Ray Fall

A global model of natural volatile organic compound emissions

Numerical assessments of global air quality and potential changes in atmospheric chemical constituents require estimates of the surface fluxes of a variety of trace gas species. We have developed a global model to estimate emissions of volatile organic compounds from natural sources (NVOC). Methane is not considered here and has been reviewed in detail elsewhere. The model has a highly resolved spatial grid (0.5° × 0.5° latitude/longitude) and generates hourly average emission estimates. Chemical species are grouped into four categories: isoprene, monoterpenes, other reactive VOC (ORVOC), and other VOC (OVOC). NVOC emissions from oceans are estimated as a function of geophysical variables from a general circulation model and ocean color satellite data. Emissions from plant foliage are estimated from ecosystem specific biomass and emission factors and algorithms describing light and temperature dependence of NVOC emissions. Foliar density estimates are based on climatic variables and satellite data. Temporal variations in the model are driven by monthly estimates of biomass and temperature and hourly light estimates. The annual global VOC flux is estimated to be 1150 Tg C, composed of 44% isoprene, 11% monoterpenes, 22.5% other reactive VOC, and 22.5% other VOC. Large uncertainties exist for each of these estimates and particularly for compounds other than isoprene and monoterpenes. Tropical woodlands (rain forest, seasonal, drought-deciduous, and savanna) contribute about half of all global natural VOC emissions. Croplands, shrublands and other woodlands contribute 10–20% apiece. Isoprene emissions calculated for temperate regions are as much as a factor of 5 higher than previous estimates.

Isoprene and monoterpene emission rate variability: Model evaluations and sensitivity analyses

The temperature dependence of monoterpene emission varies among monoterpenes, plant species, and other factors, but a simple exponential relationship between emission rate (E) and leaf temperature (T), E = Es [exp (β(T - Ts))], provides a good approximation. A review of reported measurements suggests a best estimate of β = 0.09 K-1 for all plants and monoterpenes. Isoprene emissions increase with photosynthetically active radiation up to a saturation point at 700-900 μmol m-2 s-1. An exponential increase in isoprene emission is observed at leaf temperatures of less than 30°C. Emissions continue to increase with higher temperatures until a maximum emission rate is reached at about 40°C, after which emissions rapidly decline. This temperature dependence can be described by an enzyme activation equation that includes denaturation at high temperature. -from Authors

Emissions of volatile organic compounds from vegetation and the implications for atmospheric chemistry

Vegetation provides a major source of reactive carbon entering the atmosphere. These compounds play an important role in (1) shaping global tropospheric chemistry, (2) regional photochemical oxidant formation, (3) balancing the global carbon cycle, and (4) production of organic acids which contribute to acidic deposition in rural areas. Present estimates place the total annual global emission of these compounds between approximately 500 and 825 Tg yr−1. The volatile olefinic compounds, such as isoprene and the monoterpenes, are thought to constitute the bulk of these emissions. However, it is becoming increasingly clear that a variety of partially oxidized hydrocarbons, principally alcohols, are also emitted. The available information concerning the terrestrial vegetation as sources of volatile organic compounds is reviewed. The biochemical processes associated with these emissions of the compounds and the atmospheric chemistry of the emitted compounds are discussed.

Biocontrol of Bacillus subtilis against Infection of Arabidopsis Roots by Pseudomonas syringae Is Facilitated by Biofilm Formation and Surfactin Production

Relatively little is known about the exact mechanisms used by Bacillus subtilis in its behavior as a biocontrol agent on plants. Here, we report the development of a sensitive plant infection model demonstrating that the bacterial pathogen Pseudomonas syringae pv tomato DC3000 is capable of infecting Arabidopsis roots both in vitro and in soil. Using this infection model, we demonstrated the biocontrol ability of a wild-type B. subtilis strain 6051 against P. syringae. Arabidopsis root surfaces treated with B. subtilis were analyzed with confocal scanning laser microscopy to reveal a three-dimensional B. subtilis biofilm. It is known that formation of biofilms by B. subtilis is a complex process that includes secretion of surfactin, a lipopeptide antimicrobial agent. To determine the role of surfactin in biocontrol by B. subtilis, we tested a mutant strain, M1, with a deletion in a surfactin synthase gene and, thus, deficient in surfactin production. B. subtilis M1 was ineffective as a biocontrol agent against P. syringae infectivity in Arabidopsis and also failed to form robust biofilms on either roots or inert surfaces. The antibacterial activity of surfactin against P. syringae was determined in both broth and agar cultures and also by live-dead staining methods. Although the minimum inhibitory concentrations determined were relatively high (25 μg mL-1), the levels of the lipopeptide in roots colonized by B. subtilis are likely to be sufficient to kill P. syringae. Our results collectively indicate that upon root colonization, B. subtilis 6051 forms a stable, extensive biofilm and secretes surfactin, which act together to protect plants against attack by pathogenic bacteria.

