Ecology and evolution of light-dependent and light-independent phytogenic volatile organic carbon


  • Manuel Lerdau,

    Corresponding author
    1. Ecology and Evolution, State University of New York, Stony Brook, NY 11794–5245
      Author for correspondence: Manuel Lerdau Tel: +1 631 632 6633 Fax: +1 631 632 7626 Email:
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  • Dennis Gray

    1. Ecology and Evolution, State University of New York, Stony Brook, NY 11794–5245
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Author for correspondence: Manuel Lerdau Tel: +1 631 632 6633 Fax: +1 631 632 7626 Email:



  • Summary 199

  • I. Introduction 199
  • II. Light-independent emissions 200
  • III. Light-dependent emissions 201
  • IV. Regulation of production 202
  • V. Possible functions of light-dependent VOCs 204
  • VI. Evolutionary aspects of phytogenic VOC 206
  • Acknowledgements 206

  • References 209


The low molecular weight hydrocarbons produced by plants form a uniquely exciting group of compounds. Produced by a common biosynthetic route, they play multiple and complex roles in organismal, ecological, and atmospheric processes. While some of these compounds have clearly identified functions within plants, others are made for reasons not yet fully understood. Here, both light-dependent and light-independent emissions are reviewed, together with regulation of production and possible functions of light-dependent volatile organic carbon (VOC). In addition to issues regarding the phylogenetic origins of VOC emissions, the origins of the pivotal enzymes that give rise to the observed emission phenotypes are discussed. Studies on the evolution and regulation of their production and emission provide an amazing opportunity for scientists working from the molecular to the tropospheric scales to interact.

I. Introduction

In some ways, working on questions regarding the impacts of plants on the atmosphere in the current millennium is a satisfying activity. Many of the major theoretical issues regarding the regulation of trace gas exchange at the scale of leaves, whole plants, and canopies have been successfully addressed, and major progress has been made in linking these scales for two of the most important scalars, carbon dioxide and water (Randerson et al., 1996; Thompson et al., 1996; Malmstrom et al., 1997; Randerson et al., 1999). Numerous researchers have developed and applied models of trace gas flux that incorporate both physical and physiological control processes, and, to a large extent, much of the technology necessary for testing such models is commercially available (Field et al., 1995; Waring et al., 1995). Perhaps most importantly, the scientific community has done an excellent job of persuading the funding community of the importance of this research so that some of the largest awards in ecology in both North America and Europe recently have gone to studies examining CO2 and H2O exchange between ecosystems and the atmosphere.

By contrast to this advanced state of understanding regarding CO2 and H2O exchange, we are in the early stages of developing multiscale models for the impact of plants on the atmosphere through the production and emission of volatile organic carbon (VOC). The last few years have seen major advances in our understanding of both the biological regulation of these fluxes and the massive effects these fluxes have on the redox balance of the lower atmosphere. Studies on the biochemistry, physiology, ecology, and evolution of VOC production and emission have advanced to the stage where integration across scales is, for the first time, a feasible endeavour. However, important fundamental questions remain.

The first studies on phytogenic emission of VOC's were conducted by plant physiologists seeking to understand the underlying biochemical processes of leaves (Sanadze, 1991) and the impacts that plants have on the atmosphere (Went, 1960). Very quickly, atmospheric scientists realized that these emissions played a central role in the redox balance of the atmosphere (Zimmerman et al., 1978). Studies on entire airsheds demonstrated that the dynamics of ozone, hydroxyl radical, carbon monoxide, and even methane could not be explained without inclusion of the plant-produced reactive compounds (Trainer et al., 1987; Chameides et al., 1988; Jacob & Wofsy, 1988).

The VOC whose atmospheric chemistry is best studied is isoprene (2-methyl, 1,3-butadiene) (Lerdau et al., 1997). During the daytime, isoprene is oxidized quickly, on the order of tens of minutes, by the hydroxyl radical, OH. Isoprene is so reactive that it provides the dominant sink for OH under many conditions. The initial oxidation leads to methacrolein and methyl vinyl ketone, two longer lived compounds whose further oxidation regulates ozone dynamics and whose final (on the time scale of weeks to months) oxidation leads to carbon monoxide. Carbon monoxide has an atmospheric lifetime of approximately 6 months, with its usual fate being oxidation to CO2. When isoprene oxidation occurs in a region with elevated NOx concentrations (a result of fossil fuel combustion), this oxidation to CO leads to a net production of tropospheric ozone. When, however, isoprene oxidation occurs in air with low levels of NOx, then ozone is consumed (this chemistry is thoroughly reviewed in Fuentes et al. (2000).

