Isoprene and monoterpene emission rate variability: Observations with Eucalyptus and emission rate algorithm development

Variability in the emission rates of isoprene and monoterpenes from individual leaves of Eucalyptus globulus was investigated with a laboratory gas exchange system and an environmental control leaf cuvette. For individual leaves, with constant environmental conditions, short-term (1 hour) fluctuations in isoprene emission rates were less than 3% while day-to-day fluctuations averaged 14%. Leaf-to-leaf variations were much larger (62%). Fluctuations with time and leaf-to-leaf variability in CO2 assimilation rates were of the same order as isoprene, while monoterpene variations were higher. Leaf age was identified as one of the factors contributing to leaf-to-leaf variability in CO2 assimilation and isoprene and monoterpene emission rates. Monoterpene emission rates were not influenced by light intensity or CO2 mixing ratio. The observed temperature dependence was the same for α-pinene and 1,8-cineole (an oxygenated monoterpene) and is similar to the temperature dependence of monoterpene emission rates reported by other investigators. Isoprene emissions were slightly dependent on humidity (1–3% increase in emission per 10% increase in relative humidity) and responded only to very low ( 600 ppm) CO2 mixing ratios. Isoprene emission was associated with the abaxial leaf side, which contains stomatal pores, while monoterpenes were emitted primarily from the adaxial side, which lacks stomatal pores. The temperature and light dependence of isoprene emission closely resembles relationships observed for electron transport in plant chloroplasts. For this reason, we have used a mechanistic electron transport model as the basis for an empirical isoprene emission rate model. The emission rate variation predicted by this model was within 10% of observed values for 62% of the 255 observations at light-saturated conditions and temperatures between 23° and 33°C. The entire data base includes over 600 observations at leaf temperatures ranging between 12° and 50°C and light intensities between 0 and 2000 μmol m−2 s−1. Nearly two thirds of the emission rates predicted for the entire data base were within a factor of 1.25, and 89% were within a factor of 2. The algorithms developed in this study provide a solid physiological basis for future efforts to model the biogenic flux of isoprene and monoterpenes into the atmosphere.