Foliar Monoterpenes Research Articles

Presence of Quercus Suber Soft-Leaf Defoliators on Trees with Distinct Foliar Monoterpene Emission Profiles

The cork tree, Quercus suber L., is a characteristic species of the Dehesa agrosilvopastoral system, typical of western Spain. Defoliating insects are an important component of these ecosystems. This study assessed the presence and impact of defoliators feeding on Q. suber soft leaf tissue and their relationship with foliar monoterpene emission profiles. Samplings consisted of weekly tree beating (15 times per tree per sample) during the flight period of key species. We studied 26 cork trees with known profiles of foliar monoterpene emission (13 pinene and 13 limonene chemotypes). We identified a total of 272 larvae from 9 species. The main species were Catocala nymphagoga Esper (40.8%) and Periclista andrei Know (27.6%). Notably, 70.6% of larvae were found on trees with a pinene chemotype. The combined abundance of the four key defoliator species (C. nymphagoga L., P. andrei, Bena bicolorana L., and Cyclophora punctaria L.) was 62.7% lower on limonene-chemotype trees than pinene-chemotype trees. Significant differences were found in defoliation damage between leaves with distinct terpene emission profiles. These results suggest that both the abundance and damage caused by defoliators differ with trees’ emission profiles, and this may indicate differences in palatability and/or nutritional quality between Q. suber trees with distinct foliar monoterpene emission profiles.

Growth and actual leaf temperature modulate CO2responsiveness of monoterpene emissions from holm oak in opposite ways

Abstract. Climate change can profoundly alter volatile organic compound (VOC) emissions from vegetation and thus influence climate evolution. Yet, the short- and long-term effects of elevated CO2 concentrations on emissions in interaction with temperature are not well enough understood, especially for VOCs other than isoprene. To gain additional insight, we conducted a study on holm oak, which is known for its strong foliar monoterpene emissions that are directly linked to their synthesis. We measured CO2-response curves of emissions, CO2–H2O gas exchanges and chlorophyll fluorescence at two assay temperatures (30 and 35 ∘C) on saplings of four populations grown under normal and double CO2 concentrations combined with two temperature growth regimes differing by 5 ∘C (day/night: 25/15 and 30/20 ∘C). A stepwise reduction in CO2 resulted in a decrease in emissions, occasionally preceded by an increase, with the overall decrease in emissions being greater at 35 ∘C than at 30 ∘C assay temperature. During ramping to high CO2, emissions remained mostly unchanged at 35 ∘C, whereas at 30 ∘C they often dropped, especially at the highest CO2 levels (≥ 1200 ppm). In addition to the actual leaf temperature, the high CO2 responsiveness of emissions was modulated by the plant's growth temperature with warm-grown plants being more sensitive than cool-grown plants. In contrast, growth CO2 had no significant effect on the CO2 sensitivity of emissions, although it promoted plant growth and the leaf emission factor. Correlation analyses suggest that the emission response to CO2 depended primarily on the availability of energetic cofactors produced by photosynthetic electron transport. This availability was likely limited by different processes that occurred during CO2 ramping including photooxidative stress and induction of protective and repair mechanisms as well as competition with CO2 fixation and photorespiration. In addition, feedback inhibition of photosynthesis may have played a role, especially in leaves whose emissions were inhibited only at very high CO2 levels. Overall, our results confirm an isoprene-analogous behavior of monoterpene emissions from holm oak. Emissions exhibit a non-linear response curve to CO2 similar to that currently used for isoprene emission in the MEGAN model, with no difference between major individual monoterpene species and plant chemotype. Simulations estimating the annual VOC releases from holm oak leaves at double atmospheric CO2 indicate that the observed high-CO2 inhibition is unlikely to offset the increase in emissions due to the predicted warming.

