Importance of Biotic vs Abiotic Controls on VOC Emissions from Ponderosa Pine

The emissions of VOCs, including monoterpenes (MTs) and 2-methyl-3-buten-2-ol (MBO), from ponderosa pine can be important contributors to the regional production of ozone and secondary organic aerosols in the Western United States. The goal of this study was to better characterize the influences of biotic and abiotic factors on the emissions of these compounds. Using PTR-MS coupled with measurements of photosynthesis and stomatal conductance (g_s) we generated light and temperature curves from intact needles of mature ponderosa pine trees and used abscisic acid (ABA) to reduce g_s and photosynthesis under constant light and temperature conditions. Stomatal conductance and photosynthesis were almost perfectly correlated during all our measurements, so we were unable to separate their influences. We found that increasing temperature by 10 ^oC increased emissions of both MTs and MBO by 80-120% even though g_s and photosynthesis were reduced by ~50%. Light curves performed at 30 ^oC showed that g_s and photosynthesis exhibited a strong control over MT and MBO emissions although the effect was more pronounced for MBO than MT emissions. In most cases a 60% reduction in g_s and photosynthesis caused a ~50% reduction in MBO emissions and a 5-20% reduction in MT emissions. Using ABA we were able to induce stomatal closure while maintaining a constant light and temperature environment and we found that stomatal closure due to ABA caused declines in MT and MBO emissions that were similar in magnitude to those seen in the light curves. When compared at the same light and temperature conditions, individuals with lower g_s and photosynthesis did not necessarily have lower emissions than those with higher g_s and photosynthesis, indicating that g_s and photosynthesis may not be good predictors of emissions between individuals, but within each individual the instantaneous changes in g_s and photosynthesis did appear to exert control over the emissions of VOCs. These data show that plant physiology is an important constraint in regulating the instantaneous emissions of VOCs, but also that the relationships are modified by external temperature, probably through the increased volatility of the VOCs.