Specific Root Length Research Articles

Effects of long-term water reduction and nitrogen addition on fine roots and fungal hyphae in a mixed mature Pinus koraiensis forest

PurposeThe lack of synchronous studies on fine roots and fungal hyphae makes it difficult to gain insight into changes in how mycorrhizal trees search for nutrients under reduced precipitation and increased nitrogen deposition.MethodsWe applied a modified version of the ingrowth bag approach to estimate the community-level responses of fine roots and fungal hyphae to long-term changes in nitrogen and water availability in a mixed mature forest.ResultsWater reduction, nitrogen addition and the two treatments applied in combination decreased root length density, root biomass and fungal hyphal length density, with the lowest values occurring in the combined treatment. Compared with fine roots in the control treatment, fine roots in the water reduction treatment had a thinner diameter, lower branching intensity and a greater specific root length. Fungal hyphae in the combined treatment had significantly greater diameters than in the control treatment at 0–10 cm soil depths. Root length density, root biomass, root branching intensity and hyphal length density significantly decreased with increasing soil depth. In contrast, hyphal diameter increased with increasing soil depth. Fungal hyphal length density was positively related to root length density but negatively related to hyphal diameter.ConclusionForest fine roots and fungal hyphae respond plastically to soil water and nitrogen availability. An integrative and simultaneous understanding of both root and hyphal trait plasticity can provide better insights into the resource acquisition strategies of trees experiencing environmental changes.

Site conditions more than species identity drive fine root biomass, morphology and spatial distribution in temperate pure and mixed forests

AimThe establishment of broadleaved - conifer mixtures has had positive effects on productivity. However, it is still not clear the degree to which belowground space occupation contributes to these effects and how root traits of the respective species differ between mixed and pure stands. Thus, the aim of this study was to examine how enrichment of beech forests with conifer species impacts fine root biomass and other root traits across variable site conditions. MethodsWe studied 20 mature pure and mixed stands of European beech (Fagus sylvatica), Norway spruce (Picea abies), and Douglas-fir (Pseudotsuga menziesii), and determined differences in biomass, spatial distribution, and morphology of fine roots (≤2 mm, to 60 cm soil depth) among beech – conifer mixtures and pure stands of the respective species in two regions with differing site conditions in Northern Germany. ResultsSite conditions had strong effects on fine root biomass (FRB) and fine root area index (RAI). Both FRB and RAI were significantly higher across all stand types on the sites with lower belowground resource availability. On the less favorable sites, FRB significantly increased with increasing beech proportion. In contrast to specific root area (SRA) and RAI, specific root length (SRL) was not affected by site conditions but differed significantly among species. No overyielding was observed when FRB was plotted against basal area and/or area potentially available (APA), which reflects the aboveground horizontal space configuration. Spatial fine root biomass patterns of all species in both monospecific and mixed stands changed along the vertical soil profile at both sites. FRB decreased with increasing soil depth, with the exception of beech in mixed stands. ConclusionOur data suggest that both amount and distribution of tree fine roots in pure and mixed stands are highly variable and depend primarily on-site conditions and secondarily on tree species identity. By disentangling soil and species identity effects, we conclude that beech has a remarkable potential for adjusting fine root biomass to cope with unfavorable conditions as compared to spruce and Douglas-fir.

Ontogenetic trait shifts: Seedlings display high trait variability during early stages of development

