Compensation processes of Aleppo pine (Pinus halepensis Mill.) to ozone exposure and drought stress

Nutritional changes in trees during drought-induced mortality: A comprehensive meta-analysis and a field study

Both macronutrients and micronutrients are essential for tree growth and development through participating in various ecophysiological processes. However, the impact of the nutritional status of trees on their ability to withstand drought-induced mortality remains inconclusive. We thus conducted a comprehensive meta-analysis, compiling data on 11 essential nutrients from 44 publications (493 independent observations). Additionally, a field study was conducted on Pinus sylvestris L. trees with varying drought-induced vitality loss in the "Visp" forest in southern Switzerland. No consistent decline in tree nutritional status was observed during tree mortality. The meta-analysis revealed significantly lower leaf potassium (K), iron (Fe), and copper (Cu) concentrations with tree mortality. However, the field study showed no causal relationships between nutritional levels and the vitality status of trees. This discrepancy is mainly attributed to the intrinsic differences in the two types of experimental designs and the ontogenetic stages of target trees. Nutrient reductions preceding tree mortality were predominantly observed in non-field conditions, where the study was conducted on seedlings and saplings with underdeveloped root systems. It limits the nutrient uptake capacity of these young trees during drought. Furthermore, tree nutritional responses are also influenced by many variables. Specifically, (a) leaf nutrients are more susceptible to drought stress than other organs; (b) reduced tree nutrient concentrations are more prevalent in evergreen species during drought-induced mortality; (c) of all biomes, Mediterranean forests are most vulnerable to drought-induced nutrient deficiencies; (d) soil types affect the direction and extent of tree nutritional responses. We identified factors that influence the relationship between tree nutritional status and drought survival, and proposed potential early-warning indicators of impending tree mortality, for example, decreased K concentrations with declining vitality. These findings contribute to our understanding of tree responses to drought and provide practical implications for forest management strategies in the context of global change.

Changes in growth pattern and rhizospheric soil biochemical properties of a leguminous tree species Leucaena leucocephala under long-term exposure to elevated ozone

Increasing concentrations of ground-level ozone (O₃) exert significant impacts on the plants, but there is limited data for belowground processes. We studied the effects of long-term exposure of elevated O₃ (EO₃) on plant growth parameters (plant height and biomass) and biochemical parameters (nutrients, microbial biomass and enzymatic activities) of rhizospheric soil of leguminous tree species Leucaena leucocephala. L. leucocephala seedlings were grown under ambient O₃ (AO₃) and EO₃ (+20 ppb above ambient) under Free Air Ozone Concentration Enrichment (O₃-FACE) facility and changes in plant growth and their rhizospheric soil properties were studied during 6, 12, 18 and 24 months of EO₃ exposure. L. leucocephala showed significant reductions in shoot length, root biomass, shoot biomass, leaf biomass and total biomass during 12, 18 and 24 months of exposure to EO₃. Total nutrients in rhizospheric soil like carbon and phosphorus were significantly reduced after 24 months of EO₃ exposure. Most of the available nutrients showed significant reduction after 6, 12 and 24 months of EO₃ exposure. A significant decrease was apparent in microbial biomass carbon, nitrogen and phosphorus after 6, 12, 18 and 24 months of EO₃ treatment. Significant reductions were observed in extracellular enzymatic activities (dehydrogenase, alkaline phosphatase, β-glycosidase, fluorescein diacetate, arylsulfatase, cellulase and protease) of soil after 6, 12 and 24 months of EO₃ exposure. These results suggest that increasing O₃ concentrations will directly impact L. leucocephala growth as well as have indirect impact on the nutrient contents (C, N, and P), microbial biomass and extracellular enzymatic activities of rhizospheric soil of L. leucocephala. Our results suggest that continuous increase in O₃ concentrations will have serious implications for aboveground plant growth and belowground soil fertility in this region considered as O₃ hotspot.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03215-1.