Although isoprene atmospheric chemistry has received the most attention, important roles for other VOC's have also been identified. Monoterpenes react homogeneously along pathways similar to isoprene, and they are also involved with heterogeneous reactions that can lead to aerosol production (Novakov & Penner, 1993; Fuentes et al., 2001). Methylbutenol, a compound discussed in detail below, is the main source of atmospheric acetone in the intermountain west of the United States (Goldstein & Schade, 2000). In addition to these chemical roles, VOC's have been identified as major components of cloud condensation nuclei. That is, plants can both transpire water that will later fall as rain and provide the physical basis for the cloud formation necessary for that rainfall.

Coincident with this appreciation for the roles phytogenic VOC's were playing in atmospheric processes, physiological studies began to discern the environmental regulation of these compounds (Tyson et al., 1974; Tingey et al., 1979; Tingey et al., 1981). These efforts served as a prelude to detailed efforts to understand the regulation of VOC's from the ecological through the biochemical scales (Monson & Fall, 1989; Hewitt et al., 1990b; Loreto & Sharkey, 1990; Lerdau et al., 1995). One of the most significant first results was the realization that, on time scales of seconds to hours, certain VOC’s, for example, the mono- and sesquiterpenes, were emitted independently of irradiance and with an exponential relationship to temperature that fit with simple Henry's Laws predictions of vapor pressure effects (Tyson et al., 1974; Tingey et al., 1979; Tingey et al., 1981, but see below). By contrast, emission of the single most abundant VOC, isoprene, depends closely on irradiance, with isoprene emission ceasing within minutes of leaf's being placed in darkness (Rasmussen & Jones, 1973; Jones & Rasmussen, 1975).

II. Light-independent emissions

The light-independent emissions have proven the easiest to understand from both biochemical and functional perspectives. Most of the effort on light-independent emissions has focused on mono- rather than sesquiterpenes because of the former's greater volatility and, consequently, greater importance in atmospheric processes. A simple model of production, storage, and diffusion/volatilization suffices to explain these light independent emissions from both herbaceous and woody taxa (Tyson et al., 1974; Lerdau, 1991). In this model, monoterpenes are produced and stored in a pool within a specialized structure such as a glandular trichome, resin canal, or duct. The terpenes volatilize according to their vapour pressure, which is set by three factors, the physical structure of the compound, its concentration within the pool (Henry's Law), and the temperature of that pool. The terpenes diffuse from their pool at a rate inversely proportional to the resistance along the flux pathway. Detailed physiological studies using two-sided cuvettes demonstrated that monoterpenes exited both sides of hypostomatous leaves, explaining the observed lack of stomatal regulation over emission (Guenther et al., 1991). In an elegant anatomical confirmation of this study, Schmidt et al. (1992) measured the partitioning coefficient of terpenes in cuticles and found that terpenes could pass through cuticles as they diffused to the atmosphere.

One of the important ecological predictions arising from this model is the major role that mechanical damage to plant tissues can play in regulating terpene fluxes to the atmosphere (through the effect of damage on the resistance to flux). These predictions received a detailed test through insect feeding experiments on conifers, and the results clearly show that insect damage on these conifers has the potential to alter the redox potential of the lower atmosphere (Litvak et al., 1999). In ecosystems such as northern coniferous forests, where terpene-producing taxa such as conifers often dominate and where large-scale pest outbreaks occur, insects have a mensurable impact on atmospheric chemistry during those times of year when temperatures are high enough to raise vapor pressures and cause significant volatilization.

Another prediction from this model is that significant terpene emissions ought to be seen only from those taxa with specialized storage structures. This requirement suggests that terpene emission should be predictable simply from a knowledge of plant anatomy. This prediction has been borne out in most studies of both temperate and tropical floras, where studies of leaf anatomy suffice to predict the occurrence or absence of terpenes (Fahn, 1988).

