Relationship between shoot-rooting and root-sprouting abilities and the carbohydrate and nitrogen reserves of Mediterranean dwarf shrubs

What determines root-sprouting ability: Injury or phytohormones?

PREMISE

Root-sprouting (RS) is an evolutionarily independent alternative to axillary stem branching for a plant to attain its architecture. Root-sprouting plants are better adapted to disturbance than non-RS plants, and their vigor is frequently boosted by biomass removal. Nevertheless, RS plants are rarer than plants that are not root-sprouters, possibly because they must overcome developmental barriers such as intrinsic phytohormonal balance or because RS ability is conditioned by injury to the plant body. The objective of this study was to identify whether phytohormones or injury enable RS.

METHODS

In a greenhouse experiment, growth variables, root respiration, and phytohormones were analyzed in two closely related clonal herbs that differ in RS ability (spontaneously RS Inula britannica and rhizomatous non-RS I. salicina) with and without severe biomass removal.

RESULTS

As previously reported, I. britannica is a root-sprouter, but injury did not boost its RS ability. Root respiration did not differ between the two species and decreased continuously with time irrespectively of injury, but their phytohormone profiles differed significantly. In RS species, the auxins-to-cytokinins ratio was low, and injury further decreased it.

CONCLUSIONS

This first attempt to test drivers behind different plant growth forms suggests that intrinsic phytohormone regulation, especially the auxins-to-cytokinins ratio, might be behind RS ability. Injury, causing a phytohormonal imbalance, seems to be less important in spontaneously RS species than expected for RS species in general.

Identification of Genes and Metabolic Pathways Involved in Resin Yield in Masson Pine by Integrative Analysis of Transcriptome, Proteome and Biochemical Characteristics

Masson pine (Pinus massoniana L.) is one of the most important resin-producing tree species in southern China. However, the molecular regulatory mechanisms of resin yield are still unclear in masson pine. In this study, an integrated analysis of transcriptome, proteome, and biochemical characteristics from needles of masson pine with the high and common resin yield was investigated. The results showed that chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl C), carotenoids (Car), glucose (Glu), gibberellin A9 (GA9), gibberellin A15 (GA15), and gibberellin A53 (GA53) were significantly increased, whereas fructose (Fru), jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-ILE), gibberellin A1 (GA1), gibberellin A3 (GA3), gibberellin A19 (GA19), and gibberellin A24 (GA24) were significantly decreased in the high resin yield in comparison with those in the common one. The integrated analysis of transcriptome and proteome showed that chlorophyll synthase (chlG), hexokinase (HXK), sucrose synthase (SUS), phosphoglycerate kinase (PGK), dihydrolipoamide dehydrogenase (PDH), dihydrolipoamide succinyltransferase (DLST), 12-oxophytodienoic acid reductase (OPR), and jasmonate O-methyltransferases (JMT) were consistent at the transcriptomic, proteomic, and biochemical levels. The pathways of carbohydrate metabolism, terpenoid biosynthesis, photosynthesis, and hormone biosynthesis may play crucial roles in the regulation of resin yield, and some key genes involved in these pathways may be candidates that influence the resin yield. These results provide insights into the molecular regulatory mechanisms of resin yield and also provide candidate genes that can be applied for the molecular-assisted selection and breeding of high resin-yielding masson pine.

Fire does not transform shrublands of Echinospartum horridum (Vahl) Rothm. into grasslands in the Pyrenees: Development of community structure and nutritive value after single prescribed burns

Prescribed fire has been extensively used in recent years to control woody encroachment into mountain and other grassland-dominated landscapes. In the Aragon Pyrenees, prescribed burns have been mainly used to remove the native thorny shrub Echinospartum horridum (Vahl) Rothm., whose populations are spreading to the detriment of grasslands. To study the effectiveness of the burning of E. horridum to preserve grasslands for livestock grazing, the vegetation of six sites burned 0.5, 2, 3, 6, 15 and 35 years ago was sampled and compared with that of nearby unburned shrubland (control) and grassland (reference). In addition, the nutritional quality of E. horridum was examined and compared to that of the reference grassland to evaluate to what extent shrub growth can be controlled by herbivores after burning. Initial shrub cover recovered as early as 15 years after fire, with E. horridum being dominant. Plant diversity was greatest at intermediate number of years after fire. Initial floristic composition and life-form spectrum were restored 15-35 years after burning. Echinospartum horridum exhibited early lignification that restricts its availability as a palatable forage for the first two years after burning and makes it unlikely to be consumed thereafter, highlighting the difficulty in controlling the expansion of this species by livestock herbivory. The analysis of the nutrient levels suggested an increased shortage of limiting elements, such as phosphorus or sulfur, in the mid-term after burning due to substantial nutrient losses and exports during and after the burn. Our results question the suitability and sustainability of a single prescribed burn as management tool alone to control the expansion of E. horridum and call for caution in its application for fighting shrub encroachment in the Central Pyrenees.