Variations in Leaf Traits Modulate Plant Vegetative and Reproductive Phenological Sequencing Across Arid Mediterranean Shrublands

Structural and nutrient traits of a leaf are important for understanding plant ecological strategies (e.g., drought avoidance). We studied the specific leaf area (SLA), leaf carbon content (LCC), leaf nitrogen content (LNC), leaf phosphorous content (LPC), and the phenophase sequence index (PSI) in 126 Mediterranean perennial species from predesert (SMS) and semiarid (SaMS) to subalpine (SAS), alpine cushion (AcS), and oro-Mediterranean (AjS) shrublands, which represent eight functional groups (evergreen and deciduous trees, evergreen large and half shrubs, deciduous large and half shrubs, succulents and perennial herbs). We analyzed the variation and relationships between leaf traits and PSI among shrublands, functional groups, and within species with drought-avoidance mechanisms. SLA variation of 20-60% could be ascribed to differences between functional groups and only 38-48% to different shrublands increasing from the predesert to the alpine. Alpine species display low PSI and N:P and high SLA, LNC, LPC, LCC, and C:N. On the contrary, predesert and semiarid showed high PSI and low SLA. SLA mediates the vegetative and reproductive phenological plant sequencing, high SLA is often associated with the overlapping in growth and reproductive phenophases with a seasonal reduction of vegetative growth, whereas low SLA is associated with vegetative and reproductive sequencing and a seasonal extension of vegetative growth. Species with drought-avoidance mechanisms (e.g., semideciduous species) contribute to an increase in the mean values of the SLA and LNC because these species show similar leaf and phenological patterns as the deciduous (high SLA and LNC and low PSI). The N:P indicates that only the alpine shrublands could present P limitations. The positive correlations between SLA and LPC and LNC and LPC (leaf economic spectrum) and the negative correlation between SLA and C:N were consistently maintained in the studied arid Mediterranean shrublands.

Effects of elevated ozone on maize under varying soil nitrogen levels: Biomass, nitrogen and carbon, and their allocation to kernel

Effects of ozone (O₃) on maize have been increasingly studied, but only few studies have focused on the combined impacts of O₃ and nitrogen (N) on this important crop with C₄ carbon (C) fixation. In this study, a maize cultivar with the largest acreage in China was exposed to two O₃ treatments (NF: ambient air O₃ concentration; NF60: NF plus 60 ppb O₃) and four N levels (farmers' N practice: 240 kg N ha^-1 yr^-1; 150%, 50% and 25% of farmers' N practice). Generally, O₃ and N significantly influenced biomass, N and C, but did not change their allocation to kernel. There were significant interactions between O₃ and N in stem biomass, C concentration and uptake, and leaf biomass and C uptake, with significant O₃ effects mainly occurring at N120 and N240. Based on the coefficient of determination (R²), root C:N ratio rather than the most commonly used leaf C:N ratio was the best trait to indicate maize productivity. Furthermore, O₃ significantly increased the regression slopes between root C:N ratio and kernel N uptake, kernel C uptake and plant N uptake, strengthened the correlation of C:N ratio and kernel C uptake, and weakened the correlation of C:N ratio and hundred-kernels weight. These suggest that O₃ pollution can change the relationship of C:N ratio and productivity in maize. The weak correlation between kernel harvest index (HI) and N harvest index (NHI) indicated that future breeding researches should consider how to improve the coupling between biomass and N-related nutrition allocations in crop edible parts. Our results not only are helpful to accurately estimate O₃ impacts on maize with consideration of N but also provide a new insight into the relationship between plant traits and its productivity under O₃ pollution.