Electroantennographic Responses of Cerambyx welensii Küster to Host-Related Volatiles

Wood-boring insects, such as Cerambyx welensii Küster, are involved in oak decline in Mediterranean areas. To advance our understanding of the olfactory perception of C. welensii, we recorded electroantennographic (EAG) responses from male and female antennae to 32 tree volatile organic compounds typical of emissions from its main Quercus L. hosts, and also analysed the dose-dependent response. Cerambyx welensii antennae responded to 24 chemicals. Eight odorants elicited the highest EAG responses (normalized values of over 98%): 1,8-cineole, limonene-type blend, β-pinene, pinene-type blend, sabinene, α-pinene, turpentine and (E)-2-hexenal. Cerambyx welensii exhibits a broad sensitivity to common tree volatiles. The high EAG responses to both limonene- and pinene-type blends suggest the detection of specific blends of the main foliar monoterpenes emitted by Q. suber L. and Q. ilex L. (limonene, α- and β-pinene, sabinene and myrcene), which could influence the intraspecific host choice by C. welensii, and in particular, females may be able to detect oak trees with a limonene-type chemotype. In addition, C. welensii showed high antennal activity to some odorants that characterize emissions from non-host tree species (1,8-cineole, β-pinene, α-pinene, turpentine, δ3-carene and camphene). The results obtained may be applicable to optimize monitoring and mass-trapping programmes in an integrated pest management context.

Toxicity to Douglas-Fir Tussock Moth and Foliar Concentration of Individual Monoterpenes in Douglas-Fir Following Fertilization in Thinned Stands

Tree susceptibility and suitability for herbivorous insects depends upon a wide array of chemical compounds including potential toxins such as monoterpenes. Silvicultural techniques such as thinning and fertilization may change the concentration of these compounds within tree tissue foliage. The relative toxicities to Douglas-fir tussock moth larvae of five monoterpenes commonly present in host foliage were determined in laboratory assays. Of the five monoterpenes tested, limonene and γ-terpinene were significantly more toxic to the larvae then β-pinene, 3-carene or α-pinene. We also examined the effect of three fertilization treatments applied to previously thinned stands on the concentration of foliar monoterpenes one year following stand thinning. No significant differences were detected among treatments in the concentration of individual or total monoterpenes present in foliage. The effect of stand thinning may have overwhelmed any short-term fertilization impacts.

Monoterpene emission of Quercus suber L. highly infested by Cerambyx welensii Küster

Cork oaks highly infested by Cerambyx welensii emit an amount of limonene at dusk, when C. welensii adults become active. In contrast, emissions by neighboring cork oaks free of C. welensii are dominated by pinene-type compounds. The activity of the woodborer Cerambyx welensii Kuster is a key factor in the decline of Quercus suber L. dehesas. This study aimed to estimate whether trees highly infested by C. welensii exhibited a peculiar emission profile, with known antennally active compounds. Monoterpenes were sampled in situ in 2006 (day/late evening) and 2008 (early evening) from Q. suber stratified by whether or not trees were highly infested by C. welensii and analyzed by gas chromatography. Limonene, α-pinene, β-pinene, sabinene, and myrcene accounted for over 87.2% of overall monoterpene emissions. Monoterpene composition and emission rates differed between the two groups, both during daytime and early evening, with a high presence of limonene in infested trees and dominance of pinene-type compounds in non-infested trees. This work evidenced differences in foliar monoterpene emissions between Q. suber trees highly infested by C. welensii and non-infested trees, with a high presence of limonene in the former and dominance of pinene-type compounds in non-infested trees. We hypothesize that the detection—especially during the onset of insects daily flight—of certain compounds (e.g., limonene), together with the detection of specific ratios of several monoterpenes (e.g., those of limonene to pinene-type compounds), has a role in the intraspecific host selection by C. welensii.

The role of simulated spring water stress in interactions between eastern larch and larch casebearer