How can the analysis of reserve dynamics after fire support the phenological insight of Bulbostylis paradoxa (Spreng.) Lindm (Cyperaceae)?

Carbohydrate reserves are an essential key to plant survival from disturbance. Therefore, studying the different storage organs and types of reserves makes it possible to understand the dynamics of singular plants such as Bulbostylis paradoxa (Spreng.) Lindm, which presents flowering triggered by fire in the Cerrado. Physiological response to fire frequency is detailed by measuring the plant's reserves after a fire disturbance and which carbohydrates are more available for its use. It was measured the concentrations of starch, amino acids, total soluble carbohydrates and soluble proteins in leaves (control), flowers (burning) and caudex of B. paradoxa, in unburned individuals (control), and burned individuals (annually and biennially, obtained 48 h and 15 days after fire). Starch concentrations increased at both fire frequencies in all parts of the plant, as did carbohydrate concentrations. In amino acids, an increase in the concentration of flowers from individuals burned biennially 48 h after fire was observed. The protein concentration showed a decrease in burned plants. Furthermore, the two burning frequencies and the days following the fire can influence the storage of such reserves.

Comparative root anatomy and root bud development after injury in two perennial herbs

A bud bank is a pool of dormant meristems that enable plants to resprout after injury. While the bud bank on stem organs is established prior to injury as the stem grows, the bud bank on roots is considered at least partly formed as a response to disturbance events. To date, only woody species have been examined, and the establishment of reparative buds after injury without connection to the root vascular system has been confirmed; for herbs, no data are available. We tested whether root buds are formed spontaneously or induced after plant damage by studying root anatomy following plant injury in two congeneric perennial herbs. In a pot experiment with young plants of Inula britannica (root sprouter) and I. salicina (non-root sprouter), whole aboveground biomass was removed. Roots were sampled five times at 1-week intervals after disturbance events to evaluate bud occurrence and size, root and vessel diameters, sclerenchyma areas and carbohydrate storage. Compared to non-root-sprouting I. salicina, root-sprouting I. britannica presented more secondary thickening that was connected to adventitious bud formation and improved the root storage and transport capacity necessary for resprouting. Plant injury, in contrast to expectations, did not cause increased bud formation in I. britannica, and all buds were connected to the root vascular system. No root buds were observed in I. salicina. Our study implies that plants using bud banks on roots might depend on preformed buds. Comparative studies examining more species are needed to assess the generality of our findings.

Carbohydrate storage in herbs: the forgotten functional dimension of the plant economic spectrum

BACKGROUND AND AIMS

Although the plant economic spectrum seeks to explain resource allocation strategies, carbohydrate storage is often omitted. Belowground storage organs are the centre of herb perennation, yet little is known about the role of their turnover, anatomy and carbohydrate storage in relation to the aboveground economic spectrum.

METHODS

We collected aboveground traits associated with the economic spectrum, storage organ turnover traits, storage organ inner structure traits and storage carbohydrate concentrations for ~80 temperate meadow species.

KEY RESULTS

The suites of belowground traits were largely independent of one another, but there was significant correlation of the aboveground traits with both inner structure and storage carbohydrates. Anatomical traits diverged according to leaf nitrogen concentration on the one hand and vessel area and dry matter content on the other; carbohydrates separated along gradients of leaf nitrogen concentration and plant height.

CONCLUSIONS

Contrary to our expectations, aboveground traits and not storage organ turnover were correlated with anatomy and storage carbohydrates. Belowground traits associated with the aboveground economic spectrum also did not fall clearly within the fast-slow economic continuum, thus indicating the presence of a more complicated economic space. Our study implies that the generally overlooked role of storage within the plant economic spectrum represents an important dimension of plant strategy.

