Christiane Werner

Two different strategies of Mediterranean macchia plants to avoid photoinhibitory damage by excessive radiation levels during summer drought

The adaptive strategies to high radiation and water stress of the drought tolerant evergreen sclerophylls Quercus coccifera and Arbutus unedo are compared to those of the semi-deciduous Cistus spp. (C. albidus and C. monspeliensis). Cistus spp. partially avoided drought by a marked reduction of their transpirational surface through leaf abscission during summer, when predawn water potential declined below -5.5 MPa. Chlorophyll fluorescence measurements revealed a reversible diurnal decrease of maximum photochemical efficiency of PSII (F-v/F-m), which became more accentuated during summer drought in all species. An important strategy to avoid damage by excessive radiation levels in Cistus spp. was the structural regulation of light interception through leaf angle changes, from a more horizontal orientation in spring ( 70 degrees). Horizontal orientated leaves were highly susceptible to photoinhibition, and excessive radiation often resulted in irreversible photodamage followed by leaf abscission during summer, whereas vertical leaf orientation appeared to protect the leaf from severe photoinhibition. Still, these mechanisms were not fully successful in avoiding chronic photoinhibition, and predawn F-v/F-m values remained low in Cistus spp. during summer (only exhibiting a partial overnight recovery). Evergreen sclerophylls were less susceptible to photoinhibition, and the diurnal decline in F-v/F-m remained fully reversible during drought. Structural regulation of light interception was not found to be an important strategy in these species, and only small, though significant changes in leaf angle occurred. The ecological importance of the adaptive strategies of each functional group is discussed

Characteristic patterns of chronic and dynamic photoinhibition of different functional groups in a Mediterranean ecosystem

To analyse characteristic patterns of dynamic and chronic photoinhibition within a plant community, a new technique is proposed, which is based on the long- and short-term recovery time of maximum photochemical efficiency of PSII (F v/F m) after environmental stress. Chronic photoinhibition was determined as a sustainable decrease in predawn F v/F m, occurring during periods of prolonged stress, whereas dynamic photoinhibition was assessed from the fully reversible diurnal decline in F v/F m. Applied to a Mediterranean macchia ecosystem, this definition allowed the characterization of typical annual patterns of chronic and dynamic photoinhibition. Both types of photoinhibition were highest during summer drought. However, differences emerged among the ten dominant macchia species regarding their susceptibility to chronic photoinhibition during different seasons. Chronic and dynamic photoinhibition were dependent on leaf orientation. Semi-deciduous species avoided enhanced chronic photoinhibition through a reduction of excessive light interception by vertical foliage orientation during summer, whereas evergreen sclerophylls did not exhibit pronounced structural photoprotective mechanisms. Chronic and total photoinhibition were significantly correlated with predawn and midday water potentials, respectively, and a grouping of the macchia species into three functional groups is proposed according to this relationship.

Pronounced differences in diurnal variation of carbon isotope composition of leaf respired CO2 among functional groups.

The first broad species survey of diurnal variation in carbon (C) isotope signatures of leaf dark-respired CO(2) (delta(13)C(res)) is presented here and functional differences and diurnal dynamics are linked to fractionation in different respiratory pathways, based on (13)C-labelling experiments. delta(13)C(res) was analysed with a rapid in-tube incubation technique in 16 species. A large diurnal increase in delta(13)C(res) (4-8 per thousand) occurred in evergreen, slow-growing and aromatic species and correlated significantly with cumulative photosynthesis, whereas no variation occurred in herbaceous, fast-growing plants or temperate trees. The diurnal increase in delta(13)C(res) declined almost proportionally to reductions in cumulative light and was reduced in growing compared with mature leaves. Pyruvate positional labelling provided direct evidence that functional groups differ in C allocation between respiratory pathways owing to different metabolic demands for growth, maintenance and secondary metabolism. Diurnal increase in C flux through pyruvate dehydrogenase (for investment in, for example, isoprene or aromatic compounds) combined with consistently low Krebs cycle activity resulted in pronounced increase in delta(13)C(res) in evergreen and aromatic species. By contrast, fast growing herbs with high respiratory demand exhibited no diurnal changes since C was fully respired. Hence, diurnal delta(13)C(res) pattern may provide information for C allocation in plants.

