Elevated temperature differently affects foliar nitrogen partitioning in seedlings of diverse Douglas fir provenances

A Preliminary Study on the Whole-Plant Regulations of the Shrub Campylotropis polyantha in Response to Hostile Dryland Conditions

Drylands cover more than 40% of global land surface and will continue to expand by 10% at the end of this century. Understanding the resistance mechanisms of native species is of particular importance for vegetation restoration and management in drylands. In the present study, metabolome of a dominant shrub Campylotropis polyantha in a dry-hot valley were investigated. Compared to plants grown at the wetter site, C. polyantha tended to slow down carbon (C) assimilation to prevent water loss concurrent with low foliar reactive oxygen species and sugar concentrations at the drier and hotter site. Nitrogen (N) assimilation and turn over were stimulated under stressful conditions and higher leaf N content was kept at the expense of root N pools. At the drier site, roots contained more water but less N compounds derived from the citric acid cycle. The site had little effect on metabolites partitioning between leaves and roots. Generally, roots contained more C but less N. Aromatic compounds were differently impacted by site conditions. The present study, for the first time, uncovers the apparent metabolic adaptations of C. polyantha to hostile dryland conditions. However, due to the limited number of samples, we are cautious about drawing general conclusions regarding the resistance mechanisms. Further studies with a broader spatial range and larger time scale are therefore recommended to provide more robust information for vegetation restoration and management in dryland areas under a changing climate.

Comparison of Growth and Metabolomic Profiles of Two Afforestation Cypress Species Cupressus chengiana and Platycladus orientalis Grown at Minjiang Valley in Southwest China

In recent years, afforestation has been conducted in China's hot and dry valleys. However, there is still a paucity of knowledge regarding the performance of tree species in these semi-arid regions, particularly with regard to interspecies differences. The present study compares the growth and metabolome characteristics of two widely used cypress species, namely Cupressus chengiana and Platycladus orientalis, grown at two sites with distinct climate conditions in the hot and dry Minjiang Valley in southwestern China. The findings indicate that C. chengiana trees exhibit superior growth rates compared to P. orientalis trees at both study sites. In comparison to P. orientalis trees, C. chengiana trees demonstrated a greater tendency to close their stomata in order to prevent water loss at the hotter and drier site, Llianghekou (LHK). Additionally, C. chengiana trees exhibited significantly lower hydrogen peroxide levels than P. orientalis trees, either due to lower production and/or higher scavenging of reactive oxygen species. C. chengiana trees accumulated soluble sugars as well as sugar derivatives, particularly those involved in sucrose and galactose metabolisms under stressful conditions. The species-specific differences were also reflected in metabolites involved in the tricarboxylic acid cycle, nitrogen, and secondary metabolisms. The metabolome profiles of the two species appeared to be influenced by the prevailing climatic conditions. It appeared that the trees at the drier and hotter site, LHK, were capable of efficient nitrogen uptake from the soil despite the low soil nitrogen concentration. This study is the first to compare the growth performance and metabolic profiles of two widely used tree species with high resistance to adverse conditions. In addition to the species-specific differences and adaptations to different sites, the present study also provides insights into potential management strategies to alleviate abiotic stress, particularly with regard to nitrogen nutrients, in the context of climate change.

Nitrogen fertilization stimulates nitrogen assimilation and modifies nitrogen partitioning in the spring shoot leaves of citrus (Citrus reticulata Blanco) trees

The spring shoot leaves are important sites of nitrogen (N) metabolism in citrus trees. Understanding the physiological and metabolic response of the spring shoot leaves under varying N fertilization is fundamental to the fertilization management in citrus orchards. Thus, the processes affecting N composition, the activities of N metabolism related enzymes, and the expression of relevant genes were explored in spring shoot leaves under four N levels (0, 207, 275, 413 g N tree^-1 y^-1, as N0, N207, N275, N413). The results showed that, compared with N0, N275 significantly increased total N by 24.81%, which was mainly attributed to enhancement of structural N by 30.92%, free amino acid N by 40.91% and nitrate N by 41.33%. The relative expression of nitrate reductase (NR) and glutamate dehydrogenase (GDH) under N275 increased by 19.32% and 73.48%, respectively, compared with that under N0 treatment. Compared with N0 treatment, the NR transcription level under N275 treatment increased by 381%. The relative transcription levels of NADP-GDH and GDH1 also increased with increasing N fertilization. However, compared with that under N275, the relative transcription of GDH2 under N413 treatment was inhibited. Therefore, the transcript abundance of NR, NADP-GDH,GDH1 and GDH2 affected the activities of NR and GDH and thereby contributed to the regulation of N composition in the leaves. In addition, the activities of glutamine synthetase and nitrite reductase were largely unaffected or even declined in the N207, N275 and N413 treatments compared with the N0. This study elucidated the mechanism of primary N metabolism and partitioning in citrus leaves and provided a theoretical basis for N management in citrus orchards.