Effects of Elevated Ozone Concentrations on Root Characteristics and Soil Properties of Elaeocarpus sylvestris and Michelia chapensis

The increasing concentration of surface ozone (O₃) was observed during recent decades in the world, which affects tree roots and forest soils. Meanwhile, the impact of ozone on tree roots is greatly affected by soil condition. However, there is a lack of knowledge about the possible effects of ozone on tree roots and soil processes. In this study, The influences of surface ozone (O₃) stress on the root biomass, morphology, nutrients, soil properties, and soil enzyme activity of Elaeocarpus sylvestris and Michelia chapensis seedlings were examined at four O₃ concentrations (charcoal-filtered air, 1 × O₃ air, 2 × O₃ air, and 4 × O₃ air). Elevated O₃ concentrations were found to significantly increase the root C content, N content, C/P ratio, and N/P ratio, and significantly decrease the root biomass, number of root tips, and root C/N ratio of both species. The soil organic matter content, pH, total N content, and urease and catalase activities of both species tended to increase. The limitation in root growth and responses in the root structure of E. sylvestris induced by elevated O₃ concentrations led to increased bulk density and decreased soil porosity and void ratio. These profound effects of O₃ concentrations on the roots and soil characteristics of these two species underscore the importance of research in O₃ science.

Effect of water deficit stress on an Indian wheat cultivar (Triticum aestivum L. HD 2967) under ambient and elevated level of ozone

The response of a wheat cultivar (HD 2967) under the combination of elevated ozone (O₃) and water deficit stress (WS) was evaluated in terms of morphological, physiological and yield parameters along with nutrient uptake and their redistribution to different plant parts. An open-top chamber experiment has been conducted under O₃ exposures (ambient (A) and ambient +20 ppb O₃ (E)) along with two different water regimes (well-watered; WW and water deficit with 50% of soil capacity; WS). Most of the growth parameters showed significant reductions due to elevated O₃ under both WW and WS conditions. Stomatal conductance and assimilation rate reduced significantly under the combined stress as compared to their controls (AWW). The maximum decrease in grain yield was observed under the additive effect of both the stresses of water deficit and elevated O₃ (-43.6%), followed by water deficit stress (-19.8%) and elevated O₃ (-17.9%) as compared to the control (AWW). Furthermore, the study displayed that reduced water availability has checked the uptake of nutrients in the roots, shoot and leaves, while, a higher carbon accumulation has been observed with subsequent increases in C: N and C: K ratios in the leaves. Such limitation of nutrients uptake and photosynthates availability weakened the antioxidative defense system of the test cultivar, making it more sensitive against combined stresses. Besides, the study displayed that the defense system has been remarkably suppressed under the presence of interactive stress factors, which allowed us to predict that the distribution of limited carbon pool has inverse relationship between the plant's defense system and growth.

Changes of Cinnamomum camphora root characteristics and soil properties under ozone stress in South China

High O₃ exposure affects the forest growth and soil characteristics. Although there is substantial evidence that O₃ does impose a stress on forest trees, the effects of O₃ on roots and soil of evergreen broad-leaved tree species in South China remain unknown. The effects of ozone (O₃) fumigation on the root biomass, root morphology, root nutrient, soil physical, and chemical properties were examined in Cinnamomum camphora seedlings grown under four O₃ treatments (charcoal-filtered air (CF) or O₃ at 1×, 2× and 4× ambient concentration). O₃ significantly decreased root biomass and root carbon (C). Regardless of O₃ level, elevated O₃ significantly resulted in reduced root surface area, volume, number of forks, and specific root length (SRL). The percentages of fine to total root in terms of root surface area and root volume of seedlings under the CF and 1 × O₃ treatments were significantly higher than those of seedlings under the 4 × O₃ treatment, indicating that high O₃ level impaired the growth performance of fine roots. O₃ affected root growth and structures, which increased soil bulk density and reduced soil total porosity and void ratio. The soil pH under all O₃ fumigation treatments significantly increased compared with CF treatment, whereas the organic matter significantly decreased. In conclusion, although the increased O₃ level enhanced root N and P under 2 and 4 × O₃ treatments compared with 1 × O₃ treatment as compensation mechanisms to prevent O₃-induced decrease in root C gain and root functions, O₃ still decreased the root biomass and root tips, and changed the soil physical and chemical properties.