Root Response of Moso Bamboo (Phyllostachys edulis (Carrière) J. Houz.) Seedlings to Drought with Different Intensities and Durations

The root of Moso bamboo (Phyllostachys edulis (Carrière) J. Houz.) develops extremely rapidly at seedling phase and is highly sensitive to water content in soil, but its response patterns and adaptation strategies of its root to drought are little known. The aim of this study was to investigate the response of root morphology and architecture of Moso bamboo to drought at seedling phase and then to explore the drought adaptation strategies of its root. One-year-old potted seedlings of Moso bamboo were planted under three drought treatments (control, moderate drought and severe drought) for three months. Seedling growth, specific root length (SRL), root architecture (fractal dimension (FD), root branching angle (RBA) and root topological index (TI)) and non-structural carbohydrate (NSC) concentrations in roots were measured every month. The results are as follows: (i) The dry weight of root and shoot decreased significantly under drought stress. (ii) The SRL decreased under drought stress in the early duration (the first month), and then increased in the late duration (the third month). Both FD and RBA decreased, while TI and the concentrations of NSCs increased under drought stress. (iii) The NSC concentrations were positively correlated with SRL and TI, but exhibited an inverse relationship to FD and RBA. Our results indicated that Moso bamboo seedlings formed a “steeper, simpler, expensive (low SRL and high TI)” root architecture to adapt to a short-term drought (one month), and formed a “cheaper (high SRL)” root to adapt to a long-term drought (three months). Increase of NSC concentrations supported the root architecture plasticity to some extent.

Radiation and Drought Impact Residual Leaf Conductance in Two Oak Species With Implications for Water Use Models

Stomatal closure is one of the earliest responses to water stress but residual water losses may continue through the cuticle and incomplete stomatal closure. Residual conductance (g _res ) plays a large role in determining time to mortality but we currently do not understand how do drought and shade interact to alter g _res because the underlying drivers are largely unknown. Furthermore, g _res may play an important role in models of water use, but the exact form in which g _res should be incorporated into modeling schemes is currently being discussed. Here we report the results of a study where two different oak species were experimentally subjected to highly contrasting levels of drought (resulting in 0, 50 and 80% losses of hydraulic conductivity) and radiation (photosynthetic photon flux density at 1,500 μmol m^-2 s^-1 or 35-45 μmol m^-2 s^-1). We observed that the effects of radiation and drought were interactive and species-specific and g _res correlated positively with concentrations of leaf non-structural carbohydrates and negatively with leaf nitrogen. We observed that different forms of measuring g _res , based on either nocturnal conductance under high atmospheric water demand or on the water mass loss of detached leaves, exerted only a small influence on a model of stomatal conductance and also on a coupled leaf gas exchange model. Our results indicate that, while understanding the drivers of g _res and the effects of different stressors may be important to better understand mortality, small differences in g _res across treatments and measurements exert only a minor impact on stomatal models in two closely related species.

Hydraulic and photosynthetic limitations prevail over root non-structural carbohydrate reserves as drivers of resprouting in two Mediterranean oaks

Resprouting is an ancestral trait in angiosperms that confers resilience after perturbations. As climate change increases stress, resprouting vigor is declining in many forest regions, but the underlying mechanism is poorly understood. Resprouting in woody plants is thought to be primarily limited by the availability of non-structural carbohydrate reserves (NSC), but hydraulic limitations could also be important. We conducted a multifactorial experiment with two levels of light (ambient, 2-3% of ambient) and three levels of water stress (0, 50 and 80 percent losses of hydraulic conductivity, PLC) on two Mediterranean oaks (Quercus ilex and Q. faginea) under a rain-out shelter (n = 360). The proportion of resprouting individuals after canopy clipping declined markedly as PLC increased for both species. NSC concentrations affected the response of Q. ilex, the species with higher leaf construction costs, and its effect depended on the PLC. The growth of resprouting individuals was largely dependent on photosynthetic rates for both species, while stored NSC availability and hydraulic limitations played minor and non-significant roles, respectively. Contrary to conventional wisdom, our results indicate that resprouting in oaks may be primarily driven by complex interactions between hydraulics and carbon sources, whereas stored NSC play a significant but secondary role.

Relationships between population traits, nonstructural carbohydrates, and elevation in alpine stands of Vaccinium myrtillus

PREMISE

Despite great attention given to the relationship between plant growth and carbon balance in alpine tree species, little is known about shrubs at the treeline. We hypothesized that the pattern of main nonstructural carbohydrates (NSCs) across elevations depends on the interplay between phenotypic trait plasticity, plant-plant interaction, and elevation.