High competitiveness of a resource demanding invasive acacia under low resource supply

Mechanisms controlling the successful invasion of resource demanding species into low-resource environments are still poorly understood. Well-adapted native species are often considered superior competitors under stressful conditions. Here we investigate the competitive ability of the resource demanding alien Acacia longifolia, which invades nutrient-poor Mediterranean sand dunes such as in coastal areas of Portugal. We explore the hypothesis that drought may limit invasion in a factorial competition experiment of the alien invasive versus two native species of different functional groups (Halimium halimifolium, Pinus pinea), under well-watered and drought conditions. Changes in biomass, allocation pattern, and N-uptake-efficiency (via 15N-labeling) indicated a marked drought sensitivity of the invader. However, highly efficient drought adaptations of the native species did not provide a competitive advantage under water limiting conditions. The competitive strength of H.halimifolium towards the alien invader under well-watered conditions turned into a positive interaction between both species under drought. Further, low resource utilization by native species benefited A. longifolia by permitting continued high nitrogen uptake under drought. Hence, the N-fixing invader expresses low plasticity by continuous high resource utilization, even under low resource conditions. The introduction of novel traits into a community like N-fixation and high resource use may promote A. longifolia invasiveness through changes in the physical environment, i.e., the water and nutrient cycle of the invaded sand dune system, thereby potentially disrupting the co-evolved interactions within the native plant community.

Atmospheric benzenoid emissions from plants rival those from fossil fuels

Despite the known biochemical production of a range of aromatic compounds by plants and the presence of benzenoids in floral scents, the emissions of only a few benzenoid compounds have been reported from the biosphere to the atmosphere. Here, using evidence from measurements at aircraft, ecosystem, tree, branch and leaf scales, with complementary isotopic labeling experiments, we show that vegetation (leaves, flowers, and phytoplankton) emits a wide variety of benzenoid compounds to the atmosphere at substantial rates. Controlled environment experiments show that plants are able to alter their metabolism to produce and release many benzenoids under stress conditions. The functions of these compounds remain unclear but may be related to chemical communication and protection against stress. We estimate the total global secondary organic aerosol potential from biogenic benzenoids to be similar to that from anthropogenic benzenoids (~10 Tg y−1), pointing to the importance of these natural emissions in atmospheric physics and chemistry.

Community scale 15N isoscapes : tracing the spatial impact of an exotic N2-fixing invader

Plant-plant interactions are key processes shaping plant communities, but methods are lacking to accurately capture the spatial dimension of these processes. Isoscapes, i.e. spatially continuous observations of variations in stable isotope ratios, provide innovative methods to trace the spatial dimension of ecological processes at continental to global scales. Herein, we test the usefulness of nitrogen isoscapes (δ(15) N) for quantifying alterations in community functioning following exotic plant invasion. Nitrogen introduced by an exotic N(2) -fixing acacia could be accurately traced through the ecosystem and into the surrounding native vegetation by combining native species foliar δ(15) N with spatial information regarding plant location using geostatistical methods. The area impacted by N-addition was at least 3.5-fold greater than the physical area covered by the invader. Thus, downscaling isoscapes to the community level opens new frontiers in quantifying the spatial dimension of functional changes associated with invasion and in resolving the spatial component of within-community interactions.

Carbon isotope discrimination as a tracer of functional traits in a mediterranean macchia plant community

Characterising functional plant groups with simple robust parameters of structural and functional traits is an important tool in ecological research. The reliability of carbon isotope discrimination (Δ13C) as an indicator of functional types was assessed in a highly diverse mediterranean macchia comprising drought semi-deciduous malacophylls, evergreen sclerophylls and a gymnosperm. Pronounced differences in Δ13C of 4‰ occurred: semi-deciduous species (Cistus sp. L.) showed the highest and the gymnosperm (Juniperus sp. L.) the lowest Δ13C (20.3 ± 0.5‰ and 16.2 ± 0.18‰, respectively). Across all studied species, Δ13C was correlated with (i) phenology (length of growing period) and (ii) leaf structure (leaf mass and N per area). The correlation of Δ13C with leaf water potentials, an indicator of drought stress, was species-specific and only 6 out of 11 species exhibited a significant relationship. Thus, leaf phenology governs seasonal responsiveness of Δ13C to drought, which constrains its applicability as an indicator of water use efficiency, particularly in evergreen species with short growing periods. Principal components analysis indicated the robustness of Δ13C for the classification of functional groups yielding similar results based on multiple leaf traits or solely on Δ13C. Hence Δ13C provides an ecological tracer of different functional types, integrating structural, functional and phenological attributes.

Short-term dynamics of isotopic composition of leaf-respired CO2 upon darkening: measurements and implications.