Effects of Drought Stress on Biomass, Essential Oil Content, Nutritional Parameters, and Costs of Production in Six Lamiaceae Species

Lamiaceae is one of the largest families of aromatic plants and it is characterized by the presence of external glandular structures which produce essential oils highly valued in cosmetics and medicine. Plants of Lavandula latifolia, Mentha piperita, Salvia sclarea, Salvia lavandulifolia, Thymus capitatus, and Thymus mastichina were grown for one year. In order to evaluate the effects of drought stress, plants were subjected to two water treatments (100% ETo and 70% ETo, including the rainfall during the experimental period). At the end of the experiment, the biomass, the essential oil content, and leaf nutrients concentration were assessed for each water treatment and species studied. At the end of the experiment, L. latifolia, M. piperita and T. capitatus plants showed a significant fresh weight reduction under drought stress conditions whereas the other species studied remained unchanged. With respect to dry weight, only L. latifolia plants showed a reduction under water deficit conditions. As far as essential oil content was concerned, L. latifolia and S. sclarea plants had a reduction under water deficit conditions. Leaf nutrient concentration showed different trends between species considering the nutrient assessed. The economic viability of the growth of this species will be dependent on the benefits achieved which are related to yield production obtained and the price accorded for both raw material and the essential oil extracted.

Elevated ozone affects C, N and P ecological stoichiometry and nutrient resorption of two poplar clones

The effects of elevated ozone on C (carbon), N (nitrogen) and P (phosphorus) ecological stoichiometry and nutrient resorption in different organs including leaves, stems and roots were investigated in poplar clones 546 (P. deltoides cv. '55/56' × P. deltoides cv. 'Imperial') and 107 (P. euramericana cv. '74/76') with a different sensitivity to ozone. Plants were exposed to two ozone treatments, NF (non-filtered ambient air) and NF60 (NF with targeted ozone addition of 60 ppb), for 96 days in open top chambers (OTCs). Significant ozone effects on most variables of C, N and P ecological stoichiometry were found except for the C concentration and the N/P in different organs. Elevated ozone increased both N and P concentrations of individual organs while for C/N and C/P ratios a reduction was observed. On these variables, ozone had a greater effect for clone 546 than for clone 107. N concentrations of different leaf positions ranked in the order upper > middle > lower, showing that N was transferred from the lower senescent leaves to the upper ones. This was also indicative of N resorption processes, which increased under elevated ozone. N resorption of clone 546 was 4 times larger than that of clone 107 under ambient air (NF). However, elevated ozone (NF60) had no significant effect on P resorption for both poplar clones, suggesting that their growth was only limited by N, while available P in the soil was enough to sustain growth. Understanding ecological stoichiometric responses under ozone stress is crucial to predict future effects on ecological processes and biogeochemical cycles.

Increasing aridity, temperature and soil pH induce soil C-N-P imbalance in grasslands

Due to the different degrees of controls exerted by biological and geochemical processes, climate changes are suggested to uncouple biogeochemical C, N and P cycles, influencing biomass accumulation, decomposition and storage in terrestrial ecosystems. However, the possible extent of such disruption in grassland ecosystems remains unclear, especially in China's steppes which have undergone rapid climate changes with increasing drought and warming predicted moving forward in these dryland ecosystems. Here, we assess how soil C-N-P stoichiometry is affected by climatic change along a 3500-km temperate climate transect in Inner Mongolia, China. Our results reveal that the soil from more arid and warmer sites are associated with lower soil organic C, total N and P. The ratios of both soil C:P and N:P decrease, but soil C:N increases with increasing aridity and temperature, indicating the predicted decreases in precipitation and warming for most of the temperate grassland region could lead to a soil C-N-P decoupling that may reduce plant growth and production in arid ecosystems. Soil pH, mainly reflecting long-term climate change in our sites, also contributes to the changing soil C-N-P stoichiometry, indicating the collective influences of climate and soil type on the shape of soil C-N-P balance.