Foliar traits of sessile oak (Quercus petraea Liebl) seedlings are largely determined by site properties rather than seed origin

Due to climate change, sessile oak (Quercus petraea) seedlings experience an increasing risk of drought during regeneration of forest stands by management practices. The present study was aimed at elucidating the potential of sessile oak seedlings originating from sites with different aridity and nitrogen (N) supply to acclimate to contrasting water availability. For this purpose, a free-air cross-exchange experiment was conducted between a dry and a humid forest stand with high and low soil N contents, respectively, during two consecutive years differing in aridity before harvest. Almost all structural and physiological foliar traits analyzed did not differ consistently between seed origins during both years, when cultivated at the same site. As an exception, the arid provenance upregulated foliar ascorbate contents under drought, whereas the humid provenance accumulated the phenolic antioxidants vescalagin and castalagin (VC) under favorable weather conditions and consumed VC upon drought. Apparently, differences in long-term aridity at the forest sites resulted in only few genetically fixed differences in foliar traits between the provenances. However, structural and physiological traits strongly responded to soil N contents and weather conditions before harvest. Foliar N contents and their partitioning were mostly determined by the differences in soil N availability at the sites, but still were modulated by weather conditions before harvest. In the first year, differences in aridity before harvest resulted in differences between most foliar traits. In the second year, when weather conditions at both sites were considerably similar and more arid compared to the first year, differences in foliar traits were almost negligible. This pattern was observed irrespective of seed origin. These results support the view that leaves of sessile oak seedlings generally possess a high plasticity to cope with extreme differences in aridity by immediate acclimation responses that are even better developed in plants of arid origin.

Increased CO2 and light intensity regulate growth and leaf gas exchange in tomato

Carbon dioxide concentration (CO₂ ) and light intensity are known to play important roles in plant growth and carbon assimilation. Nevertheless, the underlying physiological mechanisms have not yet been fully explored. Tomato seedlings (Solanum lycopersicum Mill. cv. Jingpeng No. 1) were exposed to two levels of CO₂ and three levels of light intensity and the effects on growth, leaf gas exchange and water use efficiency were investigated. Elevated CO₂ and increased light intensity promoted growth, dry matter accumulation and pigment concentration and together the seedling health index. Elevated CO₂ had no significant effect on leaf nitrogen content but did significantly upregulate Calvin cycle enzyme activity. Increased CO₂ and light intensity promoted photosynthesis, both on a leaf-area basis and on a chlorophyll basis. Increased CO₂ also increased light-saturated maximum photosynthetic rate, apparent quantum efficiency and carboxylation efficiency and, together with increased light intensity, it raised photosynthetic capacity. However, increased CO₂ reduced transpiration and water consumption across different levels of light intensity, thus significantly increasing both leaf-level and plant-level water use efficiency. Among the range of treatments imposed, the combination of increased CO₂ (800 µmol CO₂ mol^-1 ) and high light intensity (400 µmol m^-2 s^-1 ) resulted in optimal growth and carbon assimilation. We conclude that the combination of increased CO₂ and increased light intensity worked synergistically to promote growth, photosynthetic capacity and water use efficiency by upregulation of pigment concentration, Calvin cycle enzyme activity, light energy use and CO₂ fixation. Increased CO₂ also lowered transpiration and hence water usage.