AbstractCharacterizing variation in plant functional traits is often key to understanding community‐level processes and predicting ecosystem responses to environmental change. Trait‐based ecology has focused on interspecific trait variation, but sources and consequences of within‐species ontogenetic trait variation, particularly during early stages of development, remain understudied.Using a manipulative greenhouse experiment, we investigated trait variation during early stages of seedling development in seven dominant perennial plant species in the western United States. We examined variability in key trait values (i.e. SLA, root:shoot ratio (RSR), specific root length (SRL) and root dry matter content (RDMC)) of 20‐ to 62‐day‐old seedlings grown under low and high levels of water availability. We also compared these to compiled trait databases to assess how representative these readily available data sources are of seedling trait values.Early seedling trait values shifted greatly during early stages of development and generally differed from average plant trait database values. Trait shifts were greatest in forbs versus grasses. Overall, observed trait shifts suggested a transition from fast‐growing resource acquisitional strategies towards more slow‐growing conservative strategies over time. For example, seedling SLA decreased while RSR and RDMC increased over time.That seedling trait values aconsistently differed from trait database values indicates that plant trait database values may be poor predictors of seedling trait values. Such mismatches in species trait information could result in inaccurate predictions of community assembly outcomes or incongruities between seedling traits and environmental filters experienced by seedlings during early stages of recruitment in applied settings.We suggest that additional work is needed to characterize intraspecific trait variation across plant ontogeny, and that this information should be incorporated into studies ranging from understanding early plant growth and survival to evaluating the outcomes of ecological restoration.A freePlain Language Summarycan be found within the Supporting Information of this article.

Effect on soil water availability, rather than silicon uptake by plants, explains the beneficial effect of silicon on rice during drought

Various studies showed a decrease of drought stress specific parameters of plants after silicon (Si) fertilization. But all studies differed in soil Si concentration between the control and Si treatments. As amorphous silica (ASi) was recently found to cause a strong increase of water holding capacity and plant available water in soils, a combined effect of soil moisture and plant response due to Si addition was assumed. In this study, the influence of the soil Si content was excluded by using the same Si enriched soil for treatments of two rice lines, lsi1 mutant defective in Si uptake and its wild-type rice. Most plant parameters, such as nutrient contents, biomass, specific leaf area, specific root length, leaf water content and C allocation did not differ significantly between the genotypes neither under flooded conditions, nor under drought conditions. Only photosynthesis and stomatal conductance were slightly higher for the wild type in both drought and flooded treatments. Overall, our data showed that Si accumulation within the plant tissues has only a minor effect on plant performance under drought stress. Hence, existing studies should be reinterpreted in light of the fact that Si additions may increase soil water availability.

Chemical and morphological response of beech saplings (Fagus sylvatica L.) to an experimental soil drought gradient

European beech (Fagus sylvatica L.) is a common tree species in Central European forests but the fitness of this species under future climate conditions is still a matter of debate. Here, we study the response of transplanted beech saplings in terms of their chemical and morphological traits to different levels of soil drought in a mountainous, partially open, storm damaged Norway spruce forest. Throughfall was excluded by roof constructions in three consecutive growing seasons. Spatial soil drought gradients resulted from soil water consumption by heterogeneously distributed mature spruce trees. Individual drought stress dose (DSD) was assessed for each beech sapling using cumulative soil matric potential in the rooting zone. After three years of drought treatment, we analyzed biomass components, traits of leaves and fine roots of sixty beech saplings. In the third year, the classical permanent wilting point in the soil was exceeded for some beech saplings, however, all saplings survived the drought treatments. Several chemical plant traits responded to DSD sum of the last two years, while morphological traits and biomass components were mostly not or weakly correlated with DSD. Potassium concentration and associated C:K and N:K ratios in fine roots were most sensitive to drought stress. We assume that K and other nutrients were hardly available in the extremely dry forest floor and that the undersupply could not be compensated by nutrient uptake from the mineral soil. Increasing C concentration in leaves indicates an acclimation towards sclerophylly to maintain the hydraulic function of leaves. Increasing specific root length (SRL), specific root tip density (SRTD) and decreasing fine root diameter also indicate a morphological acclimation of saplings to increasing DSD. Weak correlations between morphological traits and DSD partly resulted from non-linear responses to increasing DSD. Maximum number of mycorrhizal root tips and maximum fine root biomass occurred at medium DSD. Our results suggest that the acclimation of chemical and morphological traits fortifies the resilience of beech saplings against soil drought in the years after transplantation. However, reduced uptake of soil nutrients caused by drought can only be partially compensated by redistribution within the sapling. An expansion of the rooting zone in the moist, nutrient-poor subsoil can improve the water uptake of beech saplings during drought periods, but this strategy hardly improves their nutrient supply.