Health and vitality assessment of two common pine species in the context of climate change in southern Europe

The Mediterranean Basin is expected to be more strongly affected by ongoing climate change than most other regions of the earth. The South-eastern France can be considered as case study for assessing global change impacts on forests. Based on non-parametric statistical tests, the climatic parameters (temperature, relative humidity, rainfall, global radiation) and forest-response indicators (crown defoliation, discoloration and visible foliar ozone injury) of two pine species (Pinus halepensis and Pinus cembra) were analyzed. In the last 20 years, the trend analyses reveal a clear hotter and drier climate along the coastline and slightly rainier inland. In the current climate change context, a reduction in ground-level ozone (O3) was found at remote sites and the visible foliar O3 injury decreased while deterioration of the crown conditions was observed likely due to a drier and warmer climate. Clearly, if such climatic and ecological changes are now being detected when the climate, in South-eastern France, has warmed in the last 20 years (+0.46-1.08°C), it can be expected that many more impacts on tree species will occur in response to predicted temperature changes by 2100 (+1.95-4.59°C). Climate change is projected to reduce the benefits of O3 precursor emissions controls leading to a higher O3 uptake. However, the drier and warmer climate should induce a soil drought leading to a lower O3 uptake. These two effects, acting together in an opposite way, could mitigate the harmful impacts of O3 on forests. The development of coordinated emission abatement strategies is useful to reduce both climate change and O3 pollution. Climate change will create additional challenges for forest management with substantial socio-economic and biological diversity impacts. However, the development of future sustainable and adaptive forest management strategies has the potential to reduce the vulnerability of forest species to climate change.

Variation in Rubisco content and activity under variable climatic factors

The main objective of the present review is to provide a compilation of published data of the effects of several climatic conditions on Rubisco, particularly its activity, state of activation, and concentration, and its influence on leaf gas exchange and photosynthesis. The environmental conditions analyzed include drought, salinity, heavy metals, growth temperature, and elevated [O3], [CO2], and ultraviolet-B irradiance. The results show conclusive evidence for a major negative effect on activity of Rubisco with increasing intensity of a range of abiotic stress factors. This decrease in the activity of Rubisco is associated with down-regulation of the activation state of the enzyme (e.g., by de-carbamylation and/or binding of inhibitory sugar phosphates) in response to drought or high temperature. On the contrary, the negative effects of low temperature, heavy metal stress (cadmium), ozone, and UV-B stress on Rubisco activity are associated with changes in the concentration of Rubisco. Notably, in response to all environmental factors, the regulation of in vivo CO2 assimilation rate was related to Rubisco in vitro parameters, either concentration and/or carboxylation, depending on the particular stress. The importance of the loss of Rubisco activity and its repercussion on plant photosynthesis are discussed in the context of climate change. It is suggested that decreased Rubisco activity will be a major effect induced by climate change, which will need to be considered in any prediction model on plant productivity in the near future.

Evidence of current impact of climate change on life: a walk from genes to the biosphere

We review the evidence of how organisms and populations are currently responding to climate change through phenotypic plasticity, genotypic evolution, changes in distribution and, in some cases, local extinction. Organisms alter their gene expression and metabolism to increase the concentrations of several antistress compounds and to change their physiology, phenology, growth and reproduction in response to climate change. Rapid adaptation and microevolution occur at the population level. Together with these phenotypic and genotypic adaptations, the movement of organisms and the turnover of populations can lead to migration toward habitats with better conditions unless hindered by barriers. Both migration and local extinction of populations have occurred. However, many unknowns for all these processes remain. The roles of phenotypic plasticity and genotypic evolution and their possible trade-offs and links with population structure warrant further research. The application of omic techniques to ecological studies will greatly favor this research. It remains poorly understood how climate change will result in asymmetrical responses of species and how it will interact with other increasing global impacts, such as N eutrophication, changes in environmental N : P ratios and species invasion, among many others. The biogeochemical and biophysical feedbacks on climate of all these changes in vegetation are also poorly understood. We here review the evidence of responses to climate change and discuss the perspectives for increasing our knowledge of the interactions between climate change and life.

Plant-soil interactions in Mediterranean forest and shrublands: impacts of climatic change

BACKGROUND

In the Mediterranean climate, plants have evolved under conditions of low soil-water and nutrient availabilities and have acquired a series of adaptive traits that, in turn exert strong feedback on soil fertility, structure, and protection. As a result, plant-soil systems constitute complex interactive webs where these adaptive traits allow plants to maximize the use of scarce resources.

