Abstract
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.
Highlights
Biospheric emissions of isoprene (2-methyl-l,3-butadiene) represent a principal control over the oxidative capacity of the continental troposphere (Brasseur and Chatfield 1991)
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 instantaneous emission rate is the product of the basal emission rate and some factor that takes into account the influence of prevailing environment on basal emissions
Summary
Biospheric emissions of isoprene (2-methyl-l,3-butadiene) represent a principal control over the oxidative capacity of the continental troposphere (Brasseur and Chatfield 1991). The basal emission rate represents the inherent capacity of a leaf to emit isoprene, independent of prevailing environmental conditions. The basal emission rate is typically measured under a standard set of environmental conditions, and is controlled by the amount of biochemical machinery and substrate available to synthesize isoprene. The primary environmental controls over the instantaneous rate are light and temperature (Tingey et al 1979, 1987; Monson and Fall 1989; Loreto and Sharkey 1990). Control by light may explain the often-reported linkage between isoprene emission rate and the instantaneous photosynthesis rate (Sanadze 1969; Rasmussen and Jones 1973; Tingey et al 1987; Monson and Fall 1989; Loreto and Sharkey 1990, 1993a; Monson et al 1991b). Using empirical measurements of the basal emission rate, biochemical models of the light dependence of photosynthetic electron transport, and Arrhenius-based models of the temperature dependence of enzymes, Guenther and co-workers have successfully simulated the instantaneous isoprene emission rate in several species (Guenther et al 1991, 1993a)
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