Recent advances in understanding the metabolic origin and the temporal dynamics in delta(13)C of dark-respired CO(2) (delta(13)C(res)) have led to an increasing awareness of the importance of plant isotopic fractionation in respiratory processes. Pronounced dynamics in delta(13)C(res) have been observed in a number of species and three main hypotheses have been proposed: first, diurnal changes in delta(13)C of respiratory substrates; second, post-photosynthetic discrimination in respiratory pathways; and third, dynamic decarboxylation of enriched carbon pools during the post-illumination respiration period. Since different functional groups exhibit distinct diurnal patterns in delta(13)C(res) (ranging from 0 to 10 per thousand diurnal increase), we explored these hypotheses for different ecotypes and environmental (i.e. growth light) conditions. Mass balance calculations revealed that the effect of respiratory substrates on diurnal changes in delta(13)C(res) was negligible in all investigated species. Further, rapid post-illumination changes in delta(13)C(res) (30 min), which increased from 2.6 per thousand to 5 per thousand over the course of the day, were examined by positional (13)C-labelling to quantify changes in pyruvate dehydrogenase (PDH) and Krebs cycle (KC) activity. We investigated the origin of these dynamics with Rayleigh mass balance calculations based on theoretical assumptions on fractionation processes. Neither the estimated changes of PDH and KC, nor decarboxylation of a malate pool entirely explained the observed pattern in delta(13)C(res). However, a Rayleigh fractionation of (12)C-discriminating enzymes and/or a rapid decline in the decarboxylation rate of an enriched substrate pool may explain the post-illumination peak in delta(13)C(res). These results are highly relevant since delta(13)C(res) is used in large-scale carbon cycle studies.

Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes

Combining C flux measurements with information on their isotopic composition can yield a process-based understanding of ecosystem C dynamics. We studied the variations in both respiratory fluxes and their stable C isotopic compositions (δ13C) for all major components (trees, understory, roots and soil microorganisms) in a Mediterranean oak savannah during a period with increasing drought. We found large drought-induced and diurnal dynamics in isotopic compositions of soil, root and foliage respiration (δ13Cres). Soil respiration was the largest contributor to ecosystem respiration (Reco), exhibiting a depleted isotopic signature and no marked variations with increasing drought, similar to ecosystem respired δ13CO2, providing evidence for a stable C-source and minor influence of recent photosynthate from plants. Short-term and diurnal variations in δ13Cres of foliage and roots (up to 8 and 4‰, respectively) were in agreement with: (1) recent hypotheses on post-photosynthetic fractionation processes, (2) substrate changes with decreasing assimilation rates in combination with increased respiratory demand, and (3) decreased phosphoenolpyruvate carboxylase activity in drying roots, while altered photosynthetic discrimination was not responsible for the observed changes in δ13Cres. We applied a flux-based and an isotopic flux-based mass balance, yielding good agreement at the soil scale, while the isotopic mass balance at the ecosystem scale was not conserved. This was mainly caused by uncertainties in Keeling plot intercepts at the ecosystem scale due to small CO2 gradients and large differences in δ13Cres of the different component fluxes. Overall, stable isotopes provided valuable new insights into the drought-related variations of ecosystem C dynamics, encouraging future studies but also highlighting the need of improved methodology to disentangle short-term dynamics of isotopic composition of Reco.

On the use of phloem sap δ13C as an indicator of canopy carbon discrimination

In this study we measured δ¹³C in various carbon pools along the basipetal transport pathway in co-occurring Pinus pinaster and Acacia longifolia trees under Mediterranean climate conditions in the field. Overall, species differences in photosynthetic discrimination resulted in more enriched δ¹³C values in the water-conserving overstory P. pinaster relative to the water-spending understory invasive A. longifolia. Post-photosynthetic fractionation effects resulted in differences in δ¹³C of water-soluble organic matter pools along the plant axis with progressive depletion in δ¹³C from the canopy to the trunk (∼6.5‰ depletion in A. longifolia and ∼0.8‰ depletion in P. pinaster). Regardless of these fractionation effects, phloem sap δ¹³C in both terminal branches and the main stem correlated well with environmental parameters driving photosynthesis for both species, indicating that phloem sap δ¹³C has potential as an integrative tracer of changes in canopy carbon discrimination (Δ¹³C). Furthermore, we illustrate that a simple model based on sap flow estimated canopy stomatal conductance (G(S)) and phloem sap δ¹³C measurements has significant potential as a tool for estimating canopy-level carbon assimilation rates.