Critical phosphorus requirements of Trifolium species: The importance of root morphology and root acclimation in response to phosphorus stress.

Differences in root morphology and acclimation to low-phosphorus (P) soil were examined among eight legume species from the Trifolium Section Tricocephalum to understand how these root attributes determine P acquisition. Ornithopus sativus was included as a highly P-efficient benchmark species. Plants were grown as microswards in pots with five rates of P supplied in a topsoil layer to mimic uneven P distribution within a field soil profile. Topsoil and subsoil roots were harvested separately to enable measurement of the nutrient-foraging responses. Critical P requirement (lowest P supply for maximum yield) varied over a threefold range, reflecting differences in root morphology and acclimation of nutrient-foraging roots to P stress. Among the species, there was a 3.2-fold range in root length density, a 1.7-fold range in specific root length, and a 2.1-fold range in root hair length. O. sativus had the lowest critical P requirement, displayed a high root length density, the highest specific root length, and the longest root hairs. Acquisition of P from P-deficient soil was facilitated by development of a large root hair cylinder (i.e. a large root-soil interface). This, in turn, was determined by the intrinsic root morphology attributes of each genotype, and the plasticity of its root morphology response to internal P stress. Root acclimation in low-P soil by all species was mostly associated with preferential allocation of mass to nutrient-foraging roots. Only O. sativus and four of the Trifolium species adjusted specific root length beneficially, and only O. sativus increased its root hair length in low-P soil.

An integrated framework of plant form and function: the belowground perspective.

Plant trait variation drives plant function, community composition and ecosystem processes. However, our current understanding of trait variation disproportionately relies on aboveground observations. Here we integrate root traits into the global framework of plant form and function. We developed and tested an overarching conceptual framework that integrates two recently identified root trait gradients with a well-established aboveground plant trait framework. We confronted our novel framework with published relationships between above- and belowground trait analogues and with multivariate analyses of above- and belowground traits of 2510 species. Our traits represent the leaf and root conservation gradients (specific leaf area, leaf and root nitrogen concentration, and root tissue density), the root collaboration gradient (root diameter and specific root length) and the plant size gradient (plant height and rooting depth). We found that an integrated, whole-plant trait space required as much as four axes. The two main axes represented the fast-slow 'conservation' gradient on which leaf and fine-root traits were well aligned, and the 'collaboration' gradient in roots. The two additional axes were separate, orthogonal plant size axes for height and rooting depth. This perspective on the multidimensional nature of plant trait variation better encompasses plant function and influence on the surrounding environment.

Functional root trait‐based classification of cover crops to improve soil physical properties

AbstractCover crop use is a well‐established soil conservation technique and has been proven effective for erosion control and soil remediation in many arable systems. Whereas the obvious protection mechanism of cover crops occurs through the canopy, plant roots perform multiple functions. It is important to consider the soil functions delivered by different root systems in order to increase the uptake of cover crops for sustainable soil and water management. A classification of cover crop root systems up to 0.6 m deep based on functional traits will allow us to better study their potential role in soil bio‐engineering, soil structural improvements for hydrological services and soil resource protection. This was a glasshouse experiment, using large 1‐m3 containers filled with loam soil, loose topsoil and compacted subsoil, in which seven cover crop species (oat, rye, buckwheat, vetch, radish, mustard, phacelia) were grown for 90 days. Root cores were taken at the end of the experiment, washed and imaged to determine root traits (total root length density, average root diameter, root specific length and root surface area) for both the topsoil and subsoil layers. Root identity was determined from a distinctive combination of single root traits and related to three soil functional variables, representing soil structural improvement, runoff mitigation and erosion control. The results showed that total root length and root surface area correlate well with aggregate stability and soil macroporosity. Buckwheat, mustard and rye had significantly greater aggregate stability, as well as 10, 8 and 7% greater microporosity, respectively, at the interface with the compacted layer when compared to the control bare soil. Furthermore, average root diameter negatively correlated with soil macroporosity, indicating that cover crops with a fine root system are more beneficial for creating pore‐space than those with thicker taproots. Selecting cover crop species with the right root traits is therefore crucial to improve soil health.Highlights Roots of cover crops are a largely unexplored frontier for bio‐engineering of agricultural soils. Combinations of root traits were identified that most improve soil characteristics. Cover crops with finer root systems were better at enhancing porosity. Root length and surface area were most important for enhancing soil structure.