Those volatile compounds that are stored in specialized storage structures serve ecological functions that are both well studied and well understood. The terpenes participate in the chemical defense of plants against herbivores and pathogens through a combination of direct toxicity, feeding inhibition, mechanical disruption, and by serving as relatively nonpolar solvents for higher molecular-weight defensive compounds that would otherwise not go into solution (Croteau, 1987). There exist a plethora of models predicting allocation to defensive compounds based on resource availability and the threat of attack, and one of the grails for studies in biosphere/atmosphere exchange has been to link these models with emissions models so that one can predict emissions based solely on the ecology of the compounds. To date, however, limitations in the successes of the ecological models at predicting concentrations have restricted our ability to link the functional ecology of the plants to their impacts on atmospheric processes.

Two exceptions to this model of light-independent terpene emissions have been found, one that occurs broadly but appears to have little relevance for atmospheric processes, and another that is geographically limited but crucial for one region. Conifers such as douglas fir (Pseudotsuga menzesii) synthesize most of their terpenes, that is, fill their storage pools, during needle expansion. This synthesis is light-dependent (Gleizes et al., 1980) and very low light-dependent emissions have been seen in expanding conifer needles (Lerdau, 1994). A more important (in atmospheric terms) and more puzzling (in terms of biology) exception occurs in the Mediterranean region of Europe, where light-dependent terpene emission has been found in oaks, Quercus (Staudt & Seufert, 1995; Bertin & Staudt, 1996). These oaks lack terpene storage structures, and their emission of monoterpenes follows very similar patterns to those found for other light-dependent compounds such as isoprene and monoterpenes (Bertin et al., 1997; Staudt et al., 1997; Staudt & Bertin, 1998; Kesselmeier & Staudt, 1999; Peñuelas & Llusia, 1999; Llusià & Peñuelas, 2000). These monoterpene emissions from live oaks are the primary source of reduced compounds in the lower atmosphere over Mediterranean Europe (Fuentes et al., 2000). In addition, several researchers have suggested the possibility that conifers emit low levels of particular monoterpenes in a light-dependent manner, but the atmospheric import of these emissions is small (Janson, 1993; Lerdau et al., 1994).

III. Light-dependent emissions

Light-dependent VOC emissions are often the most abundant VOC emissions from ecosystems on daily or longer time-scales. The vast majority of the compounds emitted in a light-dependent manner fall into one of three groups: the abovementioned monoterpenes from European live oaks; a five-carbon alcohol, 2-methyl-3-buten-2-ol (C5H10O) or methylbutenol, that is produced by some of the pine species in western North America; and isoprene (C5H8). By contrast to the VOC's that are stored in specialized structures and emitted in a light-independent manner, the function or adaptive value of these nonstored, light-dependent compounds is still under debate. Despite this uncertainty over function, the biosynthetic routes for, the environmental regulations over, and the taxonomic distribution of emission are relatively well understood. With a few exceptions, noted below, the biosynthetic pathway and environmental regulation over production and emission appear to be similar for all three compounds. In addition, there appears to be no overlap among taxa in terms of light-dependent products emitted. That is, if a taxon produces isoprene, methylbutenol, or light-dependent monoterpenes, then it produces only one of those compounds. Many taxa, however, produce both light-dependent and light-independent VOC’s, for example, Picea and Eucalyptus produce both isoprene and light-independent monoterpenes, and several Pinus species produce both methylbutenol and light-independent monoterpenes.

The biosynthetic pathway for isoprene has recently received much attention because isoprene appears to be the simplest end-product of the newly discovered methylerythritol 4-phosphate (MEP) pathway (also known as the Romer pathway in honour of its discoverer, Michel Rohmer) for its first committed intermediate; 2-methylerythritol 4-phosphate. The role of this pathway in isoprene formation has been recently reviewed (Sharkey & Yeh, 2001), and their analysis allows a detailed energetic and carbon-cost accounting of isoprene synthesis. They make the important point that isoprene synthesis by the MEP pathway is less expensive than the previously accepted biosynthetic route, the Mevalonic Acid Pathway (MVA). Isoprene synthesis by the MEP pathway requires six carbon atoms, 20 ATP, and 14 NADPH molecules as compared with a requirement of 9, 24, and 14 by the MVA pathway (Niinemets et al., 1999). The biosynthetic route of methyl butenol (MBO) has also recently been shown to be the MEP pathway, with the principal difference lying at the final step, where dimethylally pyrophosphate (DMAPP) is converted to MBO (Lichtenthaler, 1999; Zeidler & Lichtenthaler, 2001). The synthetic route for the light-dependent terpenes produced by the European oaks has not yet been identified, but they are made in chloroplasts, so the MEP pathway is the most likely candidate.

