (FSPMs) mostly resort to static descriptions of leaf spectral characteristics, which disregard the influence of leaf physiological changes over time. In many crop species, including soybean, these time-dependent physiological changes are of particular importance as leaf chlorophyll content changes with leaf age and vegetative nitrogen is remobilized to the developing fruit during pod filling. Methods PROSPECT, a model developed to estimate leaf biochemical composition from remote sensing data, is well suited to allow a dynamic approximation of leaf spectral characteristics in terms of leaf composition. In this study, measurements of the chlorophyll content index (CCI) were linked to leaf spectral characteristics within the 400–800 nm range by integrating the PROSPECT model into a soybean FSPM alongside a wavelength-specific light model. Key Results Straightforward links between the CCI and the parameters of the PROSPECT model allowed us to estimate leaf spectral characteristics with high accuracy using only the CCI as an input. After integration with an FSPM, this allowed digital reconstruction of leaf spectral characteristics on the scale of both individual leaves and the whole canopy. As a result, accurate simulations of light conditions within the canopy were obtained. Conclusions The proposed approach resulted in a very accurate representation of leaf spectral properties, based on fast and simple measurements of the CCI. Integration of accurate leaf spectral characteristics into a soybean FSPM leads to a better, dynamic understanding of the actual perceived light within the canopy in terms of both light quantity and quality. [ABSTRACT FROM AUTHOR]

Grapevines are characterized by a period of irreversible stem shrinkage around the onset of ripening of the grape berries. Since this shrinkage is unrelated to meteorological conditions or drought, it is often suggested that it is caused by the increased sink strength of the grape berries during this period. However, no studies so far have experimentally investigated the mechanisms underlying this irreversible stem shrinkage. We therefore combined continuous measurements of stem diameter variations and histology of potted 2-year-old grapevines (Vitis vinifera L. 'Boskoop Glory'). Sink strength was altered by pruning all grape clusters (treatment P), while non-pruned grapevines served as control (treatment C). Unexpectedly, our results showed irreversible post-veraison stem shrinkage in both treatments, suggesting that the shrinkage is not linked to grape berry sink strength. Anatomical analysis indicated that the shrinkage is the result of the formation of successive concentric periderm layers, and the subsequent dehydration and compression of the older bark tissues, an anatomical feature that is characteristic of Vitis stems. Stem shrinkage is hence unrelated to grape berry development, in contrast to what has been previously suggested. [ABSTRACT FROM AUTHOR]