Potential local adaptation in populations of invasive reed canary grass (Phalaris arundinacea) across an urbanization gradient.

Urban stressors represent strong selective gradients that can elicit evolutionary change, especially in non‐native species that may harbor substantial within‐population variability. To test whether urban stressors drive phenotypic differentiation and influence local adaptation, we compared stress responses of populations of a ubiquitous invader, reed canary grass (Phalaris arundinacea). Specifically, we quantified responses to salt, copper, and zinc additions by reed canary grass collected from four populations spanning an urbanization gradient (natural, rural, moderate urban, and intense urban). We measured ten phenotypic traits and trait plasticities, because reed canary grass is known to be highly plastic and because plasticity may enhance invasion success. We tested the following hypotheses: (a) Source populations vary systematically in their stress response, with the intense urban population least sensitive and the natural population most sensitive, and (b) plastic responses are adaptive under stressful conditions. We found clear trait variation among populations, with the greatest divergence in traits and trait plasticities between the natural and intense urban populations. The intense urban population showed stress tolerator characteristics for resource acquisition traits including leaf dry matter content and specific root length. Trait plasticity varied among populations for over half the traits measured, highlighting that plasticity differences were as common as trait differences. Plasticity in root mass ratio and specific root length were adaptive in some contexts, suggesting that natural selection by anthropogenic stressors may have contributed to root trait differences. Reed canary grass populations in highly urbanized wetlands may therefore be evolving enhanced tolerance to urban stressors, suggesting a mechanism by which invasive species may proliferate across urban wetland systems generally.

Fine root biomass and morphology in a temperate forest are influenced more by the nitrogen treatment approach than the rate

Global nitrogen (N) deposition patterns have profoundly affected the production and morphological structure of fine roots, which in turn changed the distribution of carbon in forests. However, traditional experiments of N addition in forests have ignored the ecological processes in the canopy, such as nitrogen retention, and the effects of these on N deposition and fine root functional traits remain unclear. In this study, we quantified the effects of the canopy (CAN) and understory N addition (UAN) on the biomass and morphology of fine roots in a temperate deciduous forest. Based on the three-way ANOVA, we found that the N treatment approach (CAN vs. UAN; p < 0.05) and sampling time (July, October, and January; p < 0.001) significantly affected the fine root biomass, specific root length, specific root surface area, and root tissue density and diameter. Through the summation of fine root biomass, it was found that CAN treatment increased fine root biomass, while UAN treatment decreased fine root biomass, and canopy N addition at 50 kg ha-1yr−1 (CAN50) significantly increased fine root biomass compared with understory N addition at 50 kg ha-1yr−1 (UAN50) in July and October (p < 0.05). Redundancy analysis (RDA) showed that fine root biomass was most affected by NH4-N and NO3-N in July. There was no consistent response of fine root morphology to the N application method and N addition rate; it was mainly affected by season (p < 0.001). Different results were obtained using different N treatment approaches. The effects of UAN on forest fine root biomass were likely overestimated compared to the effects of CAN. Our experimental results will provide a scientific basis for a more accurate prediction of the impacts of future global N deposition on fine roots in forest ecosystems.

Grazing alters species relative abundance by affecting plant functional traits in a Tibetan subalpine meadow.