Water stress can influence the ability of plants to tolerate and resist herbivory and indirectly mediate inset fitness. Larch casebearer, Coleophora laricella Hubner (Lepidoptera: Coleophoridae), is an invasive defoliator in North America where it infests eastern larch, Larix laricina (Du Roi) K. Koch (Pinaceae). Anomalous outbreaks of larch casebearer have been detected each year since 2000 in Minnesota, USA. In Minnesota, eastern larch typically occurs in peatland bogs or fens with complex hydrology. Given the potential for global climate change to alter precipitation and the seasonal flooding dynamics of eastern larch stands, we investigated the role of simulated waterlogging and drought on eastern larch–larch casebearer interactions over two years. We quantified the growth, survival, and foliar monoterpene concentrations of juvenile eastern larches in response to varying watering regimens and challenge from larch casebearer. We also quantified how watering regimen and monoterpene concentrations affected the survival of fourth instar larch casebearers to adulthood. The growth and survival of eastern larch was negatively impacted by challenge from larch casebearer, waterlogging, and drought, though the strength of responses varied between years. The monoterpene concentrations in eastern larch foliage did not change in response to water stress or challenge from larch casebearer but the within-year concentrations of several monoterpenes decreased with time. No consistent patterns emerged in the response of larch casebearer to watering regimen or monoterpenes. In summary, it appears that watering stress and defoliation do not interact to impact growth and survival of juvenile eastern larches, but rather act independently.

De novo post-illumination monoterpene burst in Quercus ilex (holm oak).

Explicit proof for de novo origin of a rare post-illumination monoterpene burst and its consistency under low O 2 , shows interaction of photorespiration, photosynthesis, and isoprenoid biosynthesis during light-dark transitions. Quercus ilex L (holm oak) constitutively emits foliar monoterpenes in an isoprene-like fashion via the methyl erythritol phosphate (MEP) pathway located in chloroplasts. Isoprene-emitting plants are known to exhibit post-illumination isoprene burst, a transient emission of isoprene in darkness. An analogous post-illumination monoterpene burst (PiMB) had remained elusive and is reported here for the first time in Q. ilex. Using 13CO2 labelling, we show that PiMB is made from freshly fixed carbon. PiMB is rare at ambient (20%) O2, absent at high (50%) O2, and becomes consistent in leaves exposed to low (2%) O2. PiMB is stronger and occurs earlier at higher temperatures. We also show that primary and secondary post-illumination CO 2 bursts (PiCO2B) are sensitive to O2 in Q. ilex. The primary photorespiratory PiCO2B is absent under both ambient and low O2, but is induced under high (>50%) O2, while the secondary PiCO2B (of unknown origin) is absent under ambient, but present at low and high O2. We propose that post-illumination recycling of photorespired CO2 competes with the MEP pathway for photosynthetic carbon and energy, making PiMB rare under ambient O2 and absent at high O2. PiMB becomes consistent when photorespiration is suppressed in Q. ilex.

Do white spruce epicuticular wax monoterpenes follow foliar patterns?

We examine the extent to which foliar monoterpenes are trapped in the epicuticular waxes as part of an investigation into their role in natural defense against folivores. We monitored concentrations in white spruce (Picea glauca (Moench) Voss) previous-year foliage and expanding foliage and their epicuticular waxes over the 2010 (14 trees) and 2011 (25 trees) growing seasons. In 2010, concentrations were low in the spring and increased over the summer; in 2011, they stayed low. The monoterpene profile of individual trees was similar between years and showed a consistent pattern over the growing season; in expanding foliage, δ-3-carene was only present in spring, whereas bornyl acetate increased over the growing season. Individual wax monoterpene profiles correlated with those of foliage, but the total concentration showed a different phenological pattern. Total content remained constant throughout the growing season on previous-year foliage but decreased on expanding foliage. Electron microscopy suggests this is due to changes in stomatal wax plugs and their role in blocking evaporation from the stomata. These findings suggest that insects contacting the leaf surface will receive accurate information from the wax chemical composition about the monoterpene mix but not about overall monoterpene levels.

Variations in foliar monoterpenes across the range of jack pine reveal three widespread chemotypes: implications to host expansion of invasive mountain pine beetle.