Climate and development modulate the metabolome and antioxidative system of date palm leaves

Date palms are remarkably tolerant to environmental stresses, but the mechanisms involved remain poorly characterized. Leaf metabolome profiling was therefore performed on mature (ML) and young (YL) leaves of 2-year-old date palm seedlings that had been grown in climate chambers that simulate summer and winter conditions in eastern Saudi Arabia. Cultivation under high temperature (summer climate) resulted in higher YL H2O2 leaf levels despite increases in dehydroascorbate reductase (DHAR) activities. The levels of raffinose and galactinol, tricarboxylic acid cycle intermediates, and total amino acids were higher under these conditions, particularly in YL. The accumulation of unsaturated fatty acids, 9,12-octadecadienoic acid and 9,12,15-octadecatrienoic acid, was lower in ML. In contrast, the amounts of saturated tetradecanoic acid and heptadecanoic acid were increased in YL under summer climate conditions. The accumulation of phenolic compounds was favored under summer conditions, while flavonoids accumulated under lower temperature (winter climate) conditions. YL displayed stronger hydration, lower H2O2 levels, and more negative δ 13C values, indicating effective reactive oxygen species scavenging. These findings, which demonstrate the substantial metabolic adjustments that facilitate tolerance to the high temperatures in YL and ML, suggest that YL may be more responsive to climate change.

Nitrogen Nutrition of European Beech Is Maintained at Sufficient Water Supply in Mixed Beech-Fir Stands

Research highlights: Interaction effects of coniferous on deciduous species have been investigated before the background of climate change. Background and objectives: The cultivation of European beech (Fagus sylvatica L.) in mixed stands has currently received attention, since the future performance of beech in mid-European forest monocultures in a changing climate is under debate. We investigated water relations and nitrogen (N) nutrition of beech in monocultures and mixed with silver-fir (Abies alba Mill.) in the Black Forest at different environmental conditions, and in the Croatian Velebit at the southern distribution limit of beech, over a seasonal course at sufficient water availability. Material and methods: Water relations were analyzed via δ13C signatures, as integrative measures of water supply assuming that photosynthesis processes were not impaired. N nutrition was characterized by N partitioning between soluble N fractions and structural N. Results: In the relatively wet year 2016, water relations of beech leaves, fir needles and roots differed by season, but generally not between beech monocultures and mixed cultivation. At all sites, previous and current year fir needles revealed significantly lower total N contents over the entire season than beech leaves. Fir fine roots exhibited higher or similar amounts of total N compared to needles. Correlation analysis revealed a strong relationship of leaf and root δ13C signatures with soil parameters at the mixed beech stands, but not at pure beech stands. While glutamine (Gln) uptake capacity of beech roots was strongly related to soil N in the monoculture beech stands, arginine (Arg) uptake capacities of beech roots were strongly related to soil N in mixed stands. Conclusions: Leaf N contents indicated a facilitative effect of silver-fir on beech on sites where soil total N concentrations where low, but an indication of competition effect where it was high. This improvement could be partially attributed to protein contents, but not to differences in uptake capacity of an individual N source. From these results it is concluded that despite similar performance of beech trees at the three field sites investigated, the association with silver-fir mediated interactive effects between species association, climate and soil parameters even at sufficient water supply.

Physiological responses of date palm (Phoenix dactylifera) seedlings to acute ozone exposure at high temperature

Vegetation in the Arabian Peninsula is facing high and steadily rising tropospheric ozone pollution. However, little is known about the impacts of elevated ozone on date palms, one of the most important indigenous economic species. To elucidate the physiological responses of date palm to peak levels of acute ozone exposure, seedlings were fumigated with 200 ppb ozone for 8 h. Net CO₂ assimilation rate, stomatal conduction, total carbon, its isotope signature and total sugar contents in leaves and roots were not significantly affected by the treatment and visible symptoms of foliar damage were not induced. Ozone exposure did not affect hydrogen peroxide and thiol contents but diminished the activities of glutathione reductase and dehydroascorbate reductase, stimulated the oxidation of ascorbate, and resulted in elevated total ascorbate contents. Total nitrogen, soluble protein and lignin contents remained unchanged upon ozone exposure, but the abundance of low molecular weight nitrogen (LMWN) compounds such as amino acids and nitrate as well as other anions were strongly diminished in leaves and roots. Other nitrogen pools did not benefit from the decline of LMWN, indicating reduced uptake and/or enhanced release of these compounds into the soil as a systemic response to aboveground ozone exposure. Several phenolic compounds, concurrent with fatty acids and stearyl alcohol, accumulated in leaves, but declined in roots, whereas total phenol contents significantly increased in the roots. Together these results indicate that local and systemic changes in both, primary and secondary metabolism contribute to the high tolerance of date palms to short-term acute ozone exposure.

