Controls of the quantum yield and saturation light of isoprene emission in different-aged aspen leaves

Impacts of elevation on plant traits and volatile organic compound emissions in deciduous tundra shrubs

The northernmost regions of our planet experience twice the rate of climate warming compared to the global average. Despite the currently low air temperatures, tundra shrubs are known to exhibit high leaf temperatures and are increasing in height due to warming, but it is unclear how the increase in height will affect the leaf temperature. To study how temperature, soil moisture, and changes in light availability influence the physiology and emissions of climate-relevant volatile organic compounds (VOCs), we conducted a study on two common deciduous tundra shrubs, Salix glauca (separating males and females for potential effects of plant sex) and Betula glandulosa, at two elevations in South Greenland. Low-elevation Salix shrubs were 45% taller, but had 37% lower rates of net CO₂ assimilation and 63% lower rates of isoprene emission compared to high-elevation shrubs. Betula shrubs showed 40% higher stomatal conductance and 24% higher glandular trichome density, in the low-elevation valley, compared to those from the high-elevation mountain slope. Betula green leaf volatile emissions were 235% higher at high elevation compared to low elevation. Male Salix showed a distinct VOC blend and emitted 55% more oxygenated VOCs, compared to females, possibly due to plant defense mechanisms. In our light response curves, isoprene emissions increased linearly with light intensity, potentially indicating adaptation to strong light. Leaf temperature decreased with increasing Salix height, at 4 °C m^-1, which can have implications for plant physiology. However, no similar relationship was observed for B. glandulosa. Our results highlight that tundra shrub traits and VOC emissions are sensitive to temperature and light, but that local variations in soil moisture strongly interact with temperature and light responses. Our results suggest that effects of climate warming, alone, poorly predict the actual plant responses in tundra vegetation.

CO2 -responsiveness of leaf isoprene emission: Why do species differ?

Leaf isoprene emission rate, I, decreases with increasing atmospheric CO₂ concentration with major implications for global change. There is a significant interspecific variability in [CO₂ ]-responsiveness of I, but the extent of this variation is unknown and its reasons are not understood. We hypothesized that the magnitude of emission reduction reflects the size and changeability of precursor pools responsible for isoprene emission (dimethylallyl diphosphate, DMADP and 2-methyl-erythritol 2,4-cyclodiphosphate, MEcDP). Changes in I and intermediate pool sizes upon increase of [CO₂ ] from 400 to 1500 μmol/mol were studied in nine woody species spanning boreal to tropical ecosystems. I varied 10-fold, total substrate pool size 37-fold and the ratio of DMADP/MEcDP pool sizes 57-fold. At higher [CO₂ ], I was reduced on average by 65%, but [CO₂ ]-responsiveness varied an order of magnitude across species. The increase in [CO₂ ] resulted in concomitant reductions in both substrate pools. The variation in [CO₂ ]-responsiveness across species scaled with the reduction in pool sizes, the substrate pool size supported and the share of DMADP in total substrate pool. This study highlights a major interspecific variation in [CO₂ ]-responsiveness of isoprene emission and conclusively links this variation to interspecific variability in [CO₂ ] effects on substrate availability and intermediate pool size.

Responses of isoprene emission and photochemical efficiency to severe drought combined with prolonged hot weather in hybrid Populus

Isoprene emissions have been considered as a protective response of plants to heat stress, but there is limited information of how prolonged heat spells affect isoprene emission capacity, particularly under the drought conditions that often accompany hot weather. Under combined long-term stresses, presence of isoprene emission could contribute to the maintenance of the precursor pool for rapid synthesis of essential isoprenoids to repair damaged components of leaf photosynthetic apparatus. We studied changes in leaf isoprene emission rate, photosynthetic characteristics, and antioxidant enzyme activities in two hybrid Populus clones, Nanlin 1388 (relatively high drought tolerance) and Nanlin 895 (relatively high thermotolerance) that were subjected to long-term (30 d) soil water stress (25% versus 90% soil field capacity) combined with a natural heat spell (day-time temperatures of 35-40 °C) that affected both control and water-stressed plants. Unexpectedly, isoprene emissions from both the clones were similar and the overall effects of drought on the emission characteristics were initially minor; however, treatment effects and clonal differences increased with time. In particular, the isoprene emission rate only increased slightly in the Nanlin 895 control plants after 15 d of treatment, whereas it decreased by more than 5-fold in all treatment × clone combinations after 30 d. The reduction in isoprene emission rate was associated with a decrease in the pool size of the isoprene precursor dimethylallyl diphosphate in all cases at 30 d after the start of treatment. Net assimilation rate, stomatal conductance, the openness of PSII centers, and the effective quantum yield all decreased, and non-photochemical quenching and catalase activity increased in both control and water-stressed plants. Contrary to the hypothesis of protection of leaf photosynthetic apparatus by isoprene, the data collectively indicated that prolonged stress affected isoprene emissions more strongly than leaf photosynthetic characteristics. This primarily reflected the depletion of isoprene precursor pools under long-term severe stress.