The secondary compounds of pines (Pinus) can strongly affect the physiology, ecology and behaviors of the bark beetles (Coleoptera: Curculionidae, Scolytinae) that feed on sub-cortical tissues of hosts. Jack pine (Pinus banksiana) has a wide natural distribution range in North America (Canada and USA) and thus variations in its secondary compounds, particularly monoterpenes, could affect the host expansion of invasive mountain pine beetle (Dendroctonus ponderosae), which has recently expanded its range into the novel jack pine boreal forest. We investigated monoterpene composition of 601 jack pine trees from natural and provenance forest stands representing 63 populations from Alberta to the Atlantic coast. Throughout its range, jack pine exhibited three chemotypes characterized by high proportions of α-pinene, β-pinene, or limonene. The frequency with which the α-pinene and β-pinene chemotypes occurred at individual sites was correlated to climatic variables, such as continentality and mean annual precipitation, as were the individual α-pinene and β-pinene concentrations. However, other monoterpenes were generally not correlated to climatic variables or geographic distribution. Finally, while the enantiomeric ratios of β-pinene and limonene remained constant across jack pine's distribution, (−):(+)-α-pinene exhibited two separate trends, thereby delineating two α-pinene phenotypes, both of which occurred across jack pine's range. These significant variations in jack pine monoterpene composition may have cascading effects on the continued eastward spread and success of D. ponderosae in the Canadian boreal forest.

The Yield of Essential Oils in Melaleuca alternifolia (Myrtaceae) Is Regulated through Transcript Abundance of Genes in the MEP Pathway

Medicinal tea tree (Melaleuca alternifolia) leaves contain large amounts of an essential oil, dominated by monoterpenes. Several enzymes of the chloroplastic methylerythritol phosphate (MEP) pathway are hypothesised to act as bottlenecks to the production of monoterpenes. We investigated, whether transcript abundance of genes encoding for enzymes of the MEP pathway were correlated with foliar terpenes in M. alternifolia using a population of 48 individuals that ranged in their oil concentration from 39 -122 mg.g DM−1. Our study shows that most genes in the MEP pathway are co-regulated and that the expression of multiple genes within the MEP pathway is correlated with oil yield. Using multiple regression analysis, variation in expression of MEP pathway genes explained 87% of variation in foliar monoterpene concentrations. The data also suggest that sesquiterpenes in M. alternifolia are synthesised, at least in part, from isopentenyl pyrophosphate originating from the plastid via the MEP pathway.

Phytochemical variation within a single plant species influences foraging behavior of deer

To determine how black‐tailed deer Odocoileus hemionus columbianus respond to phytochemical cues while browsing in heterogeneous phytochemical environments, we offered captive and free‐range deer cloned rooted cuttings and seedlings of western redcedar Thuja plicata selected for varying monoterpene content. Black‐tailed deer were thus allowed to browse among a controlled array of phytochemical cues in a series of experiments designed to evaluate foraging behavior at fine (within plot) and coarse (plot selection) scales. Within‐plot diet selection experiments demonstrated that browse preference for individual western redcedar plants was a function of foliar monoterpene concentration. Individual plant palatability combined with momentary maximization foraging strategy promoted survival of heavily defended plants. Among‐plot foraging experiments demonstrated that coarse‐scale foraging preferences were strongly influenced by distributions of high monoterpene‐containing western redcedar in available plots. Olfaction may play a significant role in both fine and coarse‐scale browse behaviors of deer as they employ a risk‐averse foraging strategy.

Ungulate herbivory of regenerating conifers in relation to foliar nutrition and terpenoid production

Ungulate browsing greatly influences regeneration dynamics of some forest ecosystems, yet the relationship between browse susceptibility and foliar chemistry of forest tree seedlings is not well understood. We applied field fertilization (15N-9P-10K controlled-release fertilizer at 0, 20, 40, and 60 g per seedling) and investigated how subsequent changes in terpenoid production and foliar nutrition influence ungulate browse preference for Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco), western hemlock ( Tsuga heterophylla Raf. Sarg.), and western redcedar ( Thuja plicata Donn ex D. Don) seedlings across four sites in northwestern Oregon, USA. Fertilization increased foliar N concentration of all three species, but above-ground growth of only Douglas-fir and western hemlock. Foliar monoterpene concentrations for western hemlock and western redcedar also increased at higher fertilization rates, while Douglas-fir monoterpene production was not affected by fertilization. Regardless of monoterpene levels, ungulate browse preference was greater for fertilized western hemlock seedlings. The opposite response, however, was observed for western redcedar at two of the four study sites where the likelihood of browse was greater for non-fertilized than fertilized seedlings. Differences in browse preference among species may depend on the type and amount of individual monoterpenes manufactured in response to fertilization. Western redcedar produce α- and β-thujone, oxygenated monoterpenes known to promote gastroenteritis and possibly inhibit microbial rumen activity, which were absent from Douglas-fir or western hemlock. Higher concentrations of α- and β-thujone associated with increasing fertilizer rate provide a plausible explanation as to why ungulates preferred non-fertilized western redcedar. Our results illustrate species-specific adaptation in browse avoidance and selective ungulate browsing behavior of individual trees as linked to foliar chemistry.