Foliar nitrogen metabolism of adult Douglas-fir trees is affected by soil water availability and varies little among provenances

The coniferous forest tree Douglas-fir (Pseudotsuga menziesii) is native to the pacific North America, and is increasingly planted in temperate regions worldwide. Nitrogen (N) metabolism is of great importance for growth, resistance and resilience of trees. In the present study, foliar N metabolism of adult trees of three coastal and one interior provenance of Douglas-fir grown at two common gardens in southwestern Germany (Wiesloch, W; Schluchsee, S) were characterized in two subsequent years. Both the native North American habitats of the seed sources and the common garden sites in Germany differ in climate conditions. Total and mineral soil N as well as soil water content were higher in S compared to W. We hypothesized that i) provenances differ constitutively in N pool sizes and composition, ii) N pools are affected by environmental conditions, and iii) that effects of environmental factors on N pools differ among interior and coastal provenances. Soil water content strongly affected the concentrations of total N, soluble protein, total amino acids (TAA), arginine and glutamate. Foliar concentrations of total N, soluble protein, structural N and TAA of trees grown at W were much higher than in trees at S. Provenance effects were small but significant for total N and soluble protein content (interior provenance showed lowest concentrations), as well as arginine, asparagine and glutamate. Our data suggest that needle N status of adult Douglas-fir is independent from soil N availability and that low soil water availability induces a re-allocation of N from structural N to metabolic N pools. Small provenance effects on N pools suggest that local adaptation of Douglas-fir is not dominated by N conditions at the native habitats.

Woody-plant ecosystems under climate change and air pollution-response consistencies across zonobiomes?

Forests store the largest terrestrial pools of carbon (C), helping to stabilize the global climate system, yet are threatened by climate change (CC) and associated air pollution (AP, highlighting ozone (O3) and nitrogen oxides (NOx)). We adopt the perspective that CC-AP drivers and physiological impacts are universal, resulting in consistent stress responses of forest ecosystems across zonobiomes. Evidence supporting this viewpoint is presented from the literature on ecosystem gross/net primary productivity and water cycling. Responses to CC-AP are compared across evergreen/deciduous foliage types, discussing implications of nutrition and resource turnover at tree and ecosystem scales. The availability of data is extremely uneven across zonobiomes, yet unifying patterns of ecosystem response are discernable. Ecosystem warming results in trade-offs between respiration and biomass production, affecting high elevation forests more than in the lowland tropics and low-elevation temperate zone. Resilience to drought is modulated by tree size and species richness. Elevated O3 tends to counteract stimulation by elevated carbon dioxide (CO2). Biotic stress and genomic structure ultimately determine ecosystem responsiveness. Aggrading early- rather than mature late-successional communities respond to CO2 enhancement, whereas O3 affects North American and Eurasian tree species consistently under free-air fumigation. Insect herbivory is exacerbated by CC-AP in biome-specific ways. Rhizosphere responses reflect similar stand-level nutritional dynamics across zonobiomes, but are modulated by differences in tree-soil nutrient cycling between deciduous and evergreen systems, and natural versus anthropogenic nitrogen (N) oversupply. The hypothesis of consistency of forest responses to interacting CC-AP is supported by currently available data, establishing the precedent for a global network of long-term coordinated research sites across zonobiomes to simultaneously advance both bottom-up (e.g., mechanistic) and top-down (systems-level) understanding. This global, synthetic approach is needed because high biological plasticity and physiographic variation across individual ecosystems currently limit development of predictive models of forest responses to CC-AP. Integrated research on C and nutrient cycling, O3-vegetation interactions and water relations must target mechanisms' ecosystem responsiveness. Worldwide case studies must be subject to biostatistical exploration to elucidate overarching response patterns and synthesize the resulting empirical data through advanced modelling, in order to provide regionally coherent, yet globally integrated information in support of internationally coordinated decision-making and policy development.

Thermotolerance and heat stress responses of Douglas-fir and ponderosa pine seedling populations from contrasting climates

