About 90% of the plastic garbage remains in terrestrial ecosystems, and increasing evidence highlights the exposure of crops to plastic particles. However, the potential bioaccumulation of ...microplastics by plants and their effects on plants' physiology remains unexplored. Here, we evaluated the adsorption, potential uptake, and physiological effects of polyethylene (PE) microbeads in an experimental hydroponic culture of maize. Using isotope analysis, taking advantage of the different carbon isotope composition (δ13C) of fossil-derived PE and C4 plants (e.g., maize), we estimated that about 30% of the carbon in the rhizosphere of microplastic-exposed plants was derived from PE. Still, we did not find evidence of PE translocation to the shoots. Plastic bioaccumulation in the rhizosphere caused a significant decline in transpiration, nitrogen content, and growth. Our results indicate that plastic particles may accumulate in the rhizosphere, impairing water and nutrient uptake, and eventually reaching root eaters. Due to the implications for food production and livestock feeding, our findings encourage further research on the mechanism leading to the bioaccumulation of microplastics on the surface of belowground tissues.
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•Different carbon isotope composition (δ13C) of fossil-derived PE and C4 plants allows accurate estimations of polymers uptake.•Microplastics (3 μm) accumulate on the roots of plants, but do not translocate to aboveground tissues.•Microplastics on the rhizosphere can impair water and nutrient uptake, and eventually reach root eaters.•Isotope labelling is a promising tool for the assessment of organic polymer uptake/adsorption into living organisms.
The relevance of interspecific variation in the use of plant water sources has been recognized in drought‐prone environments. By contrast, the characterization of intraspecific differences in water ...uptake patterns remains elusive, although preferential access to particular soil layers may be an important adaptive response for species along aridity gradients. Stable water isotopes were analysed in soil and xylem samples of 56 populations of the drought‐avoidant conifer Pinus halepensis grown in a common garden test. We found that most populations reverted to deep soil layers as the main plant water source during seasonal summer droughts. More specifically, we detected a clear geographical differentiation among populations in water uptake patterns even under relatively mild drought conditions (early autumn), with populations originating from more arid regions taking up more water from deep soil layers. However, the preferential access to deep soil water was largely independent of aboveground growth. Our findings highlight the high plasticity and adaptive relevance of the differential access to soil water pools among Aleppo pine populations. The observed ecotypic patterns point to the adaptive relevance of resource investment in deep roots as a strategy towards securing a source of water in dry environments for P. halepensis.
To date, studies on terrestrial plant ecology and evolution have focused primarily on the trade-off patterns in the allocation of metabolic production to roots and shoots in individual plants and the ...scaling of whole-plant respiration. However, few empirical studies have investigated the root : shoot ratio by considering scaling whole-plant respiration at various sizes throughout ontogeny.
Here, using a whole-plant chamber system, we measured the respiration rates, fresh mass and surface area of entire roots and shoots from 377 Fagus crenata individuals, from germinating seeds to mature trees, collected from five different Japanese provenances. Non-linear regression analysis was performed for scaling of root and shoot respiration, fresh mass and surface area with body size.
Whole-plant respiration increased rapidly in germinating seeds. In the seedling to mature tree size range, the scaling of whole-plant respiration to whole-plant fresh mass was expressed as a linear trend on the log-log coordinates (exponent slightly greater than 0.75). In the same body size range, root and shoot respiration vs. whole-plant fresh mass were modelled by upward-convex (exponent decreased from 2.35 to 0.638) and downward-convex trends (exponent increased from -0.918 to 0.864), respectively. The root fraction in whole-plant respiration, fresh mass and surface area shifted continuously throughout ontogeny, increasing in smaller seedlings during early growth stages and decreasing in larger trees.
Our results suggest a gradual shift in allocation priorities of metabolic energy from roots in seedlings to shoots in mature trees, providing insights into how roots contribute to shoot and whole-plant growth during ontogeny. The models of root : shoot ratio in relation to whole-plant physiology could be applied in tree growth modelling, and in linking the different levels of ecological phenomena, from individuals to ecosystems.
Stable isotopes are extensively used as tracers for the study of plant-water sources. Isotope-ratio infrared spectroscopy (IRIS) offers a cheaper alternative to isotope-ratio mass spectroscopy ...(IRMS), but its use in studying plant and soil water is limited by the spectral interference caused by organic contaminants. Here, we examine two approaches to cope with contaminated samples in IRIS: on-line oxidation of organic compounds (MCM) and post-processing correction.
