Ecological controls over monoterpene emissions from Douglas-fir

Ecology, Dec, 1995 by Manuel Lerdau, Pamela Matson, Ray Fall, Russell Monson

INTRODUCTION

Monoterpene emission by plants is one of the principal factors regulating the oxidative capacity of the atmosphere (Seiler and Conrad 1987, Singh and Zimmerman 1992). The annual rate of monoterpene emission from vegetation is estimated at 120-150 Tg/yr of carbon, representing [approximately]0.1-0.3% of global net primary productivity, and approximately one-half the sum of anthropogenic plus biogenic methane emissions. Unlike methane, however, monoterpenes are extremely reactive in the troposphere and so exist at very low atmospheric concentrations (Muller 1992). This high reactivity (primarily with hydroxyl radical and ozone in sunlight, and nitrate radical at night) gives monoterpenes a critical role in determining concentrations of tropospheric ozone and carbon monoxide (both important pollutants and greenhouse gases), in producing organic nitrates and weak organic acids, and in controlling the atmospheric lifetime of methane, another greenhouse gas (Jacob and Wofsy 1988). Despite the importance of this biogenic emission in atmospheric chemistry, relatively little is known of the biological controls over monoterpene emissions.

Monoterpenes are 10-carbon hydrocarbons produced by a variety of flowering plants (most commonly in the Myrtaceae, Asteraceae, and Lamiaceae) and nearly all conifers (Banthorpe and Charlwood 1982). They are synthesized through the mevalonic acid pathway and are stored internally, either in specialized ducts or canals, or externally in glandular hairs (Croteau 1987). Within plants, monoterpenes are part of the chemical defense system, functioning as feeding deterrents to many mammals and generalist insects (but see Raffa 1991 for a discussion of adaptations by specialized insects), and as solvents for higher molecular weight hydrocarbons and organic acids (Hanover 1966, Berryman 1972, Loomis and Croteau 1973, Lorio 1986, Lewinsohn et al. 1993).

Previous studies of monoterpene emissions have concentrated on the effects of leaf temperature upon emissions over the time scale of minutes to days (Tyson et al. 1974, Dement et al. 1975, Zimmerman 1979, Tingey et al. 1980, 1991, Lamb et al. 1985, Guenther et al. 1991, 1993, Janson 1993). These studies have shown that temperature effects are sufficient to explain short-term variations in monoterpene emissions from any individual plant; they have not, however, addressed questions of why different plants, even of the same species, often have different emission rates at the same temperatures, or why emissions vary seasonally (Flyckt 1979, Yokouchi and Ambe 1984).

One factor that could cause between-plant variation in monoterpene emission rate is the difference in concentration within plant tissues (Lerdau 1991, Tingey et al. 1991). Concentration could affect emissions both by directly affecting partial pressure in the vapor phase according to Henry's Law, and by affecting diffusion as the monoterpenes move from the resin canals to intercellular air spaces; concentration is linearly related to diffusion rate across a surface. An increase in diffusion rate from the resin canal to the interior of the leaf would lead to an increase in the vapor pressure gradient from leaf to atmosphere.

A number of factors can control chemical concentrations in plant tissues, including production and turnover rates. Because monoterpenes are defensive compounds, controls over their production can be evaluated in terms of resource allocation theory. This body of theory deals with the controls of plant allocation of resources to different types of compounds, e.g., resource gathering (photosynthetic tissues) and resource protecting (defensive compounds). The carbon/nutrient balance hypothesis (C.N.B.H.) (Bryant et al. 1983) and the growth differentiation balance hypothesis (G.D.B.H.) (Loomis 1932, as cited in Lorio 1986) both address how the relative availabilities of resources affect their allocation to the production of new tissue and to the defense of existing tissues. The C.N.B.H. predicts that when a resource, such as nitrogen, is scarce, a plant will allocate proportionately more of an abundant resource, such as carbon, to the acquisition of the scarce resource, and also will use the abundant resource in the construction of defense. When nitrogen is abundant, a plant will allocate less carbon toward defense and more toward growth. The G.D.B.H., in contrast, uses two physiological assumptions: (1) that whenever all necessary resources for growth are available, growth processes will be favored over defensive compound production, and (2) that growth is more sensitive to the availability of belowground resources than is assimilation of new carbon. From these assumptions comes the prediction that when nitrogen is moderately scarce, growth will be more constrained than carbon fixation, leading to a surplus of carbon and, thus, an increase in the allocation of carbon to defense.

These theories suggest that differences in the relative availability of carbon, water, or nitrogen should cause differences in allocation to growth (cell division and enlargement) and defense. As such, they provide a framework for evaluating the factors controlling variability in plant monoterpene pools and emission rates. To date, however, the studies that have used defense theories to examine the relationship between monoterpene concentrations and variations in light, nutrient, or carbon dioxide availabilities have found conflicting results. Some found the predicted negative relationship between resources and monoterpene concentrations (Clark and Menary 1980, Mihaliak and Lincoln 1985, 1989, Mihaliak et al. 1987). Others showed no relationship between nitrogen availability and monoterpene concentrations (Muzika et al. 1989, Johnson and Lincoln 1990, Reichardt et al. 1991) Two studies have shown a positive relationship between nitrogen and monoterpene concentrations (Bjorkman et al. 1991, McCullough and Kulmon 1991). Each of these studies examined monoterpene concentrations at one point in time; none, however, examined the relationship in the context of plant phenology or seasonality.