Evidence for glutathione peroxidase activities in cultured plant cells

Extracts from cultured plant cells of spinach, maize and sycamore and from Lemna plants contain detectable glutathione peroxidase activity, using either hydrogen peroxide or t-butyl hydroperoxide as substrates. Using extracts from cultured maize cells, two peaks of glutathione peroxidase activity could be resolved by a combination of gel filtration and ion exchange chromatography. One peak was eluted along with glutathione transferase activity; the second was distinct from both glutathione transferase and ascorbic acid peroxidase, and was active with both hydrogen peroxide and organic hydroperoxides. It seems likely that at least two enzymes with glutathione peroxidase activity exist in higher plant cells.

Increased CO2 uncouples growth from isoprene emission in an agriforest ecosystem.

The emission of isoprene from the leaves of forest trees is a fundamental component of biosphere–atmosphere interactions, controlling many aspects of photochemistry in the lower atmosphere. As almost all commercial agriforest species emit high levels of isoprene, proliferation of agriforest plantations has significant potential to increase regional ozone pollution and enhance the lifetime of methane, an important determinant of global climate. Here we show that growth of an intact Populus deltoides plantation under increased CO2 (800 µmol mol-1 and 1,200 µmol mol-1) reduced ecosystem isoprene production by 21% and 41%, while above-ground biomass accumulation was enhanced by 60% and 82%, respectively. Exposure to increased CO2 significantly reduced the cellular content of dimethylallyl diphosphate, the substrate for isoprene synthesis, in both leaves and leaf protoplasts. We identify intracellular metabolic competition for phosphoenolpyruvate as a possible control point in explaining the suppression of isoprene emission under increased CO2. Our results highlight the potential for uncoupling isoprene emission from biomass accumulation in an agriforest species, and show that negative air-quality effects of proliferating agriforests may be offset by increases in CO2.

Leaf methanol — the simplest natural product from plants

Current interest in the chemistry of the atmosphere has led to the identification of methanol as one of the major organic compounds in forest air and in the troposphere. Analysis of the emissions of volatile organic compounds from leaves has revealed that most plants emit methanol, especially during early stages of leaf expansion — it is probably produced as a by-product of pectin metabolism during cell wall synthesis, and a fraction of this pool is then emitted through stomata during transpiration. There is also evidence that leaf methanol supports an abundant population of epiphytic bacterial methylotrophs. Application of methanol to leaves enhances the growth and yield of C 3 plants, although the mechanism responsible for this phenomenon is uncertain.

Volatile organic compounds emitted after leaf wounding: On‐line analysis by proton‐transfer‐reaction mass spectrometry

Volatile organic compounds (VOCs) released from vegetation, including wound-induced VOCs, can have important effects on atmospheric chemistry. The analytical methods for measuring wound-induced VOCs, especially the hexenal family of VOCs (hexenals, hexenols, and hexenyl esters), are complicated by their chemical instability and the transient nature of their formation after leaf and stem wounding. Here we demonstrate that formation and emission of hexenal family compounds can be monitored on-line using proton-transfer-reaction mass spectrometry (PTR-MS), avoiding the need for preconcentration or chromatography. These measurements allow direct analysis of the rapid emission of the parent compound, (Z)-3-hexenal, within 1-2 s of wounding of aspen leaves and then its disappearance and the appearance of its metabolites including (E)-2-hexenal, hexenols, and hexenyl acetates. Similar results were seen in wounded beech leaves and clover. The emission of hexenal family compounds was proportional to the extent of wounding, was not dependent on light, occurred in attached or detached leaves, and was greatly enhanced as detached leaves dried out. Emission of (Z)-3-hexenal from detached drying aspen leaves averaged 500 m gCg 21 (dry leaf weight). Leaf wound compounds were not emitted in a nitrogen atmosphere but were released within seconds of reintroduction of oxygen; this indicates that there are not large pools of hexenyl compounds in leaves. The PTR-MS method also allows the simultaneous detection of less abundant hexanal family VOCs including hexanal, hexanol, and hexyl acetate and VOCs formed in the light (isoprene) or during anoxia (acetaldehyde). PTR- MS may be a useful tool for the analysis of VOC emissions resulting from grazing, herbivory, and other physical damage to vegetation, from harvesting of crops, and from senescing leaves.