Response of isoprene emission from poplar saplings to ozone pollution and nitrogen deposition depends on leaf position along the vertical canopy profile

We investigated isoprene (ISO) emission and gas exchange in leaves from different positions along the vertical canopy profile of poplar saplings (Populus euramericana cv. '74/76'). For a growing season, plants were subjected to four N treatments, control (NC, no N addition), low N (LN, 50 kg N ha^-1year^-1), middle N (MN, 100 kg N ha^-1year^-1), high N (HN, 200 kg N ha^-1year^-1) and three O₃ treatments (CF, charcoal-filtered ambient air; NF, non-filtered ambient air; NF + O₃, NF + 40 ppb O₃). Our results showed the effects of O₃ and/or N on standardized ISO rate (ISO_rate) and photosynthetic parameters differed along with the leaf position, with larger negative effects of O₃ and positive effects of N on ISO_rate and photosynthetic parameters in the older leaves. Expanded young leaves were insensitive to both treatments even at very high O₃ concentration (67 ppb as 10-h average) and HN treatment. Significant O₃ × N interactions were only found in middle and lower leaves, where ISO_rate declined by O₃ just when N was limited (NC and LN). With increasing light-saturated photosynthesis and chlorophyll content, ISO_rate was reduced in the upper leaves but on the contrary increased in middle and lower leaves. The responses of ISO_rate to AOT40 (accumulated exposure to hourly O₃ concentrations > 40 ppb) and POD_Y (accumulative stomatal uptake of O₃ > Y nmol O₃ m^-2 PLA s^-1) were not significant in upper leaves, but ISO_rate significantly decreased with increasing AOT40 or POD_Y under limited N supply in middle leaves but at all N levels in lower leaves. Overall, ISO_rate changed along the vertical canopy profile in response to combined O₃ and N exposure, a behavior that should be incorporated into multi-layer canopy models. Our results are relevant for modelling regional isoprene emissions under current and future O₃ pollution and N deposition scenarios.

Shoot Characterization of Isoprene and Ocimene-Emitting Transgenic Arabidopsis Plants under Contrasting Environmental Conditions

Isoprenoids are among the most abundant biogenic volatile compounds (VOCs) emitted by plants, and mediate both biotic and abiotic stress responses. Here, we provide for the first time a comparative analysis of transgenic Arabidopsis lines constitutively emitting isoprene and ocimene. Transgenic lines and Columbia-0 (Col-0) Arabidopsis were characterized under optimal, water stress, and heat stress conditions. Under optimal conditions, the projected leaf area (PLA), relative growth rate, and final dry weight were generally higher in transgenics than Col-0. These traits were associated to a larger photosynthetic capacity and CO2 assimilation rate at saturating light. Isoprene and ocimene emitters displayed a moderately higher stress tolerance than Col-0, showing higher PLA and gas-exchange traits throughout the experiments. Contrasting behaviors were recorded for the two overexpressors under water stress, with isoprene emitters showing earlier stomatal closure (conservative behavior) than ocimene emitters (non-conservative behavior), which might suggest different induced strategies for water conservation and stress adaptation. Our work indicates that (i) isoprene and ocimene emitters resulted in enhanced PLA and biomass under optimal and control conditions and that (ii) a moderate stress tolerance is induced when isoprene and ocimene are constitutively emitted in Arabidopsis, thus providing evidence of their role as a potential preferable trait for crop improvement.