Another similarity that all three light-dependent compounds share is their flux pathway, from site-of-synthesis to point-of-exit. All three are made in chloroplasts and diffuse through the chloroplast membrane, through the cellular membrane, into the intercellular air space and out through stomata. This stomatal flux was first shown conclusively for isoprene in experiments conducted in Russ Monson's and Ray Fall's labs in the late 1980s (Guenther et al., 1991). Interestingly, the vapour pressure of isoprene in the intercellular air space is always so far below saturation that, unlike in the case of water vapour flux, stomatal conductance does not regulate emissions under steady-state conditions. In addition, and in contrast to CO2 flux, there are no feedbacks between isoprene concentration and metabolism; this lack of feedback further reduces a role for stomata in regulating isoprene biosynthesis and emission.

Similar results have recently been obtained for MBO emission from pines, where steady-state emission rates are completely divorced from stomatal behavior across a wide range of conductances (D. Gray, unpublished). However, on a time scale of tens of minutes to a few hours, large reductions in stomatal conductance can lead to parallel declines in MBO emission. Over time these declines in emission are reversed and emissions re-equilibrate at levels comparable to those present before stomatal closure occurred (Fig. 1). This time-lag between stomatal change and flux compensation supports the idea first put forward by Monson et al. (1995) that, as long as the internal vapor pressure of the VOC is below its saturation vapor pressure, then stomatal conductance only affects flux in nonequilibrium conditions.

Figure 1.

Response of methyl butenol (MBO) emission (solid symbols) and stomatal conductance (open symbols) in Pinus ponderosa through time. The vertical dashed line shows when the needles were severed from the branch to induce water stress and stomatal closure. Measurements to the left of the vertical dashed line were made on intact needles. Measurements to the right were on severed needles. Methods as described in Gray et al. (in press ).

Because, as noted above, plants lack specialized structures for storing their light-dependent VOC’s, rates of production tie closely to concentrations in intercellular air-spaces. This lack of storage, combined with reduced stomatal control, facilitates modeling light-dependent VOC fluxes at large spatial scales because one does not need to introduce significant lag terms between the biosynthetic physiology and the emission and because effects of light and temperature can be modeled directly, without considering indirect effects of these parameters on stomata. Of course, efforts to model emissions on very short time scales, seconds to tens of seconds, must account for the nonequilibrial possibilities, but on longer time scales the equilibrium models suffice.

IV. Regulation of production

As with any other metabolic process, the production of light-dependent VOC's by plants must be evaluated across multiple biological and temporal scales. The first studies of isoprene emission demonstrated that flux rates were very sensitive to changes in incident temperature and light (Zimmerman, 1979). The shapes of these response curves are nearly identical at these short time-scales for all taxa that have been studied, with light response curves approximating hyperbolas, and the temperature response curves exponential with a clear maximum (Fig. 2). However, differences in the intercepts of these curves and subtle differences in their shapes lend insight into the physiological and ecological regulation of production and emission.

Figure 2.

Response of isoprene emission measured as gas chromatograph (GC) peak areas to light (a) or temperature (b). Data are from measurements made on Quercus kellogii (Gray unpublished). Methods as described in Gray et al. (in press ).

The light environment in which leaves develop affects the exact shape of their light response curves, with studies of multiple taxa demonstrating that shade-adapted leaves show a curve that clearly saturates (rectangular hyperbola) and sun-leaves do not show saturation through full-sun intensities (Lerdau & Throop, 2000). Similarly, the exact shape of the temperature response curve depends upon the rate at which temperature changes, with very rapid changes leading to a temperature response curve that resembles Michaelis–Menten kinetics but slower changes leading to differently shaped responses (Singsaas & Sharkey, 2000). The physiological regulation of these fluxes has recently been reviewed in detail by Sharkey & Yeh (2001) and is not repeated here. One particularly interesting aspect of the temperature response is that in the temperature region below saturation, the change in isoprene emission rate with temperature is well above the change the change that would be expected based on changes in vapour pressure alone; that is, the exponential increase in emission must be due to changes in biological activity as well as changes in vapor pressure.

