ENVIRONMENTAL AND DEVELOPMENTAL CONTROLS OVER THE SEASONAL PATTERN OFISOPRENE EMISSION FROM ASPEN LEAVES

Isoprene emission from plants represents one of the principal biospher ic controls over the oxidative capacity of the continental troposphere . In the study reported here, the seasonal pattern of isoprene emissio n, and its underlying determinants, were studied for aspen trees growi ng in the Rocky Mountains of Colorado, The springtime onset of isopren e emission was delayed for up to 4 weeks following leaf emergence, des pite the presence of positive net photosynthesis rates. Maximum isopre ne emission rates were reached approximately 6 weeks following leaf em ergence. During this initial developmental phase, isoprene emission ra tes were negatively correlated with leaf nitrogen concentrations. Duri ng the autumnal decline in isoprene emission, rates were positively co rrelated with leaf nitrogen concentration. Given past studies that dem onstrate a correlation between leaf nitrogen concentration and isopren e emission rate, we conclude that factors other than the amount of lea f nitrogen determine the early-season initiation of isoprene emission. The late-season decline in isoprene emission rate is interpreted as d ue to the autumnal breakdown of metabolic machinery and loss of leaf n itrogen. In potted aspen trees, leaves that emerged in February and de veloped under cool, springtime temperatures did not emit isoprene unti l 23 days after leaf emergence. Leaves that emerged in July and develo ped in hot, midsummer temperatures emitted isoprene within 6 days. Lea ves that had emerged during the cool spring, and had grown for several weeks without emitting isoprene, could be induced to emit isoprene wi thin 2 h of exposure to 32 degrees C. Continued exposure to warm tempe ratures resulted in a progressive increase in the isoprene emission ra te. Thus, temperature appears to be an important determinant of the ea rly season induction of isoprene emission. The seasonal pattern of iso prene emission was examined in trees growing along an elevational grad ient in the Colorado Front Range (1829-2896 m). Trees at different ele vations exhibited staggered patterns of bud-break and initiation of ph otosynthesis and isoprene emission in concert with the staggered onset of warm, springtime temperatures. The springtime induction of isopren e emission could be predicted at each of the three sites as the time a fter bud break required for cumulative temperatures above 0 degrees C to reach approximately 400 degree days. Seasonal temperature acclimati on of isoprene emission rate and photosynthesis rate was not observed. The temperature dependence of isoprene emission rate between 20 and 3 5 degrees C could be accurately predicted during spring and summer usi ng a single algorithm that describes the Arrhenius relationship of enz yme activity. From these results, it is concluded that the early seaso n pattern of isoprene emission is controlled by prevailing temperature and its interaction with developmental processes. The late-season pat tern is determined by controls over leaf nitrogen concentration, espec ially the depletion of leaf nitrogen during senescence. Following earl y-season induction, isoprene emission rates correlate with photosynthe sis rates. During the season there is little acclimation to temperatur e, so that seasonal modeling simplifies to a single temperature-respon se algorithm.