METHODS

We studied the pattern of NSCs (i.e., glucose, fructose, sucrose, and starch) in alpine stands of Vaccinium myrtillus (above treeline) across an elevational gradient. In the same plots, we measured key growth traits (i.e., anatomical stem features) and shrub cover, evaluating putative relationships with NSCs.

RESULTS

Glucose content was positively related with altitude, but negatively related with shrub cover. Sucrose decreased at high altitude and in older populations and increased with higher percentage of vascular tissue. Starch content increased at middle and high elevations and in stands with high shrub cover. Moreover, starch content was negatively related with the number of xylem rings and the percentage of phloem tissue, but positively correlated with the percentage of xylem tissue.

CONCLUSIONS

We found that the increase in carbon reserves across elevations was uncoupled from plant growth, supporting the growth limitation hypothesis, which postulates NSCs accumulate at high elevation as a consequence of low temperature. Moreover, the response of NSC content to the environmental stress caused by elevation was buffered by phenotypic plasticity of plant traits, suggesting that, under climate warming conditions, shrub expansion due to enhanced plant growth would be pronounced in old but sparse stands.

Comparative transcriptome analysis reveals the molecular regulation underlying the adaptive mechanism of cherry (Cerasus pseudocerasus Lindl.) to shelter covering

BACKGROUND

Rain-shelter covering is widely applied during cherry fruit development in subtropical monsoon climates with the aim of decreasing the dropping and cracking of fruit caused by excessive rainfall. Under rain-shelter covering, the characteristics of the leaves and fruit of the cherry plant may adapt to the changes in the microclimate. However, the molecular mechanism underlying such adaptation remains unclear, although clarifying it may be helpful for improving the yield and quality of cherry under rain-shelter covering.

RESULTS

To better understand the regulation and adaptive mechanism of cherry under rain-shelter covering, 38,621 and 3584 differentially expressed genes were identified with a combination of Illumina HiSeq and single-molecule real-time sequencing in leaves and fruits, respectively, at three developmental stages. Among these, key genes, such as those encoding photosynthetic-antenna proteins (Lhca and Lhcb) and photosynthetic electron transporters (PsbP, PsbR, PsbY, and PetF), were up-regulated following the application of rain-shelter covering, leading to increased efficiency of light utilization. The mRNA levels of genes involved in carbon fixation, namely, rbcL and rbcS, were clearly increased compared with those under shelter-free conditions, resulting in improved CO₂ utilization. Furthermore, the transcription levels of genes involved in chlorophyll (hemA, hemN, and chlH) and carotenoid synthesis (crtB, PDS, crtISO, and lcyB) in the sheltered leaves peaked earlier than those in the unsheltered leaves, thereby promoting organic matter accumulation in leaves. Remarkably, the expression levels of key genes involved in the metabolic pathways of phenylpropanoid (PAL, C4H, and 4CL) and flavonoid (CHS, CHI, F3'H, DFR, and ANS) in the sheltered fruits were also up-regulated earlier than of those in the unsheltered fruits, conducive to an increase in anthocyanin content in the fruits.

CONCLUSIONS

According to the physiological indicators and transcriptional expression levels of the related genes, the adaptive regulation mechanism of cherry plants was systematically revealed. These findings can help understand the effect of rain-shelter covering on Chinese cherry cultivation in rainy regions.

Belowground Bud Bank Distribution and Aboveground Community Characteristics along Different Moisture Gradients of Alpine Meadow in the Zoige Plateau, China

The belowground bud bank plays an important role in plant communities succession and maintenance. In order to understand the response of the bud bank to the sod layer moisture, we investigated the bud bank distribution, size, and composition of six different water gradient alpine meadows through excavating in the Zoige Plateau. The results showed: (1) The alpine meadow plant belowground buds were mainly distributed in the 0–10 cm sod layer, accounting for 74.2%–100% of the total. The total bud density of the swamp wetland and degraded meadow was the highest (16567.9 bud/m3) and the lowest (4839.5 bud/m3). (2) A decrease of the moisture plant diversity showed a trend of increasing first and then decreasing. Among six alpine meadows the swamp meadow plant diversity was the highest, and species richness, Simpson, Shannon–Wiener, and Pielou were 10.333, 0.871, 0.944, and 0.931, respectively. (3) The moisture was significantly positively correlated with the total belowground buds and short rhizome bud density. There were significant positive correlations with sod layer moisture and tiller bulb bud density. This study indicates that the moisture affected bud bank distribution and composition in the plant community, and the results provide important information for predicting plant community succession in the alpine meadow with future changes in precipitation patterns.