Domestic livestock grazing has caused dramatic changes in plant community composition across the globe. However, the response of plant species abundance in communities subject to grazing has not often been investigated through a functional lens, especially for belowground traits. Grazing directly impacts aboveground plant tissues, but the relationships between above‐ and belowground traits, and their influence on species abundance are also not well known. We collected plant trait and species relative abundance data in the grazed and nongrazed meadow plant communities in a species‐rich subalpine ecosystem of the Qinghai–Tibet Plateau. We measured three aboveground traits (leaf photosynthesis rate, specific leaf area, and maximum height) and five belowground traits (root average diameter, root biomass, specific root length, root tissue density, and specific root area). We tested for shifts in the relationship between species relative abundance and among all measured traits under grazing compared with the nongrazed meadow. We also compared the power of above‐ and belowground traits to predict species relative abundance. We observed a significant shift from a resource conservation strategy to a resource acquisition strategy. Moreover, this resource conservation versus resource acquisition trade‐off can also determine species relative abundance in the grazed and nongrazed plant communities. Specifically, abundant species in the nongrazed meadow had aboveground and belowground traits that are associated with high resource conservation, whereas aboveground and belowground traits that are correlated with high resource acquisition determined species relative abundance in the grazed meadow. However, belowground traits were found to explain more variances in species relative abundance than aboveground traits in the nongrazed meadow, while aboveground and belowground traits had comparable predictive power in the grazed meadow. We show that species relative abundance in both the grazed and the nongrazed meadows can be predicted by both aboveground traits and belowground traits associated with a resource acquisition versus conservation trade‐off. More importantly, we show that belowground traits have higher predictive power of species relative abundance than aboveground traits in the nongrazed meadow, whereas in the grazed meadows, above‐ and belowground traits had comparable high predictive power.

Organic mulching alters the composition, but not the diversity, of rhizosphere bacterial and fungal communities

Organic mulching is an effective forest management technique that provides carbon and nutrient sources to soil ecosystems, thereby improving the soil environment and promoting plant growth. Although the importance of rhizosphere microbiomes in plant and soil ecosystem functions has been widely recognised, the effect of organic mulching on rhizosphere microorganisms and the underlying mechanisms are unclear. We performed a field experiment in a 15-year-old Ligustrum lucidum forest of urban green space. The diversity and composition of the rhizosphere bacterial and fungal communities following organic mulching were assessed by combining 16S ribosomal RNA and internal transcribed spacer amplicon sequencing. The correlations between microbial diversity, composition, and fine-root traits, as well as rhizosphere soil properties, were also analysed. The results showed that organic mulching did not significantly affect the diversity of the rhizosphere bacterial or fungal communities on the whole, but it increased the bacterial diversity after 6 months, with a 20-cm-thick mulch layer showing a greater effect than 5- or 10-cm layers. Organic mulching significantly altered the rhizosphere bacterial and fungal community composition; after 6 months of mulching, the community compositions were significantly associated with fine-root traits (specific root length, nitrogen, and phosphorus concentration) and enzyme (urease and dehydrogenase) activity. Moreover, alterations in the bacterial and fungal communities occurred at the order level within each mulching stage. Bacterial diversity is affected by fungal diversity and rhizosphere soil properties (water content and organic carbon) in time-dependent manners. Hence, organic mulching appears to directly affect the fungal composition while indirectly affecting the bacterial composition via influencing rhizosphere soil properties (dissolved organic carbon and peroxidase activity). Our study suggests that organic mulching affects the rhizosphere bacterial and fungal community composition through different pathways; however, the underlying mechanisms, including the effects of time and soil layers, require further exploration combined with multi-index measurements and long-term dynamic monitoring.

Responses of Collembola communities to mixtures of wheat varieties: A trait-based approach

The genetic diversity of cultivated crops has decreased continuously since the beginning of the 20th century, because of the gradual replacement of genetically heterogeneous traditional varieties by new genetically homogenous varieties, grown in monospecific stands. The resulting agro-ecosystems are now considered as unsustainable. Increasing within-field genetic crop diversity by using a mixture of varieties could increase the sustainability of these agro-ecosystems. This could also potentially increase non-crop biodiversity. In the present study we used an experimental approach to test whether the number of wheat varieties (genetic diversity), the number of functional groups (functional diversity), the composition of functional groups and wheat traits influence (1) species richness and abundance of Collembola, and (2) species and trait assemblages of Collembola. A total of 104 plots were seeded with either monocultures or mixtures of 2, 4 and 8 wheat varieties. Soil cores were collected in each plot to extract Collembola and measure soil features. The number of wheat varieties and the functional wheat diversity did not influence the abundance and species richness of Collembola. The sensitivity of wheat to septoria leaf blotch was positively related to abundance and species richness of Collembola, while specific root length was positively related to collembolan species richness. Wheat traits related to sensitivity to fungal diseases, (more especially septoria leaf blotch) and characteristics of aerial parts and roots impacted collembolan species and trait assemblages, but these effects were weak. Soil features, especially the proportion of coarse silt, were also influential. Our study did not show a favorable impact of wheat genetic diversity on soil Collembola, which might result from their low abundance. Nevertheless, it suggests correlations between some variety traits and the species richness and abundance of Collembola.