Role of Volatile and Non-Volatile Plant Secondary Metabolites in Host Tree Selection by Christmas Beetles

Individual Eucalyptus trees in south-eastern Australia vary considerably in susceptibility to herbivores. On the one hand, studies with insect herbivores have suggested that variation in the concentrations of foliar monoterpenes is related to variation in susceptibility. On the other, studies with marsupial folivores have suggested that variation in the concentrations of sideroxylonals (a group of formylated phloroglucinol compounds) is responsible for variation in susceptibility. We examined relative importance of sideroxylonals and 1,8-cineole (a dominant monoterpene) in host tree selection by Christmas beetles (Anoplognathus species: Coleoptera: Scarabaeidae) by using no-choice experiments, choice/no-choice experiments, and manipulative experiments in which concentrations of sideroxylonals or 1,8-cineole were altered. We used two species of host Eucalyptus, one species of non-host Eucalyptus, and three species of non-host non-Eucalyptus trees. Leaf consumption by Christmas beetles was negatively correlated with the concentrations of sideroxylonals and 1,8-cineole. Artificial increases in the concentration of sideroxylonals or 1,8-cineole reduced leaf consumption by Christmas beetles. An artificial reduction in foliar monoterpenes had no effect on leaf consumption by the beetles when leaves contained high or very low concentrations of sideroxylonals. However, when the concentration of sideroxylonals was moderate, a reduction in the foliar monoterpenes increased leaf consumption by the beetles. Therefore, monoterpenes such as 1,8-cineole may be used as a negative cue by Christmas beetles. The pattern of food consumption on non-host Eucalyptus species and non-host non-Eucalyptus species suggest that both positive and negative cues may be used by Christmas beetles to select host trees.

Silvicultural Attempts to Induce Browse Resistance in Conifer Seedlings

A multiyear study was conducted to determine if soil amendment combined with topical application of elemental sulfur could be employed to reduce deer browse damage to four conifer species. Fertilizer and sulfur were applied to conifer seedlings at seven sites near Corvallis, OR. Growth and browse damage data were collected for all seedlings over a period of 17 months. Additionally, foliar concentrations of monoterpenes and simple carbohydrates were assessed in western redcedar (Thuja plicata) seedlings over a period of three years. Fertilization and sulfur treatments had a moderate impact on growth and no influence on browse damage or the chemical responses. Over the course of the study, browse damage diminished while foliar monoterpene concentrations increased in redcedar. It appears that silvicultural manipulation via sulfur application and/or soil amendment cannot accelerate or alter the ontogenetical changes that may naturally defend seedlings against mammalian herbivores. In a brief trial with captive deer, redcedar browse resistance was influenced by seedling maturation, but not monoterpene content. Other maturation effects may yield significant browse protection to young seedlings.

A Biochemical Interpretation of Terpene Chemotypes in Melaleuca alternifolia

The variation of foliar monoterpenes in the Australian Tea Tree (Melaleuca alternifolia) has been of significant interest both to the essential oil industry as well as to ecologists. The majority of studies on leaf chemistry have been aimed directly towards obtaining oil of higher quality or quantity. In the current study, we aimed to understand how molecular mechanisms contribute to the chemical variability of this species, based on chemical analysis of the leaf oils from a biochemical perspective. Correlations between monoterpenes across the species as well as within chemotypes show strong, persistent patterns, which enable us to establish groups based on possible common biosynthetic origins. We found that three distinct enzymes corresponding to these groups: a sabinene-hydrate synthase, a 1,8-cineole synthase, and a terpinolene synthase may be sufficient to explain all six chemotypes in M. alternifolia.