Effect of the population density on belowground bud bank of a rhizomatous clonal plant Leymus secalinus in Mu Us sandy land

Clonal propagation is the main strategy for clonal plants to adapt to wind-sand habitat, and underground bud bank could reflect the potential ability of clonal propagation. However, the effects of population density on belowground bud bank are unknown, hindering efforts in the process of dune stabilization. We investigated the horizontal density and vertical distribution of belowground bud bank of a typical rhizomatous grass Leymus secalinus, and soil water content in four dune types with different population density (dune type I: 11.2 ± 1.7 no. m^-2, type II: 24.2 ± 2.6 no. m^-2, type III: 40.0 ± 4.0 no. m^-2, and type IV: 53.5 ± 7.2 no. m^-2) in Mu Us sandy land. Our results showed that (1) total bud density of population increased markedly with increasing population density, but it did not exhibit significant difference between dune types III and IV, where density was about 130 buds m^-2; and tiller bud density of population first increased, then decreased, and reached a maximum in dune type III. (2) Total bud density per individual in dune type III was significantly larger than that in other dune types (P < 0.05), whereas rhizome and tiller bud density per individual did not show significant differences in dune types II, III and IV (P > 0.05). (3) Buds tended to be concentrated at 10-30 cm soil layer in all dune types, and be buried deeper in dune types III and IV than that in dune types I and II. (4) No pronounced relationship was shown between bud density and soil water content in 10-30 cm soil layer with increasing population density. Our results suggest that moderate population density (40.0 ± 4.0 no. m^-2) significantly increase the bud bank density of L. secalinus population and individual. Soil water content was not the main factor responsible for the density of L. secalinus bud bank. These results can provide important information for implementation of effective sand fixation measures and species selection for desertification control in semiarid sandy land ecosystems.

Antioxidative enzymes and expression of rbcL gene as tools to monitor heavy metal-related stress in plants

The aim of the study was to evaluate sensitivity and potential applications of selected biomarkers in phytoremediation under complex heavy metal contamination in Sinapis alba L., Robinia pseudoacacia L. and Lupinus luteus L as a potential tools in effective phytoremediation management. The toxicity assessment was conducted using selected measurement endpoints, both classical and advanced, i.e., germination index, roots length, guaiacol peroxidase activity (GPX), chlorophyll and protein content, the amount of total phenolic compounds (TPC) and level of expression of one of the ribulose-bisphosphate carboxylase genes (rbcL). Moreover, the influence of organic additives: cattle, horse manure, and vermicompost on lowering plant abiotic stress caused by complex heavy metal contamination was studied to assess the possible applications of selected stress markers in large scale phytoremediation planning. The results demonstrated the beneficial effects of selected soil additives on plant development. The 5% difference in the quantity of applied amendment caused statistically significant differences in GPX, TPC, chlorophyll content and expression level of rbcL. Among all endpoints, GPX activity, chlorophyll, and phenolic compounds content, as well as the expression of rbcL, turned out to be the most reliable assays for determination of the type and dosage of selected soil amendments (fertilizers) in the assisted phytoremediation process. Selected markers can be used to achieve the desired level of plant abiotic stress and consequently photosynthesis efficiency and CO₂ sequestration. The results showed, that presented assays can be used in different taxonomical groups such as Fabaceae for planning effective phytoremediation process.

Non-structural carbohydrate dynamics associated with drought-induced die-off in woody species of a shrubland community

BACKGROUND AND AIMS

The relationship between plant carbon economy and drought responses of co-occurring woody species can be assessed by comparing carbohydrate (C) dynamics following drought and rain periods, relating these dynamics to species' functional traits. We studied nine woody species coexisting in a continental Mediterranean shrubland that experienced severe drought effects followed by rain.

METHODS

We measured total non-structural carbohydrates (NSC) and soluble sugars (SS) in roots and stems during drought and after an autumn rain pulse in plants exhibiting leaf loss and in undefoliated ones. We explored whether their dynamics were related to foliage recovery and functional traits (height [H], specific leaf area [SLA], wood density [WD]).