Differing Trade-Off Patterns of Tree Vegetative Organs in a Tropical Cloud Forest.

Functional trait ecology demonstrates the significance of the leaf economics spectrum in understanding plants’ trade-off between acquisitive and conservative resource utilization. However, whether trait variations of different vegetative organs are coordinated and whether the plant economics spectrum is characterized by more than one vegetative organ remain controversial. To gain insights into these questions, within a tropical cloud forest in Hainan Island, a total of 13 functional traits of 84 tree species were analyzed here, including leaf, stem and root traits. By using standardized major axis (SMA) regression and principal components analysis, we examined the trait variations and correlations for deciphering plants’ trade-off pattern. We found decreases of leaf phosphorus content, leaf nitrogen content and specific leaf area and increases of leaf mass per unit area (LMA), wood density and leaf thickness along the first principal component, while there were decreases of specific root length and specific root area and increases of root tissue density along the second principal component. Root phosphorus and nitrogen contents were significantly positively associated with the phosphorus and nitrogen contents of both stem and leaf. Wood density was significantly positively associated with LMA and leaf thickness, but negatively associated with leaf thickness and specific leaf area. Our results indicate that, in the tropical cloud forest, there is a “fast–slow” economic spectrum characterized by leaf and stem. Changes of nutrient trait are coordinated, whereas the relationships of morphological traits varied independently between plant above- and below-ground parts, while root nutrient traits are decoupled from root morphological traits. Our findings can provide an insight into the species coexistence and community assembly in high-altitude tropical forests.

Correlative associations of photosynthetic and rooting attributes of sesame: Drought-induced reversed associations are corrected upon salicylic acid exposure

Concomitant rooting and photosynthetic responses of crop plants to water availability and plant growth regulators lack clarity. Three pot-grown sesame genotypes were subjected to three irrigation regimes, ranging from control to severe moisture stress, and two levels of salicylic acid (SA) (0 and 0.6 mM) to address this lacuna of data. Drought led to decreases in gas exchange, water relation, and rooting attributes, and shoot dry mass of the three genotypes in different extents, but it resulted in increases in substomatal CO2 concentration, proline concentration, and root/shoot dry mass. Cumulative root length, root length density, specific root length, and root surface tended to increase in genotype Yekta upon experiencing stress, in contrast to more or less decreasing tendencies in rooting characters of genotypes Shiraz and Naz-Takshakhe. Such genotype-specificity led to the reversal of correlations between gas exchange attributes and rooting characteristics under drought conditions. External SA led to enhancements in gas exchange, water relation, and rooting attributes, and dry mass; the degree of the enhancements tended to be greater in the stress-stricken plants. These findings indicate that genotype Yekta benefits from rooting characteristics with drought tolerance significance and SA is effective in enhancing photosynthetic, physiological and rooting attributes, drought tolerance, and growth of sesame under drought circumstances.