SCOPE

It is necessary to review the current bibliography to highlight the most know characteristic mechanisms underlying Mediterranean plant-soil feed-backs and identify the processes that merit further research in order to reach an understanding of the plant-soil feed-backs and its capacity to cope with future global change scenarios. In this review, we characterize the functional and structural plant-soil relationships and feedbacks in Mediterranean regions. We thereafter discuss the effects of global change drivers on these complex interactions between plants and soil.

CONCLUSIONS

The large plant diversity that characterizes Mediterranean ecosystems is associated to the success of coexisting species in avoiding competition for soil resources by differential exploitation in space (soil layers) and time (year and daily). Among plant and soil traits, high foliar nutrient re-translocation and large contents of recalcitrant compounds reduce nutrient cycling. Meanwhile increased allocation of resources to roots and soil enzymes help to protect against soil erosion and to improve soil fertility and capacity to retain water. The long-term evolutionary adaptation to drought of Mediterranean plants allows them to cope with moderate increases of drought without significant losses of production and survival in some species. However, other species have proved to be more sensitive decreasing their growth and increasing their mortality under moderate rising of drought. All these increases contribute to species composition shifts. Moreover, in more xeric sites, the desertification resulting from synergic interactions among some related process such as drought increases, torrential rainfall increases and human driven disturbances is an increasing concern. A research priority now is to discern the effects of long-term increases in atmospheric CO2 concentrations, warming, and drought on soil fertility and water availability and on the structure of soil communities (e.g. shifts from bacteria to fungi) and on patching vegetation and root-water uplift (from soil to plant and from soil deep layers to soil superficial layers) roles in desertification.

Evaluation of the effect of pollution and fungal disease on Pinus radiata pollen allergenicity

BACKGROUND

Pollutants and other stressing factors like mold infection might increase the production of pathogen-related proteins in plants. Since this is invoked as one of the causes for the high prevalence of allergic diseases in developed countries, we aimed to determine the potential effect of environmental pollution, with or without mold infection of the trees, on the allergenic potency of pine pollen (Pinus radiata).

METHODS

Pine pollen samples were recovered from three selected areas: low polluted (A), highly polluted (B) and highly polluted and infected with fungi (Spheropsis sapinea) (C). The allergenic potency of pollen from areas A, B or C were compared in vivo in 35 pine pollen-allergic patients by skin prick test and specific IgE (sIgE) quantification. Pollen was also analyzed in vitro by SDS-PAGE immunoblotting, RAST inhibition and cDNA-AFLP (amplified fragment length polymorphism) to compare differences in proteins and mRNA expression.

RESULTS

The allergenic potency measured by prick test, sIgE and RAST inhibition was greater in pollen A, which was exposed to smaller amounts of NO(x), PM(10) and SO(2) but greater amounts of O(3). No differences were found in IgE-binding bands in immunoblotting or densitometry of the bands. In cDNA-AFLP, three homologous transcript-derived fragments were expressed in samples B only, with an expressed sequence tag related with stress-regulated gene expression.

CONCLUSIONS

A greater allergenic potency, in terms of skin tests and sIgE, is observed in pine pollen coming from unpolluted areas. We consider that this fact might be related to a higher exposure to ozone, resulting in a greater expression of allergenic proteins.

What can enzymes of C₄ photosynthesis do for C₃ plants under stress?

Phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), and pyruvate, phosphate dikinase (PPDK) participate in the process of concentrating CO₂ in C₄ photosynthesis. Non-photosynthetic counterparts of these enzymes, which are present in all plants, play important roles in the maintenance of pH and replenishment of Krebs cycle intermediates, thereby contributing to the biosynthesis of amino acids and other compounds and providing NADPH for biosynthesis and the antioxidant system. Enhanced activities of PEPC and/or NADP-ME and/or PPDK were found in plants under various types of abiotic stress, such as drought, high salt concentration, ozone, the absence of phosphate and iron or the presence of heavy metals in the soil. Moreover, the activities of all of these enzymes were enhanced in plants under biotic stress caused by viral infection. The functions of PEPC, NADP-ME and PPDK appear to be more important for plants under stress than under optimal growth conditions.