Temperature and the frequency and intensity of heat waves are predicted to increase throughout the 21st century. Germinant seedlings are expected to be particularly vulnerable to heat stress because they are in the boundary layer close to the soil surface where intense heating occurs in open habitats. We quantified leaf thermotolerance and whole-plant physiological responses to heat stress in first-year germinant seedlings in two populations each of Pinus ponderosa P. and C. Lawson (PIPO) and Pseudotsuga menziesii (Mirb.) Franco (PSME) from climates with contrasting precipitation and temperature regimes. Thermotolerance of detached needles was evaluated using chlorophyll fluorescence (FV/FM, FO) and electrolyte leakage. PSME was more heat tolerant than PIPO according to both independent assessments of thermotolerance. Following exposure of whole seedlings to a simulated heat wave at 45 °C for 1 h in a growth chamber, we monitored FV/FM, photosynthesis, stomatal conductance, non-structural carbohydrates (NSCs) and carbon isotope ratios (δ13C) for 14 days. Heat treatment induced significant reductions in FV/FM in both species and a transient reduction in photosynthetic gas exchange only in PIPO 1 day after treatment. Heat treatment induced an increase in glucose + fructose concurrent with a decrease in starch in both species, whereas total NSC and sucrose were not affected by heat treatment. The negative relationship between glucose + fructose and starch observed in treated plants may be due to the conversion of starch to glucose + fructose to aid recovery from heat-induced damage. Populations from drier sites displayed greater δ13C values than those from wetter sites, consistent with higher intrinsic water-use efficiency and drought resistance of populations from drier climates. Thermotolerance and heat stress responses appeared to be phenotypically plastic and representative of the environment in which plants were grown, whereas intrinsic water-use efficiency appeared to reflect ecotypic differentiation and the climate of origin.

Immediate and potential long-term effects of consecutive heat waves on the photosynthetic performance and water balance in Douglas-fir

The frequency and intensity of climatic extremes, such as heat waves, are predicted to increase globally, with severe implications for terrestrial carbon and water cycling. Temperatures may rise above critical thresholds that allow trees to function optimally, with unknown long-term consequences for forest ecosystems. In this context, we investigated how photosynthetic traits and the water balance in Douglas-fir are affected by exposure to three heat waves with temperatures about 12°C above ambient. Photosynthetic carboxylation efficiency (V_cmax) was mostly unaffected, but electron transport (J_max) and photosynthetic rates under saturating light (A_sat) were strongly influenced by the heat waves, with lagging limitations on photosynthesis still being observed six weeks after the last heat wave. We also observed lingering heat-induced inhibitions on transpiration, minimum stomatal conductance, and night-time stomatal conductance (g_s-night). Results from the stomatal models used to calculate minimum stomatal conductance were similar to g_s-night and indicated changes in leaf morphology, e.g. stomatal occlusions and alterations in epicuticular wax. Our results show Douglas-fir's ability to restrict water loss following heat stress, but at the price of reduced photosynthetic performance. Such limitations indicate potential long-term restrictions that heat waves can impose on tree development and functioning under extreme climatic conditions.

A coastal and an interior Douglas fir provenance exhibit different metabolic strategies to deal with drought stress

Drought is a major environmental stress affecting growth and vitality of forest ecosystems. In the present study, foliar nitrogen (N) and carbon (C) metabolism of two Douglas fir (Pseudotsuga menziesii) provenances with assumed different drought tolerance were investigated. We worked with 1-year-old seedlings of the interior provenance Fehr Lake (FEHR) originating from a dry environment and the coastal provenance Snoqualmie (SNO) from a more humid origin. Total C and N, structural N and the concentrations of soluble protein, total amino acids (TAAs) and individual amino acids as well as the relative abundance of polar, low-molecular-weight metabolites including antioxidants were determined in current-year needles exposed either to 42 days of drought or to 42 days drought plus 14 days of rewatering. The seedlings reacted in a provenance-specific manner to drought stress. Coastal provenance SNO showed considerably increased contents of TAAs, which were caused by increased abundance of the quantitatively most important amino acids arginine, ornithine and lysine. Additionally, the polyamine putrescine accumulated exclusively in drought-stressed trees of this provenance. In contrast, the interior provenance FEHR showed the opposite response, i.e., drastically reduced concentrations of these amino acids. However, FEHR showed considerably increased contents of pyruvate-derived and aromatic amino acids, and also higher drought-induced levels of the antioxidants ascorbate and α-tocopherol. In response to drought, both provenances produced large amounts of carbohydrates, such as glucose and fructose, most likely as osmolytes that can readily be metabolized for protection against osmotic stress. We conclude that FEHR and SNO cope with drought stress in a provenance-specific manner: the coastal provenance SNO was mainly synthesizing N-based osmolytes, a reaction not observed in the interior provenance FEHR; instead, the latter increased the levels of scavengers of reactive oxygen species. Our results underline the importance of provenance-specific reactions to abiotic stress.