Effect of water exchange rate on interspecies competition between submerged macrophytes: functional trait hierarchy drives competition

The “competition-trait similarity” and “competition-trait hierarchy” hypotheses can be used to predict competitive outcomes between terrestrial plant species in response to environmental variation. However, their validity in aquatic plants remains poorly understood, particularly in terms of variation in the water exchange rate (WER). We compared competitive outcomes and variation in functional traits in two pairs of submerged macrophytes (Vallisneria natans vs. Myriophyllum aquaticum and V. natans vs. Myriophyllum spicatum) at three levels of WER using the replacement series method. We found that the relative competitive ability of V. natans consistently decreased with increasing WER and decreasing the planting proportion of V. natans. Under these conditions, V. natans experienced reduced plant nutrient content and the inhibition of dissolved carbon dioxide absorption, which eventually shifted this species from a stronger to weaker competitor. With increasing WER and decreasing the planting proportion of V. natans, the below-:above-ground biomass ratio and root diameter increased, while the root length and specific root length decreased, for V. natans. In contrast, all of these traits exhibited the opposite patterns for the co-cultured Myriophyllum species. In addition, the branch number per plant mass increased in V. natans but exhibited no change in co-cultured Myriophyllum species under the same conditions. Our results indicate that WER affected the outcome of interspecies competition between submerged macrophyte species. Moreover, the relative competitive ability of each species within a pair was more strongly linked to species competition-trait hierarchy than to competition-trait similarity.

Coordination of root growth with root morphology, physiology and defense functions in response to root pruning in Platycladus orientalis

IntroductionRoot pruning is commonly used to facilitate seedling transplantation for the restoration of degraded or damaged ecosystems. However, little is known about how root growth coordinates morphology, physiology and defense functions following root pruning. ObjectivesWe aim to elucidate whether and how root growth trades off with defense functioning after pruning. MethodsSeedlings of Platycladus orientalis, a tree species widely used in forest restoration, were subjected to root pruning treatment. A suite of root growth, morphological and physiological traits were measured after pruning in combination with proteomic analysis. ResultsRoot growth was insensitive to pruning until at 504 h with a significant increase of 16.8%, whereas root physiology was activated rapidly after pruning. Key root morphological traits, such as root diameter, specific root length and root tissue density, showed no response to the pruning treatment. Plant defense syndromes such as reactive oxygen species-scavenging enzymes and defensive phytohormones such as jasmonic acid and abscisic acid, were recruited at six hours after pruning and recovered to the unpruned levels at 504 h. Compared with the controls, 271, 360 and 106 proteins were differentially expressed at 6, 72 and 504 h after root pruning, respectively. These proteins, associated with defense function, showed temporal patterns similar to the above defense syndromes. ConclusionOur results suggest a root growth-defense tradeoff following root pruning in P. orientalis. This tradeoff was potentially due to the significant increase of indole-3-acetic acid, the phytohormone stimulating root branching, which occurred soon after pruning. Together, these results provide a holistic understanding of how root growth is coordinated with root morphology, physiology, and defense in response to root pruning.

Silicon application and plant growth promoting rhizobacteria consisting of six pure Bacillus species alleviate salinity stress in cucumber (Cucumis sativus L)

As water quality and availability decreases in many parts of the world, salinity is becoming a major challenge that reduces crop yield, even in soilless cultivation systems. Therefore, novel strategies are needed to promote plant salt tolerance in these systems. We hypothesized that the non-essential element silicon (Si) and plant-growth promoting Bacillus spp. can alleviate salt stress of cucumber (Cucumis sativus L.) grown in hydroponics. We tested this hypothesis by growing cucumber seedlings with and without salt stress (75 mM NaCl) and with and without 1.5 mM Si and an inoculum of six rhizosphere Bacillus species in a full-factorial design. Seedlings were grown in a climate room for two weeks in independent deep-water culture containers. The applied salt stress strongly reduced plant biomass, whereas Si application under salt stress resulted in a substantial increase in cucumber shoot and root biomass. This beneficial impact of Si was also observed in increased plant height, leaf area, specific leaf area, root length, specific root length, root surface area and root volume. The Bacillus species increased root dry weight, specific leaf area as well as specific root length. In seedlings grown under salt stress, Si application increased shoot and root Si concentration, whereas Cl− concentration was reduced in the plant shoots. A reduction in Cl− concentration of the shoots was also apparent in the Bacillus treatment. Under non-stress conditions, neither Si nor Bacillus species affected plant growth parameters. However, shoot mineral content was affected as Si application reduced shoot Cl−and Ca2+ concentrations, and inoculation with Bacillus species decreased K concentration. We conclude that Si does promote salt stress alleviation during the early growth stage of cucumber grown in deep water culture and this has implications for soilless crop production. Seed inoculation with Bacillus species showed a beneficial trend for some plant growth characteristics and nutrient status under high salinity, although not as pronounced as for Si.