Visible and microscopic needle alterations of mature Aleppo pine (Pinus halepensis) trees growing on an ozone gradient in eastern Spain

Visible injuries and 42 microscopic features of tissue and cell structure were quantified in needles of mature Aleppo pine (Pinus halepensis) growing at four field sites located on a natural ozone gradient in eastern Spain. Principal component analysis was used to find out if the forest sites differed from each other, to determine the reasons for the site differences and to evaluate the relations between the parameters studied. In previous-year needles, the first principal component (PC) was described by changes typical of long-term ozone stress: high occurrence of microscopic changes indicating increased defence and faint chlorotic mottling, but low occurrence of ultrastructural changes related to photosynthesis and its storage products. The second PC was described by needle ageing or ontological senescence. Statistical differences between the sites in terms of ozone stress were found and were in line with measured ozone concentrations and the values of the ozone exposure index, AOT40. Symptoms of ozone stress were mild, i.e., not related to severe tissue damage. Results suggested that the faint chlorotic mottling can be attributed to certain forms of condensed tannins or small chloroplasts. In addition, a coastal site differed from mountainous sites by having a more mesomorphic needle anatomy.

Metabolic-dependent changes in plant cell redox power after ozone exposure

The tropospheric level of the phytotoxic air pollutant ozone has increased considerably during the last century, and is expected to continue to rise. Long-term exposure of higher plants to low ozone concentrations affects biochemical processes prior to any visible symptoms of injury. The current critical level of ozone used to determine the threshold for damaging plants (biomass loss) is still based on the seasonal sum of the external concentration above 40 nl.l(-1) (AOT40). Taking into account stomatal conductance and the internal capacity of leaf defences, a more relevant concept should be based upon the 'effective ozone flux', the balance between the stomatal flux and the intensity of cellular detoxification. The large decrease in the Rubisco/PEPc ratio reflects photosynthetic damage from ozone, and a large increase in activity of cytosolic PEPc, which allows increased malate production. Although the direct detoxification of ozone (and ROS produced from its decomposition) is carried out primarily by cell wall ascorbate, the existing level of this antioxidant is not sufficient to indicate the degree of cell sensitivity. In order to regenerate ascorbate, NAD(P)H is needed as the primary supplier of reducing power. It is hypothesised that increased activity of the catabolic pathways and associated shunts (glucose-6-phosphate dehydrogenase, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase, isocitrate dehydrogenase and malic enzyme) can provide sufficient NAD(P)H to maintain intracellular detoxification. Thus, measurement of the level of redox power would contribute to determination of the 'effective ozone dose', serving ultimately to improve the ozone risk index for higher plants.

Physiology-phenology interactions in a productive semi-arid pine forest

This study explored possible advantages conferred by the phase shift between leaf phenology and photosynthesis seasonality in a semi-arid Pinus halepensis forest system, not seen in temperate sites. Leaf-scale measurements of gas exchange, nitrogen and phenology were used on daily, seasonal and annual time-scales. Peak photosynthesis was in late winter, when high soil moisture, mild temperatures and low leaf vapour pressure deficit (D(L)) allowed high rates associated with high water- and nitrogen-use efficiencies. Self-sustained new needle growth through the dry and hot summer maximized photosynthesis in the following wet season, without straining carbon storage. Low rates of water loss were associated with increasing sensitivity of stomatal conductance (g(s)) to soil moisture below a relative extractable water (REW) of 0.4, and decreased g(s )sensitivity to D(L) below REW of approx. 0.2. This response was captured by the modified Ball-Berry (Leuning) model. While most physiological parameters and responses measured were typical of temperate pines, the photosynthesis-phenological phasing contributed to high productivity under warm-dry conditions. This contrasts with reported effects of short-term periodical droughts and could lead to different predictions of the effect of warming and drying climate on pine forest productivity.