KEY RESULTS

During drought, NSC concentrations were overall lower in stems and roots of plants experiencing leaf loss, while SS decreases were smaller. Roots had higher NSC concentrations than stems. After the rain, NSC concentrations continued to decrease, while SS increased. Green foliage recovered after rain, particularly in plants previously experiencing higher leaf loss, independently of NSC concentrations during drought. Species with lower WD tended to have more SS during drought and lower SS increases after rain. In low-WD species, plants with severe leaf loss had lower NSC relative to undefoliated ones. No significant relationship was found between H or SLA and C content or dynamics.

CONCLUSIONS

Our community-level study reveals that, while responses were species-specific, C stocks overall diminished in plants affected by prolonged drought and did not increase after a pulse of seasonal rain. Dynamics were faster for SS than NSC. We found limited depletion of SS, consistent with their role in basal metabolic, transport and signalling functions. In a scenario of increased drought under climate change, NSC stocks in woody plants are expected to decrease differentially in coexisting species, with potential implications for their adaptive abilities and community dynamics.

Leaf non-structural carbohydrate allocation and C:N:P stoichiometry in response to light acclimation in seedlings of two subtropical shade-tolerant tree species

Light availability greatly affects plant growth and development. In shaded environments, plants must respond to reduced light intensity to ensure a regular rate of photosynthesis to maintain the dynamic balance of nutrients, such as leaf non-structural carbohydrates (NSCs), carbon (C), nitrogen (N) and phosphorus (P). To improve our understanding of the nutrient utilization strategies of understory shade-tolerant plants, we compared the variations in leaf NSCs, C, N and P in response to heterogeneous controlled light conditions between two subtropical evergreen broadleaf shade-tolerant species, Elaeocarpus sylvestris (E. sylvestris) and Illicium henryi (I. henryi). Light intensity treatments were applied at five levels (100%, 52%, 33%, 15% and 6% full sunlight) for 30 weeks to identify the effects of reduced light intensity on leaf NSC allocation patterns and leaf C:N:P stoichiometry characteristics. We found that leaf soluble sugar, starch and NSC concentrations in E. sylvestris showed decreasing trends with reduced light intensity, whereas I. henryi presented slightly increasing trends from 100% to 15% full sunlight and then significant decreases at extremely low light intensity (6% full sunlight). The soluble sugar/starch ratio of E. sylvestris decreased with decreasing light intensity, whereas that of I. henryi remained stable. Moreover, both species exhibited increasing trends in leaf N and P concentrations but limited leaf N:P and C:P ratio fluctuations with decreasing light intensity, revealing their adaptive strategies for poor light environments and their growth strategies under ideal light environments. There were highly significant correlations between leaf NSC variables and C:N:P stoichiometric variables in both species, revealing a trade-off in photosynthesis production between leaf NSC and carbon allocation. Thus, shade-tolerant plants readjusted their allocation of leaf NSCs, C, N and P in response to light acclimation. Redundancy analysis showed that leaf morphological features of both E. sylvestris and I. henryi affected their corresponding leaf nutrient traits. These results improve our understanding of the dynamic balance between leaf NSCs and leaf C, N and P components in the nutritional metabolism of shade-tolerant plants.

KEY MESSAGE

Two species of understory shade-tolerant plants responded differently to varying light intensities in terms of leaf non-structural carbohydrate allocation and the utilization of carbon, nitrogen and phosphorus to balance nutritional metabolism and adapt to environmental stress.

Is the scaling relationship between carbohydrate storage and leaf biomass in meadow plants affected by the disturbance regime?

BACKGROUND AND AIMS

Below-ground carbohydrate storage is considered an adaptation of plants aimed at regeneration after disturbance. A theoretical model by Iwasa and Kubo was empirically tested which predicted (1) that storage of carbohydrates scales allometrically with leaf biomass and (2) when the disturbance regime is relaxed, the ratio of storage to leaf biomass increases, as carbohydrates are not depleted by disturbance.

METHODS

These ideas were tested on nine herbaceous species from a temperate meadow and the disturbance regime was manipulated to create recently abandoned and mown plots. Just before mowing in June and at the end of the season in October, plants with below-ground organs were sampled. The material was used to assess the pool of total non-structural carbohydrates and leaf biomass.

KEY RESULTS

In half of the cases, a mostly isometric relationship between below-ground carbohydrate storage and leaf biomass in meadow plants was found. The ratio of below-ground carbohydrate storage to leaf biomass did not change when the disturbance regime was less intensive than that for which the plants were adapted.

CONCLUSIONS

These findings (isometric scaling relationship between below-ground carbohydrate storage and leaf biomass; no effect of a relaxed disturbance regime) imply that storage in herbs is probably governed by factors other than just the disturbance regime applied once in a growing season.