These clear and consistent temperature and light responses lead to the question of what regulates emission at constant light and temperature levels (what has been termed the standardized emission rate, or SER). As noted above, for those VOC's that are volatilizing from stored pools, the answer is fairly straightforward: the vapor pressure as determined by the chemical structure of the compound, its concentration, and the temperature of the leaf are sufficient to predict the fluxes (Lerdau et al., 1994; Lerdau et al., 1995). For light-dependent VOC’s, the question can be rephrased, ‘What regulates the production rate at constant light and temperature levels?’ The first factor to be explicitly examined in this regard was leaf nitrogen concentration, which was shown to be a very good predictor of isoprene emission under controlled environment conditions (Harley et al., 1994; Litvak et al., 1996). Similar results appear to hold for those pines producing MBO (D. Gray et al., unpublished).

The consistency of the relationship between leaf N and standardized isoprene emission rate leads to the possibility that other physiological processes that scale with leaf N can be correlated with standardized isoprene. This relationship has been shown both within taxa in the fertilization experiments described above and across taxa in surveys conducted in both dry and wet tropical forests (Lerdau & Keller, 1997; Lerdau & Throop, 1999). This strong consistent relationship opens up the possibility that isoprene emission on large spatial scales can be modeled using already extant, and well tested, models of carbon exchange such as the CASA model (Field et al., 1995; Malmstrom et al., 1997; DeFries et al., 1999). We do not yet have the data to speak to the question of whether or not the other light-dependent VOC's have such a consistent relationship between leaf N and standardized emission rate.

Also, as noted above, the light environment in which leaves develop is one of the main determinants of their SER, with leaves developing in full sun having higher SER than leaves that develop in shade. Similar patterns have been seen with respect to photosynthetic capacity. In the case of photosynthesis, most of the effect of light environment on photosynthesis can be understood in terms of the effect of light environment on specific leaf area, so that shade and sun leaves from the same plant tend to have similar photosynthetic rates when those rates are expressed on mass-basis rather than on an area basis (Reich et al., 1997; Reich et al., 1998; Reich et al., 1999). With isoprene, however, a true change in the biochemical potential is seen, with sun leaves having higher isoprene SER on both mass and areal bases (Harley et al., 1996; Harley et al., 1997). The isoprene SER as a proportion of photosynthetic capacity increases with the light environment in which the leaf develops for many taxa (Lerdau & Throop, 2000). This biochemical change is concordant with the idea that isoprene's function may be more significant for leaves developing in high light environments than for leaves developing in low light environments.

In addition to developmental environment, the ontogenetic state of the leaf plays a large role in determining isoprene emission capacity, with leaves of broadleafed plants not producing isoprene until they have reached full expansion (Grinspoon et al., 1991; Kuzma & Fall, 1993). This pattern has been seen in several different taxa from temperate and tropical regions and has been linked to the onset of activity of the enzyme isoprene synthase (Grinspoon et al., 1991; Kuzma & Fall, 1993). Somewhat surprisingly, this developmental pattern does not hold true for conifers, where isoprene emission in spruce and MBO emission in ponderosa pine begin while the needles are still expanding (Fig. 3). This difference between conifers and broadleafed plants can be understood in light of the proposal that isoprene, and by extension MBO, are involved with high temperature tolerance (Sharkey & Singsaas, 1995). Broadleafed plants tend to see their highest leaf temperatures after full expansion both because of seasonal effects and because of boundary layer changes. By contrast, conifers have their highest boundary layer resistances while the needles are still packed closely together in the expanding state; the entire fascicle acts to create a boundary layer before the needles expand and spread apart. This high boundary layer could be involved with needle heating under clear-sky conditions. Clearly, detailed studies of whole fascicle temperature relations are necessary to test this hypothesis.

Figure 3.

Ontogeny of needle growth (black bars) and methyl butenol (MBO) emission (gray bars) in Pinus ponderosa . Data are averages taken from four trees. Error bars represent standard deviations. (Data from Gray et al . unpublished). Methods as described in Gray et al. (in press ).