We assessed these methods compared to IRMS across 136 samples of xylem and soil water, and a set of ethanol– and methanol–water mixtures.
A post-processing correction significantly improved IRIS accuracy in both natural samples and alcohol dilutions, being effective with concentrations up to 8% of ethanol and 0.4% of methanol. MCM outperformed the post-processing correction in removing methanol interference, but did not effectively remove interference for high concentrations of ethanol.
By using both approaches, IRIS can overcome with reasonable accuracy the analytical uncertainties associated with most organic contaminants found in soil and xylem water. We recommend the post-processing correction as the first choice for analysis of samples of unknown contamination. Nevertheless, MCM can be more effective for evaluating samples containing contaminants responsible for strong spectral interferences at low concentrations, such as methanol.
The mechanistic understanding of isotope fractionation processes is increasing but we still lack detailed knowledge of the processes that determine the isotopic composition of the tree-ring archive ...over the long term. Especially with regard to the path from leaf photosynthate production to wood formation, post-assimilation fractionations/processes might cause at least a partial decoupling between the leaf isotope signals that record processes such as stomatal conductance, transpiration and photosynthesis, and the wood or cellulose signals that are stored in the paleophysiological record. In this review, we start from the rather well understood processes at the leaf level such as photosynthetic carbon isotope fractionation, leaf water evaporative isotope enrichment and the issue of the isotopic composition of inorganic sources (CO2 and H2O), though we focus on the less explored 'downstream' processes related to metabolism and transport. We further summarize the roles of cellulose and lignin as important chemical constituents of wood, and the processes that determine the transfer of photosynthate (sucrose) and associated isotopic signals to wood production. We cover the broad topics of post-carboxylation carbon isotope fractionation and of the exchange of organic oxygen with water within the tree. In two case studies, we assess the transfer of carbon and oxygen isotopic signals from leaves to tree rings. Finally we address the issue of different temporal scales and link isotope fractionation at the shorter time scale for processes in the leaf to the isotopic ratio as recorded across longer time scales of the tree-ring archive.
Winter‐drought induced forest diebacks in the low‐latitude margins of species' distribution ranges can provide new insights into the mechanisms (carbon starvation, hydraulic failure) underlying ...contrasting tree reactions. We analysed a winter‐drought induced dieback at the Scots pine's southern edge through a dual‐isotope approach (Δ13C and δ18O in tree‐ring cellulose). We hypothesized that a differential long‐term performance, mediated by the interaction between CO2 and climate, determined the fates of individuals during dieback. Declining trees showed a stronger coupling between climate, growth and intrinsic water‐use efficiency (WUEi) than non‐declining individuals that was noticeable for 25 years prior to dieback. The rising stomatal control of water losses with time in declining trees, indicated by negative Δ13C‐δ18O relationships, was likely associated with their native aptitude to grow more and take up more water (suggested by larger tracheid lumen widths) than non‐declining trees and, therefore, to exhibit a greater cavitation risk. Freeze‐thaw episodes occurring in winter 2001 unveiled such physiological differences by triggering dieback in those trees more vulnerable to hydraulic failure. Thus, WUEi tightly modulated growth responses to long‐term warming in declining trees, indicating that co‐occurring individuals were differentially predisposed to winter‐drought mortality. These different performances were unconnected to the depletion of stored carbohydrates.
This work examined a winter‐drought induced dieback at the Scots pine's southern edge through a dual‐isotope approach (Δ13C and δ18O in tree‐ring cellulose). Declining trees showed a stronger coupling between climate, growth and intrinsic water‐use efficiency (WUEi) than non‐declining individuals, which was likely associated with their native aptitude to grow more, take up more water and, also, exhibit a greater cavitation risk. This indicated that co‐occurring individuals were differentially predisposed to winter‐drought mortality.
Summary
We compiled hydrogen and oxygen stable isotope compositions (δ2H and δ18O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2H was more closely ...correlated with δ2H of xylem water or atmospheric vapour, whereas leaf water δ18O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ2H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ18O. We next expressed leaf water as isotopic enrichment above xylem water (Δ2H and Δ18O) to remove the impact of xylem water isotopic variation. For Δ2H, leaf water still correlated with atmospheric vapour, whereas Δ18O showed no such correlation. This was explained by covariance between air relative humidity and the Δ18O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2H and 18O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.