Stem CO2 efflux in six co-occurring tree species: underlying factors and ecological implications

Stem respiration plays a role in species coexistence and forest dynamics. Here we examined the intra- and inter-specific variability of stem CO2 efflux (E) in dominant and suppressed trees of six deciduous species in a mixed forest stand: Fagus sylvatica L., Quercus petraea [Matt.] Liebl, Quercus pyrenaica Willd., Prunus avium L., Sorbus aucuparia L. and Crataegus monogyna Jacq. We conducted measurements in late autumn. Within species, dominants had higher E per unit stem surface area (Es ) mainly because sapwood depth was higher than in suppressed trees. Across species, however, differences in Es corresponded with differences in the proportion of living parenchyma in sapwood and concentration of non-structural carbohydrates (NSC). Across species, Es was strongly and NSC marginally positively related with an index of drought tolerance, suggesting that slow growth of drought-tolerant trees is related to higher NSC concentration and Es . We conclude that, during the leafless period, E is indicative of maintenance respiration and is related with some ecological characteristics of the species, such as drought resistance; that sapwood depth is the main factor explaining variability in Es within species; and that the proportion of NSC in the sapwood is the main factor behind variability in Es among species.

Fast growth involves high dependence on stored resources in seedlings of Mediterranean evergreen trees

BACKGROUND AND AIMS

The carbon (C) and nitrogen (N) needed for plant growth can come either from soil N and current photosynthesis or through remobilization of stored resources. The contribution of remobilization to new organ growth on a whole-plant basis is quite well known in deciduous woody plants and evergreen conifers, but this information is very limited in broadleaf evergreen trees. This study compares the contribution of remobilized C and N to the construction of new organs in spring, and assesses the importance of different organs as C and N sources in 1-year-old potted seedlings of four ecologically distinct evergreen Mediterranean trees, namely Quercus ilex, Q. coccifera, Olea europaea and Pinus hapelensis.

METHODS

Dual (13)C and (15)N isotope labelling was used to unravel the contribution of currently taken up and stored C and N to new growth. Stored C was labelled under simulated winter conditions. Soil N was labelled with the fertilization during the spring growth.

KEY RESULTS

Oaks allocated most C assimilated under simulated winter conditions to coarse roots, while O. europaea and P. halepensis allocated it to the leaves. Remobilization was the main N source (>74 %) for new fine-root growth in early spring, but by mid-spring soil supplied most of the N required for new growth (>64 %). Current photosynthesis supplied >60 % of the C in new fine roots by mid-spring in most species. Across species, the proportion of remobilized C and N in new shoots increased with the relative growth rate. Quercus species, the slowest growing trees, primarily used currently acquired resources, while P. halepensis, the fastest growing species, mainly used reserves. Increases in the amount of stored N increased N remobilization, which fostered absolute growth both within and across species. Old leaves were major sources of remobilized C and N, but stems and roots also supplied considerable amounts of both in all species except in P. halepensis, which mainly relied on foliage formed in the previous growing season to supply stored resources.

CONCLUSIONS

Seedlings of Mediterranean evergreen trees have distinct C and N storage physiologies, with relative growth rate driving the contribution of remobilized resources to new growth. These differences may reduce competition and facilitate species coexistence.

Potential bud bank responses to apical meristem damage and environmental variables: matching or complementing axillary meristems?

Soil nutrients, dormant axillary meristem availability, and competition can influence plant tolerance to damage. However, the role of potential bud banks (adventitious meristems initiated only after injury) is not known. Examining Central European field populations of 22 species of short-lived monocarpic herbs exposed to various sources of damage, we hypothesized that: (1) with increasing injury severity, the number of axillary branches would decrease, due to axillary meristem limitation, whereas the number of adventitious shoots (typically induced by severe injury) would increase; (2) favorable environmental conditions would allow intact plants to branch more, resulting in stronger axillary meristem limitation than in unfavorable conditions; and (3) consequently, adventitious sprouting would be better enabled in favorable than unfavorable conditions. We found strong support for the first hypothesis, only limited support for the second, and none for the third. Our results imply that whereas soil nutrients and competition marginally influence plant tolerance to damage, potential bud banks enable plants to overcome meristem limitation from severe damage, and therefore better tolerate it. All the significant effects were found in intraspecific comparisons, whereas interspecific differences were not found. Monocarpic plants with potential bud banks therefore represent a distinct strategy occupying a narrow environmental niche. The disturbance regime typical for this niche remains to be examined, as do the costs associated with the banks of adventitious and axillary reserve meristems.