Recent work on light-dependent VOC SER has identified another control for MBO and isoprene production. While it has been known for almost a decade that enzyme activity in the case of isoprene synthase requires a high temperature episode for induction and that without such an episode leaves do not emit isoprene (Sharkey & Loreto, 1993), ambient temperature appears to play a role in determining SER for both isoprene and MBO (Sharkey et al., 1999; Schade et al., 2000). The exact temporal dynamics of this effect are not known; however, it clearly acts with somewhere between a 3 and 24 h lag. Leaves clearly raise or lower their light dependent VOC SER based on the temperature they experience over some, as yet not exactly known, time interval. Whether this effect is caused by temperature-induced changes in enzyme activities, metabolic pool sizes, or both is not known.

V. Possible functions of light-dependent VOCs

The similarities in biosynthetic pathway, chemical structure, physiological regulation, and the fact that no taxa produce more than one kind of light dependent VOC all suggest that these compounds serve similar or identical functions in the plants that produce them. Most functional studies have focused on the compound with the widest taxonomic distribution, isoprene, but it is likely that results from these isoprene studies will be applicable to MBO and the light dependent monoterpenes.

The early research on isoprene production from Soviet Georgia suggested that isoprene was involved in reduction of carbon dioxide by a second carboxylase (references in Sanadze, 1991). This second carboxylase was never found, and isoprene-researchers moved on quickly to other explanations. The positive relationship between isoprene production and oxygen concentration led to the second explanation for isoprene production; isoprene was hypothesized to be a by-product of photorespiration (Rasmussen & Jones, 1973). However, detailed physiological studies in the late 1980s disproved this putative function (Hewitt et al., 1990b).

The current functional explanations for isoprene production all stem from the observation that isoprene production tends to increase under high light and temperature conditions well beyond the point at which photosynthesis saturates or starts to decline. These explanations all propose that, in some way, isoprene protects plants from extreme conditions. The protection models fall into two categories. The first are those that postulate that the protection isoprene confers comes through the energy and/or carbon required to produce isoprene, and the second are based upon the idea that some aspect, physical, chemical, or both, of the isoprene molecule in some way protects the plant.

The energy/carbon-dump models are conceptually similar to those models proposing that photorespiration serves an adaptive purpose by acting as a sink for excess reductant during times when electron transport chain activity outstrips the carbon reduction activity of the Calvin-Benson Cycle (Ogren, 1984). This model, first proposed by R. Fall in 1991 and elaborated a decade later by B. Logan and colleagues (Logan et al., 2000) suggests that the nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) required for isoprene synthesis is a sufficiently large quantity to justify the energy and carbon costs of maintaining the biochemical machinery during nonpathological conditions. This argument has been criticized because these quantities are so small compared to the energetic savings a plant can make by using the violaxanthin/zeaxanthin cycle; similarly, the carbon-dump component is much smaller than the amount that can be saved by small changes in RuBisCO activity (Sharkey & Yeh, 2001).

The physiochemical protection models are based on either isoprene's interactions with membrane bi-layers and the impacts of those interactions on the membrane's stability, or on the chemistry of isoprene reactions with oxidizing species within the leaf. Early studies on the interactions of plant-produced hydrocarbons and ozone demonstrated that these reactions led to the production of highly toxic organic peroxides. These organic peroxides caused greater toxicity than the ozone itself, a result that leads to VOC-producing plants to being more sensitive to ozone than nonproducing plants (Hewitt et al., 1990a). A detailed model of these interactions suggests that organic peroxide production should be a ubiquitous outcome of ozone hydrocarbon interactions (Salter & Hewitt, 1992). Recent studies involving isoprene–ozone interactions have suggested that, under certain extreme circumstances of acute ozone exposure, isoprene production may protect tissues, and these results suggest that further study along these lines is essential (Loreto et al., 2001). It may be possible that isoprene (and other light dependent VOC’s) are involved with protection against other important oxidants (Zeidler et al., 1997). Recent work by Peñuelas & Llusia (2002) provides further evidence that VOC's may be involved in protection against oxidizing agents.

