Volatile organic compounds emitted by plants have long been recognized as having important influences on tropospheric chemistry, most notably in contributing to the production of tropospheric ozone and aerosols. One such compound, methylbutenol (MBO), was recently identified as a major component of the volatiles emitted by ponderosa pine and several other species of pine in western North America. On short time scales, MBO emissions increase with light intensity in parallel with photosynthetic responses, but emissions and photosynthesis show opposite responses to increases in temperature. We investigate the response of MBO emission to water stress, ambient temperature, and the aging of needles in field grown Pinus ponderosa (ponderosa pine). Photosynthetic rates and MBO production capacities of P. ponderosa were measured over the course of a season on saplings at a site that experienced summer drought and at a site that received supplementary water. In addition, a water-stress alleviation experiment was performed in which drought-stressed P. ponderosa seedlings were re-watered at the end of the season and pre-/post-alleviation photosynthetic rates and MBO production capacities were compared. Over the season photosynthesis declined while MBO production capacity tracked changes in ambient temperature linearly both over the entire season and on a day-to-day basis. Although severe water stress reduced photosynthetic rates, there was no difference in the response of MBO production capacity to ambient temperature under either drought-stressed or well-watered conditions, and there was no change in MBO production capacity following water stress alleviation. MBO emission declined with needle age. The correlation between MBO production capacity and ambient temperature is consistent with the view that MBO may provide a protection against high temperature stress similar to that suggested for isoprene.