Seasonal variations in emission rates and composition of terpenoids emitted from Chamaecyparis formosensis (Cupressaceae) of different ages

Chamaecyparis formosensis (Cupressaceae) is among the most precious endemic conifers in Taiwan. Field study was conducted on seasonal variations in emission rates and compositions of terpenoids from this tree species of two different ages. A total of 21 terpenoids were detected, of which there were 13 monoterpenoids (MTs), 4 sesquiterpenoids (STs), and 4 diterpenoids (DTs). MTs dominated the emissions in both saplings and adult trees and produced more than 80% of terpene emissions. Contrasting seasonal pattern between saplings and adult trees was found. Total actual emissions from saplings were higher in cold seasons (range, 64.40 ± 13.18 to 140.74 ± 18.90 ng g^-1 h^-1) than in warm seasons (range, 55.63 ± 15.84 to 63.48 ± 11.85 ng g^-1 h^-1). Photosynthetically active radiation (PAR) was found to be the most important factor affecting terpene emissions from saplings. On the contrary, higher emissions were found in warm seasons for adult trees (range, 101.49 ± 12.29 to 181.35 ± 80.15 ng g^-1 h^-1), and the emissions were mainly in response to temperature. Some compounds in C. formosensis of both ages (e.g., β-myrcene, α-terpinene, trans-β-ocimene, terpinen-4-ol, α-cedrene and trans-β-farnesene) showed comparably higher contents in cold seasons. Results presented here provide important fundamental information for better understanding of forest bathing and estimating air quality in Taiwan.

Alternative Carbon Sources for Isoprene Emission

Isoprene and other plastidial isoprenoids are produced primarily from recently assimilated photosynthates via the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. However, when environmental conditions limit photosynthesis, a fraction of carbon for MEP pathway can come from extrachloroplastic sources. The flow of extrachloroplastic carbon depends on the species and on leaf developmental and environmental conditions. The exchange of common phosphorylated intermediates between the MEP pathway and other metabolic pathways can occur via plastidic phosphate translocators. C1 and C2 carbon intermediates can contribute to chloroplastic metabolism, including photosynthesis and isoprenoid synthesis. Integration of these metabolic processes provide an example of metabolic flexibility, and results in the synthesis of primary metabolites for plant growth and secondary metabolites for plant defense, allowing effective use of environmental resources under multiple stresses.

In Planta Recapitulation of Isoprene Synthase Evolution from Ocimene Synthases

Isoprene is the most abundant biogenic volatile hydrocarbon compound naturally emitted by plants and plays a major role in atmospheric chemistry. It has been proposed that isoprene synthases (IspS) may readily evolve from other terpene synthases, but this hypothesis has not been experimentally investigated. We isolated and functionally validated in Arabidopsis the first isoprene synthase gene, AdoIspS, from a monocotyledonous species (Arundo donax L., Poaceae). Phylogenetic reconstruction indicates that AdoIspS and dicots isoprene synthases most likely originated by parallel evolution from TPS-b monoterpene synthases. Site-directed mutagenesis demonstrated invivo the functional and evolutionary relevance of the residues considered diagnostic for IspS function. One of these positions was identified by saturating mutagenesis as a major determinant of substrate specificity in AdoIspS able to cause invivo a dramatic change in total volatile emission from hemi- to monoterpenes and supporting evolution of isoprene synthases from ocimene synthases. The mechanism responsible for IspS neofunctionalization by active site size modulation by a single amino acid mutation demonstrated in this study might be general, as the very same amino acidic position is implicated in the parallel evolution of different short-chain terpene synthases from both angiosperms and gymnosperms. Based on these results, we present a model reconciling in a unified conceptual framework the apparently contrasting patterns previously observed for isoprene synthase evolution in plants. These results indicate that parallel evolution may be driven by relatively simple biophysical constraints, and illustrate the intimate molecular evolutionary links between the structural and functional bases of traits with global relevance.

Fading of wound-induced volatile release during Populus tremula leaf expansion

The release of stress-driven volatiles throughout leaf development has been little studied. Therefore, we subjected poplar leaves during their developmental stage (from 2 days to 2 weeks old) to wounding by a single punch hole, and measured online the wound-induced volatile organic compound emissions. Our study shows that the emission of certain volatile compounds fades with increasing leaf age. Among these compounds we found lipoxygenase products (LOX products), acetaldehyde, methyl benzoate, methyl salicylate, and mono- and sesquiterpenes. In parallel, we studied the fading of constitutive emissions of methanol during leaf maturation, as well as the rise in isoprene constitutive emission during leaf maturation and its relationship to leaf photosynthetic capacity. We found highly significant relationships between leaf chlorophyll content, photosynthetic capacity, and leaf size during leaf ageing. As the level of constitutive defences increases with increasing leaf age, the strength of the volatile signal is expected to be gradually reduced. The higher elicitation of volatile organic compound emissions (especially LOX products) in younger leaves could be an evolutionary defence against herbivory, given that younger leaves are usually more subjected to infestation and herbivory.