Interannual and seasonal changes in the soil exchange rates of monoterpenes and other VOCs in a Mediterranean shrubland

SummaryInformation about soil VOC inventories and exchange rates in different soils is very scarce. Seasonality of soil VOC exchange rates is also largely unknown, despite the increasing interest in some soil volatile compounds, such as monoterpenes, because of their important role in soil ecology. We aimed to explore and quantify soil VOC exchange rates in a Mediterranean shrubland and their seasonality. Measurements of soil VOC exchange were taken using GC‐MS and PTR‐MS techniques, together with soil temperature, soil moisture and soil CO2 efflux measurements, during two annual campaigns with contrasting precipitation. Methanol, acetic acid, ethyl acetate, acetaldehyde, acetone, C3 and C4 carbonyls (such as methyl ethyl ketone), α‐pinene and limonene, showed the highest emission rates. Maximum soil monoterpene emission rates were very low (0.003 nmol m−2 s−1) compared with foliar monoterpene emission rates. The emission rates of the other VOCs were also low (maximum 0.8 nmol m−2 s−1) except for methanol (1.2 nmol m−2 s−1). Maximum soil uptake rates for some VOCs, such as methanol and acetonitrile (ranging from −0.1 to −0.5 nmol m−2 s−1) were, however, comparable with foliar uptake rates. Further studies are needed to corroborate these results and the possible importance of the soil VOC sink in regional chemistry‐climate models. Long‐term severe drought increased soil monoterpene emission rates in this Mediterranean shrubland. The increases seem to be linked to changes in the soil’s physical properties induced by low soil moisture. Unlike monoterpenes, other soil VOC emission rates decreased when soil moisture was low. The results suggest a seasonal control of soil temperature on the emission rates of monoterpenes and other VOCs. The emission rates increase with soil temperature. Positive correlations between the VOC exchange rates and the soil CO2 fluxes suggest that phenology of roots and microorganisms also controls seasonal changes in soil VOCs in this Mediterranean shrubland.

Causes and Consequences of Host Expansion by Mnesampela privata

The autumn gum moth, Mnesampela privata, utilizes several species of Eucalyptus planted outside regions of endemism within Australia. We investigated whether foliar monoterpene composition influenced oviposition in the field on the natural primary host (E. globulus) and a novel host (E. rubida), both characterized by nonstructural epicuticular waxes. In the laboratory, oviposition preferences of females for species and families of known host, novel hosts, and non-hosts that were characterized by both nonstructural and structural waxes but also varied in foliar concentrations of the purportedly toxic plant secondary metabolite (sideroxylonal) were studied. Although M. privata laid as many eggs on trees of two families of E. rubida as they did on trees of two families of E. globulus, there were significant differences in the numbers of clutches of eggs laid. When combined with data for oviposition on another five families of E. globulus, we found a negative relationship between mean numbers of eggs and foliar concentration of alpha-pinene but a positive relationship between egg numbers and the concentration of alpha-terpineol. The field data suggest that female M. privata are just as willing to lay eggs on novel hosts with comparable foliar monoterpene compositions to those of the primary host, especially if they produce nonstructural epicuticular waxes. Oviposition assays in the laboratory endorse this mechanism of host plant hierarchy and support the long-held assumption of the host primacy of E. globulus. In laboratory assays, some larvae pupated on all hosts (except Corymbia eximia) but the number completing larval development was greater on hosts with softer leaves. Larval survival was also reduced on hosts with high concentrations of sideroxylonal but only if those hosts also had modest to high concentrations of monoterpenes. Larval survival was high on a host (E. macarthurii) with a high concentration of sideroxylonal but with virtually zero monoterpene content. This suggests that the monoterpene content of a host could antagonize the effect on M. privata larvae of its sideroxylonal content. The larval food plant most affected the fitness of female rather than male pupae. Of the known host expansion events, all have occurred in mixed species plantations. The co-occurrence in these plantations of either the primary host or other highly ranked species probably explains the eventual expansion onto the neighboring species of Eucalyptus and Corymbia.