The protection hypothesis that has received the most attention is the idea that isoprene production/emission protects plants against high temperature damage (Sharkey & Singsaas, 1995). The thermal protection hypothesis (TPH) arose from the observation that leaves emitting isoprene were able to withstand high temperatures better than leaves where isoprene production was inhibited. These initial observations were later refined and extended using several fluorescence-based techniques for a variety of taxa. It is noteworthy that the TPH is the only explanation that has been evaluated for another light dependent VOC; monoterpenes can protect oaks against high temperature damage (Loreto et al., 1998). While there is no formal model for how isoprene or other light dependent VOC may protect membranes, the effect is postulated to occur through a physical interaction based on membrane structure and the presence of unsaturated bonds in the light dependent VOC; recent work has shown that only alkenes provide protection against high temperatures, and alkanes do not increase membrane stability (Sharkey et al., 2001).

The TPH can be set within the larger framework of plant responses to high temperatures (Fig. 4). Plants have different responses for high temperature episodes that occur at different time scales. On long time scales, high temperatures lead to a turnover in species composition at the community scale. On shorter time scales, high temperatures (and associated water stress) are correlated with phenological changes such as leaf senescence. On time scales of days to weeks, membrane fatty acid composition can change. On time scales as short as hours, high temperatures lead to the induction of chaperonin proteins involved with heat stress. On time scales in the range of tens of minutes, plants can use changes in leaf turgor to modulate leaf temperature; by wilting they can reduce radiative load and lower leaf temperature. On slightly shorter time scales, changes in stomatal conductance can be an effective mechanism for reducing leaf temperature through evaporative cooling. On very short time-scales, however, plants can receive very high radiative loads that raise leaf temperatures into lethal ranges at time scales faster than stomatal response times (Sharkey, 1996). Isoprene emission, with its almost instantaneous response to temperature, is an effective means improving a leaf's ability to tolerate these short-term but extreme radiative events.

Figure 4.

Responses of plants to high temperatures stresses at multiple time scales.

There have, to date, not yet been any large-scale surveys of thermal stress tolerance across light dependent VOC emitting and nonemitting taxa. However, studies in a species-poor ecosystem at Harvard Forest in Petersham, MA, USA, have demonstrated that the temperature tolerance of the three dominant taxa correlates with their rate of isoprene emission; red oak is a large emitter and tolerates high temperatures well, yellow birch emits at a lower level and is more sensitive, and red maple does not emit isoprene and shows the least tolerance for high temperature episodes (Bassow & Bazzaz, 1997; Bassow & Bazzaz, 1998).

VI. Evolutionary aspects of phytogenic VOC

The role of light-independent VOC’s, the mono- and sesquiterpenes, as taxonomic characters has received detailed attention because their presence in specialized storage structures has facilitated their use as characters in systematic studies. Only recently has attention shifted to the evolution of the VOC's themselves. The consensus is that the enzymes responsible for the first steps in the MVA and MEP pathways are primitive for green plants. Indeed, many bacteria contain these initial enzymes (Sharkey & Yeh, 2001). By contrast, the terpene synthase enzymes, the ones responsible for converting geranyl pyrophophate to simple terpenes appear to have arisen multiple times in different groups, and recent studies of both sequence and genetic architecture data suggest independent origins in several angiosperm families as well a single independent origin in gymnosperms (Trapp & Croteau, 2001a; Trapp & Croteau, 2001b).

The patterns of evolutionary acquisitions and losses for light dependent isoprenoid emissions are, as yet, poorly understood. However, because all green plants produce DMAPP in the chloroplast via the MEP pathway, it is clear that all plants are but one enzyme away (isoprene synthase or methylbutenol synthase) from being able to produce either isoprene or MBO. The question of how many times have plants acquired one of these enzymes, and how many times have they lost them remains unanswered. The number of enzymes necessary to produce the monoterpenes made by the Mediterranean live oaks is not known, but mapping light dependent monoterpene emission onto the phylogeny of oaks provides a clear indication that these light dependent VOC's evolved once in this clade (Harley et al., 1999).

Methylbutenol emission appears to be restricted taxonomically to some members of the genus Pinus; and has yet to be detected in any angiosperm, or related gymnosperms (e.g. spruce, fir, cedar, juniper). Work by Harley et al. (1998) and data collected by Gray (Table 1) show that within the pines MBO emission is found only in certain hard pines (subgenus Pinus) distributed in western North America. However, attempts to understand the evolution of MBO emission in a phylogenetic context are hampered by the confusion over the taxonomic position and relatedness of many species of pine (especially those found in Mesoamerica) and incomplete taxonomic screening for MBO. A robust phylogeny for the conifers would go far toward helping us understand the evolution of MBO.