Spectacular Oscillations in Plant Isoprene Emission under Transient Conditions Explain the Enigmatic CO2 Response

Plant isoprene emissions respond to light and temperature similarly to photosynthesis, but CO₂ dependencies of isoprene emission and photosynthesis are profoundly different, with photosynthesis increasing and isoprene emission decreasing with increasing CO₂ concentration due to reasons not yet understood. We studied isoprene emission, net assimilation rate, and chlorophyll fluorescence under different CO₂ and O₂ concentrations in the strong isoprene emitter hybrid aspen (Populus tremula × Populus tremuloides), and used rapid changes in ambient CO₂ or O₂ concentrations or light level to induce oscillations. As isoprene-emitting species support very high steady-state chloroplastic pool sizes of the primary isoprene substrate, dimethylallyl diphosphate (DMADP), which can mask the effects of oscillatory dynamics on isoprene emission, the size of the DMADP pool was experimentally reduced by either partial inhibition of isoprenoid synthesis pathway by fosmidomycin-feeding or by changes in ambient gas concentrations leading to DMADP pool depletion in intact leaves. In feedback-limited conditions observed at low O₂ and/or high CO₂ concentration under which the rate of photosynthesis is governed by the limited rate of ATP and NADPH formation due to low chloroplastic phosphate levels, oscillations in photosynthesis and isoprene emission were repeatedly induced by rapid environmental modifications in both partly fosmidomycin-inhibited leaves and in intact leaves with in vivo reduced DMADP pools. The oscillations in net assimilation rate and isoprene emission in feedback-inhibited leaves were in the same phase, and relative changes in the pools of photosynthetic metabolites and DMADP estimated by in vivo kinetic methods were directly proportional through all oscillations induced by different environmental perturbations. We conclude that the oscillations in isoprene emission provide direct experimental evidence demonstrating that the response of isoprene emission to changes in ambient gas concentrations is controlled by the chloroplastic reductant supply.

Scaling of photosynthesis and constitutive and induced volatile emissions with severity of leaf infection by rust fungus (Melampsora larici-populina) in Populus balsamifera var. suaveolens

Fungal infections result in decreases in photosynthesis, induction of stress and signaling volatile emissions and reductions in constitutive volatile emissions, but the way different physiological processes scale with the severity of infection is poorly known. We studied the effects of infection by the obligate biotrophic fungal pathogen Melampsora larici-populina Kleb., the causal agent of poplar leaf rust disease, on photosynthetic characteristics, and constitutive isoprene and induced volatile emissions in leaves of Populus balsamifera var. suaveolens (Fisch.) Loudon. exhibiting different degrees of damage. The degree of fungal damage, quantified by the total area of chlorotic and necrotic leaf areas, varied between 0 (noninfected control) and ∼60%. The rates of all physiological processes scaled quantitatively with the degree of visual damage, but the scaling with damage severity was weaker for photosynthetic characteristics than for constitutive and induced volatile release. Over the whole range of damage severity, the net assimilation rate per area (AA) decreased 1.5-fold, dry mass per unit area 2.4-fold and constitutive isoprene emissions 5-fold, while stomatal conductance increased 1.9-fold and dark respiration rate 1.6-fold. The emissions of key stress and signaling volatiles (methanol, green leaf volatiles, monoterpenes, sesquiterpenes and methyl salicylate) were in most cases nondetectable in noninfested leaves, and increased strongly with increasing the spread of infection. The moderate reduction in AA resulted from the loss of photosynthetically active biomass, but the reduction in constitutive isoprene emissions and the increase in induced volatile emissions primarily reflected changes in the activities of corresponding biochemical pathways. Although all physiological alterations in fungal-infected leaves occurred in a stress severity-dependent manner, modifications in primary and secondary metabolic pathways scaled differently due to contrasting operational mechanisms.