Methanol Emission from Leaves (Enzymatic Detection of Gas-Phase Methanol and Relation of Methanol Fluxes to Stomatal Conductance and Leaf Development)

We recently reported the detection of methanol emissions from leaves (R. MacDonald, R. Fall [1993] Atmos Environ 27A: 1709–1713). This could represent a substantial flux of methanol to the atmosphere. Leaf methanol production and emission have not been investigated in detail, in part because of difficulties in sampling and analyzing methanol. In this study we used an enzymatic method to convert methanol to a fluorescent product and verified that leaves from several species emit methanol. Methanol was emitted almost exclusively from the abaxial surfaces of hypostomatous leaves but from both surfaces of amphistomatous leaves, suggesting that methanol exits leaves via stomates. The role of stomatal conductance was verified in experiments in which stomates were induced to close, resulting in reduced methanol. Free methanol was detected in bean leaf extracts, ranging from 26.8 [mu]g g-1 fresh weight in young leaves to 10.0 [mu]g g-1 fresh weight in older leaves. Methanol emission was related to leaf development, generally declining with increasing leaf age after leaf expansion; this is consistent with volatilization from a cellular pool that declines in older leaves. It is possible that leaf emission could be a major source of methanol found in the atmosphere of forests.

Environmental and developmental controls over the seasonal pattern of isoprene emission from aspen leaves

Isoprene emission from plants represents one of the principal biospheric controls over the oxidative capacity of the continental troposphere. In the study reported here, the seasonal pattern of isoprene emission, and its underlying determinants, were studied for aspen trees growing in the Rocky Mountains of Colorado. The springtime onset of isoprene emission was delayed for up to 4 weeks following leaf emergence, despite the presence of positive net photosynthesis rates. Maximum isoprene emission rates were reached approximately 6 weeks following leaf emergence. During this initial developmental phase, isoprene emission rates were negatively correlated with leaf nitrogen concentrations. During the autumnal decline in isoprene emission, rates were positively correlated with leaf nitrogen concentration. Given past studies that demonstrate a correlation between leaf nitrogen concentration and isoprene emission rate, we conclude that factors other than the amount of leaf nitrogen determine the early-season initiation of isoprene emission. The late-season decline in isoprene emission rate is interpreted as due to the autumnal breakdown of metabolic machinery and loss of leaf nitrogen. In potted aspen trees, leaves that emerged in February and developed under cool, springtime temperatures did not emit isoprene until 23 days after leaf emergence. Leaves that emrged in July and developed in hot, midsummer temperatures emitted isoprene within 6 days. Leaves that had emerged during the cool spring, and had grown for several weeks without emitting isoprene, could be induced to emit isoprene within 2 h of exposure to 32°C. Continued exposure to warm temperatures resulted in a progressive increase in the isoprene emission rate. Thus, temperature appears to be an important determinant of the early season induction of isoprene emission. The seasonal pattern of isoprene emission was examined in trees growing along an elevational gradient in the Colorado Front Range (1829–2896 m). Trees at different elevations exhibited staggered patterns of bud-break and initiation of photosynthesis and isoprene emission in concert with the staggered onset of warm, springtime temperatures. The springtime induction of isoprene emission could be predicted at each of the three sites as the time after bud break required for cumulative temperatures above 0°C to reach approximately 400 degree days. Seasonal temperature acclimation of isoprene emission rate and photosynthesis rate was not observed. The temperature dependence of isoprene emission rate between 20 and 35°C could be accurately predicted during spring and summer using a single algorithm that describes the Arrhenius relationship of enzyme activity. From these results, it is concluded that the early season pattern of isoprene emission is controlled by prevailing temperature and its interaction with developmental processes. The late-season pattern is determined by controls over leaf nitrogen concentration, especially the depletion of leaf nitrogen during senescence. Following early-season induction, isoprene emission rates correlate with photosynthesis rates. During the season there is little acclimation to temperature, so that seasonal modeling simplifies to a single temperature-response algorithm.