Relationships between plant–soil feedbacks and functional traits

Abstract Plant–soil feedbacks (PSF) and functional traits are two active but not well theoretically integrated areas of research. However, PSF and traits are both affected by life‐history evolution, so the two should theoretically be related. We provide a conceptual framework to link plant functional traits to two types of PSF metrics, and hypothesize that individual PSF (plant performance in conspecific vs. heterospecific soil) should be related to the fast–slow trait spectrum, whereas pairwise PSF (the sum of the individual feedbacks for two species growing in each other's soils) should be related to trait dissimilarity. We performed meta‐analyses to test these hypotheses by compiling two datasets, one dataset consisting of individual PSF values and plant trait values (specific leaf area, SLA; leaf N concentration, LNC; specific root length, SRL; fine root diameter, FRD; plant height; seed mass), and the second consisting of pairwise PSF values and trait dissimilarity. Our meta‐analyses showed that individual PSF values were more negative in faster‐growing species with greater SLA, LNC and SRL, supporting the growth–defence trade‐off hypothesis. Plant height was positively correlated with individual PSF, perhaps because large, long‐lived plants defend against pathogens better than smaller, shorter‐lived plants. We also found that larger‐seeded species had more positive or less negative PSF, likely reflecting greater tolerance of soil pathogens. The direction of relationships between trait dissimilarity and pairwise PSF varied with trait identity. Dissimilarities in SRL and FRD were negatively correlated with pairwise PSF while height dissimilarity was positively correlated with pairwise PSF. The contrasting relationships may reflect distinct links between trait dissimilarity and niche and fitness differences. Synthesis. Our results demonstrate how an integration of PSF and trait‐based approaches can advance plant community ecology.

Root Functional Trait and Soil Microbial Coordination: Implications for Soil Respiration in Riparian Agroecosystems.

Predicting respiration from roots and soil microbes is important in agricultural landscapes where net flux of carbon from the soil to the atmosphere is of large concern. Yet, in riparian agroecosystems that buffer aquatic environments from agricultural fields, little is known on the differential contribution of CO2 sources nor the systematic patterns in root and microbial communities that relate to these emissions. We deployed a field-based root exclusion experiment to measure heterotrophic and autotrophic-rhizospheric respiration across riparian buffer types in an agricultural landscape in southern Ontario, Canada. We paired bi-weekly measurements of in-field CO2 flux with analysis of soil properties and fine root functional traits. We quantified soil microbial community structure using qPCR to estimate bacterial and fungal abundance and characterized microbial diversity using high-throughput sequencing. Mean daytime total soil respiration rates in the growing season were 186.1 ± 26.7, 188.7 ± 23.0, 278.6 ± 30.0, and 503.4 ± 31.3 mg CO2-C m–2 h–1 in remnant coniferous and mixed forest, and rehabilitated forest and grass buffers, respectively. Contributions of autotrophic-rhizospheric respiration to total soil CO2 fluxes ranged widely between 14 and 63% across the buffers. Covariation in root traits aligned roots of higher specific root length and nitrogen content with higher specific root respiration rates, while microbial abundance in rhizosphere soil coorindated with roots that were thicker in diameter and higher in carbon to nitrogen ratio. Variation in autotrophic-rhizospheric respiration on a soil area basis was explained by soil temperature, fine root length density, and covariation in root traits. Heterotrophic respiration was strongly explained by soil moisture, temperature, and soil carbon, while multiple factor analysis revealed a positive correlation with soil microbial diversity. This is a first in-field study to quantify root and soil respiration in relation to trade-offs in root trait expression and to determine interactions between root traits and soil microbial community structure to predict soil respiration.