How growth conditions affect the capacity of Quercus ilex leaves to emit monoterpenes

Summary • The effects of climate conditions on foliar monoterpene emissions from Quercus ilex saplings were studied in new leaves grown at two temperature regimes combined with two light regimes and by acclimatizing them to new regimes once leaves were mature. • Both high light and warmth strongly promoted the foliar emission capacity, ES (i.e. the steady-state emission rate measured at standard conditions), of developing and mature leaves, but did not significantly change the emission composition. The ES values of leaves adapted to warm and high-light conditions were one order of magnitude higher than that of leaves adapted to cool and shaded conditions. • The acclimatizations of ES in mature leaves to new regimes proceeded over a few days to a few weeks and were often paralleled by changes in photosynthesis. Under unchanged regimes, the ES of mature leaves remained stable in high-light adapted leaves and decreased somewhat in shade adapted leaves. • The findings suggest that the spatial variation in foliar ES within a tree canopy results from the direct influence of the leaf microclimate rather than from leaf structural properties, and that in the course of the year, the ES of leaves will repeatedly be upregulated and downregulated according to the prevailing temperature and light conditions, which may be accompanied by a slight attenuation of ES related to leaf age.

A model coupling foliar monoterpene emissions to leaf photosynthetic characteristics in Mediterranean evergreen Quercus species

Summary A model was developed and parameterized to simulate monoterpene emission rates per unit leaf area (E) dependent on foliar photosynthetic potentials and activity of monoterpene synthases in the evergreen sclerophylls Quercus coccifera L. and Q. ilex L. Assuming that total activity of monoterpene synthases controls the pathway flux, the product of leaf dry mass per unit area E was culculated as the fraction of total electron flow used for monoterpene synthesis (ɛ), the rate of photosynthetic electron transport (J) per unit leaf dry mass and the reciprocal of the electron cost of monoterpene synthesis. In the model, light effects on E result from light responses of J, and the temperature relationship of E combines temperature dependencies of J and the specific activity of monoterpene synthase (SS). Having determined J from leaf photosynthesis data and deriving an estimate of SS from in vitro laboratory measurements, good fits to diurnal time‐courses of monoterpenoid emission rates were obtained using a single leaf‐dependent coefficient, the total monoterpene synthase activity in the leaves. Our analysis demonstrates that using J as a surrogate of E leads to a realistic description of E, especially under stress conditions where the previous models fail. However, analysis of daily time courses of E indicated that storage of monoterpenoids in nonspecific leaf compartments might alter the correspondence between monoterpenoid synthesis and emission rates, especially after rapid environmental change.

Testing the ecophysiological basis for the control of monoterpene concentrations in thinned and unthinned balsam fir stands across different drainage classes.

Stand thinning across different soil drainage types was used to test the ecophysiological basis for the control of foliar monoterpenes in current-year foliage of balsam trees [Abies balsamea (L.) P. Mill.] the year following treatment. Photosynthetic capacity (A max) was greater on mesic sites than on sub-hygric and hydric sites (1.81, 1.33 and 0.88 µmol m-2 s-1, respectively) and greater in mid-July than at the end of June (1.86 vs 0.77 µmol m-2 s-1). Foliar N was greater on thinned plots than control plots (2.6 vs 2.2g m-2); starch was greater on mesic control plots than mesic thinned plots (16.3 vs 10.9g m-2); and total soluble sugars (TSS) were higher in mid-July than at the end of June (16.5 vs 12.8g m-2). Total monoterpenes, as well as five individual monoterpenes, showed a complex interaction between site drainage class, thinning treatment and time of sampling (P<0.05). For all sites and dates combined, total monoterpenes were negatively correlated with specific leaf area (r 2=0.46), and positively but weakly correlated with A max (r 2=0.35), foliar N (r 2=0.29) and TSS (r 2=0.24). Total monoterpenes were not correlated with the different measures of carbon to nitrogen ratio as predicted by the carbon-nutrient balance hypothesis. Our results offer partial support to the different hypotheses proposing that foliage having greater resource availability, enzymatic machinery and potential for building storage structures will have greater monoterpene content. However, the strength of the relationships with monoterpenes often varied with sampling date, suggesting that phenological development may play a strong role in determining which factors control monoterpenes at a given time.