Submediterranean forests are considered an ecotone between Mediterranean and Eurosiberian ecosystems, and are very sensitive to global change. A decline of Scots pine (Pinus sylvestris L.) and a ...related expansion of oak species (Quercus spp.) have been reported in the Spanish Pre-Pyrenees. Although this has been associated with increasing drought stress, the underlying mechanisms are not fully understood, and suitable monitoring protocols are lacking. The aim of this study is to bring insight into the physiological mechanisms anticipating selective decline of the pines, with particular focus on carbon and water relations. For this purpose, we performed a sampling campaign covering two growing seasons in a mixed stand of P. sylvestris and Quercus subpyrenaica E.H del Villar. We sampled seasonally twig xylem and soil for water isotope composition (δ18O and δ2H), leaves for carbon isotope composition (δ13C) and stems to quantify non-structural carbohydrates (NSC) concentration, and measured water potential and leaf gas exchange. The first summer drought was severe for both species, reaching low predawn water potential (-2.2 MPa), very low stomatal conductance (12 ± 1.0 mmol m-2 s-1) and near-zero or even negative net photosynthesis, particularly in P. sylvestris (-0.6 ± 0.34 μmol m-2 s-1 in oaks, -1.3 ± 0.16 μmol m-2 s-1 in pines). Hence, the tighter stomatal control and more isohydric strategy of P. sylvestris resulted in larger limitations on carbon assimilation, and this was also reflected in carbon storage, showing twofold larger total NSC concentration in oaks than in pines (7.8 ± 2.4% and 4.0 ± 1.3%, respectively). We observed a faster recovery of predawn water potential after summer drought in Q. subpyrenaica than in P. sylvestris (-0.8 MPa and -1.1 MPa, respectively). As supported by the isotopic data, this was probably associated with a deeper and more reliable water supply in Q. subpyrenaica. In line with these short-term observations, we found a more pronounced negative effect of steadily increasing drought stress on long-term growth in pines compared with oaks. All these observations confer evidence of early warning of P. sylvestris decline and indicate the adaptive advantage of Q. subpyrenaica in the area.
Aims
Harsh edaphic environments harbor species with different soil affinities. Plant’s responses to specific edaphic constraints may be compromised against responses to prevalent stresses shared with ...other semi-arid environments. We expect that species with high edaphic affinity may show traits to overcome harsh soil properties, while species with low affinity may respond to environmental constraints shared with arid environments.
Methods
We quantified the edaphic affinity of 12 plant species co-occurring in gypsum outcrops and measured traits related to plant responses to specific gypsum constraints (rooting and water uptake depth, foliar accumulation of Ca, S and Mg), and traits related to common constraints of arid environments (water use efficiency, macronutrients foliar content).
Results
Plants in gypsum outcrops differed in their strategies to face edaphic limitations. A phylogenetic informed PCA segregated species based on their foliar Ca and S accumulation and greater water uptake depths, associated with plant responses to specific gypsum limitations. Species’ gypsum affinity explained this segregation, but traits related to water or nutrient use efficiency did not contribute substantially to this axis.
Conclusions
Plant’s specializations to respond to specific edaphic constraints of gypsum soils do not limit their ability to deal with other non-specific environmental constraints.
Background and Aim
Drought is the main abiotic stress affecting Mediterranean forests. Root systems are responsible for water uptake, but intraspecific variability in tree root morphology is poorly ...understood mainly owing to sampling difficulties. The aim of this study was to gain knowledge on the adaptive relevance of rooting traits for a widespread pine using a non-invasive, high-throughput phenotyping technique.
Methods
Ground-Penetrating Radar (GPR) was used to characterize variability in coarse root features (depth, diameter and frequency) among populations of the Mediterranean conifer
Pinus halepensis
evaluated in a common garden. GPR records were examined in relation to aboveground growth and climate variables at origin of populations.
Results
Variability was detected for root traits among 56 range-wide populations categorized into 16 ecotypes. Root diameter decreased eastward within the Mediterranean basin. In turn, root frequency, but not depth and diameter, decreased following a northward gradient. Root traits also varied with climatic variables at origin such as the ratio of summer to annual precipitation, summer temperature or solar radiation. Particularly, root frequency increased with aridity, whereas root depth and diameter were maximum for ecotypes occupying the thermal midpoint of the species distribution range.
Conclusion
GPR is a high-throughput phenotyping tool that allows detection of intraspecific variation in root traits of
P. halepensis
and its dependencies on eco-geographic characteristics at origin, thereby informing on the adaptive relevance of root systems for the species. It is also potentially suited for inferring population divergence in resource allocation above- and belowground in forest genetic trials.
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