Adult root structure of Mediterranean shrubs: relationship with post-fire regenerative syndrome

Life-history attributes can impose differences on root system structures and properties related to nutrient and water uptake. Here, we assess whether plants with different post-fire regenerative strategies (resprouters, seeders and seeder-resprouters) differ in the topological and morphological properties of their root systems (external path, altitude, magnitude, topological index, specific root length, root length, root-to-shoot biomass ratio, length of the main axis of the root system and link length). To achieve these objectives, we sampled individuals from eight woody species in a shrubland located in the western Mediterranean Basin. We sampled the adult root systems using manual field excavation with the aid of an air compressor. The results indicate that resprouters have a higher root-to-shoot ratio, confirming their higher ability to store water, starch and nutrients and to invest in the belowground biomass. Moreover, this pattern would allow them to explore deeper parts of the soil layers. Seeder species would benefit from a higher specific root length, pointing to increased relative root growth and water uptake rates. This study confirms that seeders and resprouters may differ in nutrient and water uptake ability according to the characteristics of their root system. Species that can both resprout and establish seedlings after fire had different patterns of root system structure; in particular, root:shoot ratio was more similar to resprouters and specific root length was closer to seeders, supporting the distinct functional performance of this type of species.

Relationship between leaf traits and fire-response strategies in shrub species of a mountainous region of south-eastern Australia

BACKGROUND AND AIMS

Resprouting and seed recruitment are important ways in which plants respond to fire. However, the investments a plant makes into ensuring the success of post-fire resprouting or seedling recruitment can result in trade-offs that are manifested in a range of co-occurring morphological, life history and physiological traits. Relationships between fire-response strategies and other traits have been widely examined in fire-prone Mediterranean-type climates. In this paper, we aim to determine whether shrubs growing in a non-Mediterranean climate region exhibit relationships between their fire-response strategy and leaf traits.

METHODS

Field surveys were used to classify species into fire-response types. We then compared specific leaf area, leaf dry-matter content, leaf width, leaf nitrogen and carbon to nitrogen ratios between (a) obligate seeders and all other resprouters, and (b) obligate seeders, facultative resprouters and obligate resprouters.

KEY RESULTS

Leaf traits only varied between fire-response types when we considered facultative resprouters as a separate group to obligate resprouters, as observed after a large landscape-scale fire. We found no differences between obligate seeders and obligate resprouters, nor between obligate seeders and resprouters considered as one group.

CONCLUSIONS

The results suggest that facultative resprouters may require a strategy of rapid resource acquisition and fast growth in order to compete with species that either resprout, or recruit from seed. However, the overall lack of difference between obligate seeders and obligate resprouters suggests that environmental factors are exerting similar effects on species' ecological strategies, irrespective of the constraints and trade-offs that may be associated with obligate seeding and obligate resprouting. These results highlight the limits to trait co-occurrences across different ecosystems and the difficulty in identifying global-scale relationships amongst traits.

Effect of abandonment and plant classification on carbohydrate reserves of meadow plants

We studied the effect of cessation of management on carbohydrate reserves of plants in meadows with different environmental characteristics and plant composition. We recorded storage carbohydrates and seasonal changes for 40 plant species. We asked whether there are differences in responses of carbohydrate reserves in forbs versus graminoids and in plants storing starch versus plants storing osmotically active carbohydrates. We analysed belowground organs before the meadows were mown and at the end of the vegetation season in mown versus recently abandoned plots. Whereas starch and fructans were widely distributed, raffinose family oligosaccharides were the main carbohydrate reserves of the Lamiaceae and Plantago lanceolata. Properties of carbohydrate reserves differed between forbs and graminoids but no difference was found between plants storing starch versus osmotically active carbohydrates. Graminoids had lower carbohydrate concentrations than forbs. We observed a positive effect of mowing on carbohydrate concentrations of graminoids in the dry, calcium-rich meadow and higher seasonal fluctuations of these values in the acid, wet meadow, suggesting that local factors and/or the species pool affect carbohydrate reserves. Despite local conditions, graminoids represent a distinct functional group in meadows from the point of view of their storage economy. We suggest that as well as growth, storage processes should also be considered for understanding the functioning of meadow plant communities.