Table 1.  Methylbutenol emission from Pinus species
SubgenusSubsectionSpeciesEmitter/ Non-EmitterMeasured Fluxes (µg carbon m−2 s−1)
  1. Emission status was determined by gas chromatography of the headspace of needle containing vials or the exhaust gas of a photosynthesis cuvette system. Vials were incubated in sunlight for 30–90 min before measurement. Fluxes represent steady state measurements taken on cut branches. Analytical methods described in Schade et al. (2000) in the units µg carbon m−2 s−1. NS, taxon not sampled. (Data from Gray et al. unpublished).

  uncinata (mugo)NS 
  brutia var eldaricanonemitter 
  contorta var bolanderiemitter3.3
  contorta var contortaemitter 
  contorta var latifoliaemitter 
  contorta var murrayanaemitter 
PinusPonderosaePonderosa Group  
  cooperi var ornelasiemitter0.6
  ponderosa var ponderosaemitter5–20
  ponderosa var arizonicaemitter0.7–1
  ponderosa var scopulorumemitter0.4–3.2
  Montezuma Group  
  Pseudostrobus Group  
  pseudostrobus var esteveziinonemitter0
  pseudostrobus var oaxacanaemitter 
  Sabiniana Group  
  radiata var binatanonemitter 
 OocarpaeOocarpae Group  
  Teocote Group  
  strobiformis (ayacahuite)nonemitter 

Nonetheless, by combining the extant screening data and best phylogenetic evidence available, it is clear that MBO emission either evolved multiple times or was secondarily lost on many occasions. Within monophyletic subsections (sensuPrice et al., 1998), one finds a mixture of emitting and nonemitting species amongst those sampled. This combination is most notable in the subsection contortae in which P. contorta emits strongly and neither P. banksiana nor P. virginiana show any detectable emission. Likewise, the subsections Ponderosae, Oocarpae, and Attenuatae contain a number of strongly emitting pines as well as nonemitters. One also finds clear phylogenetic evidence placing the nonemitting subsection Australes nested within a clade containing the emitting subsections Attenuatae and Oocarpae. In addition to these phylogenetic patterns, it is interesting to note that the on the whole and within a clade the highest emitting species tend to possess more northerly distributions.

Isoprene emission, while far more prevalent than MBO emission, also has a spotty distribution among both vascular and nonvascular plants (Harley et al., 1999), suggesting multiple origins or multiple losses of the character. For example, isoprene production is common to the mosses, but absent from liverworts and hornworts, ferns contain both emitting and nonemitting taxa (Hanson et al., 1999). Among the seed plants, isoprene occurs in only a few gymnosperm taxa and thus is presumably derived for them, but it appears to be primitive for the dicots among the angiosperms (Harley et al., 1999).

In addition to issues regarding the phylogenetic origins of VOC emissions, questions regarding the origins of the pivotal enzymes that give rise to the observed emission phenotypes are virtually untouched. The evolutionary origins of the isoprene and MBO synthases (which are both terpene synthase enzymes) remain open questions; however, recent DNA sequence data has shown that the isoprene synthase gene from poplar (Miller et al., 2001) shares strong sequence homology with a monoterpene synthase. This similarity is noteworthy in that, relative to the monoterpene synthases, the isoprene synthases that have been studied are inefficient enzymes with surprisingly large Km values for their substrates. Could isoprene synthase have its origin in a monoterpene synthase that has switched from using the 10 carbon substrate GPP to using the 5 carbon substrate DMAPP? Such a possibility is intriguing because monoterpene synthases forming the same products appear to have evolved independently in widely separated groups (conifers and angiosperms). Only a cross-taxon comparative study of isoprene synthase and monoterpene synthase genes will address whether isoprene synthase had its origin in a single event, evolved repeatedly from functionally similar but unrelated monoterpene synthases, or whether plants have co-opted different terpene synthases in the evolution of isoprene synthase and the development of isoprene production capability.


ML was supported by fellowships from Harvard University (Bullard and Hrdy) during preparation of this manuscript. DG received a NASA ESS Fellowship for much of the empirical work reported. We thank two reviewers and Rich Norby for very helpful comments on an earlier version.