Growth and actual leaf temperature modulate CO 2 responsiveness of monoterpene emissions from holm oak in opposite ways

[摘要] Climate change can profoundly alter volatile organic compound (VOC) emissions fromvegetation and thus influence climate evolution. Yet, the short- andlong-term effects of elevated CO 2 concentrations on emissions ininteraction with temperature are not well enough understood, especially for VOCsother than isoprene. To gain additional insight, we conducted a study onholm oak, which is known for its strong foliar monoterpene emissions thatare directly linked to their synthesis. We measured CO 2 -response curvesof emissions, CO 2 –H 2 O gas exchanges and chlorophyll fluorescenceat two assay temperatures (30 and 35  ∘ C) on saplings of fourpopulations grown under normal and double CO 2 concentrations combinedwith two temperature growth regimes differing by 5  ∘ C (day/night:25/15 and 30/20  ∘ C). A stepwise reduction in CO 2 resulted ina decrease in emissions, occasionally preceded by an increase, with theoverall decrease in emissions being greater at 35  ∘ C than at 30  ∘ C assay temperature. During ramping to high CO 2 , emissionsremained mostly unchanged at 35  ∘ C, whereas at 30  ∘ Cthey often dropped, especially at the highest CO 2 levels ( ≥  1200 ppm). In addition to the actual leaf temperature, the highCO 2  responsiveness of emissions was modulated by the plant's growthtemperature with warm-grown plants being more sensitive than cool-grownplants. In contrast, growth CO 2 had no significant effect on theCO 2 sensitivity of emissions, although it promoted plant growth and theleaf emission factor. Correlation analyses suggest that the emissionresponse to CO 2 depended primarily on the availability ofenergetic cofactors produced by photosynthetic electron transport. Thisavailability was likely limited by different processes that occurred duringCO 2  ramping including photooxidative stress and induction of protectiveand repair mechanisms as well as competition with CO 2  fixation andphotorespiration. In addition, feedback inhibition of photosynthesis mayhave played a role, especially in leaves whose emissions were inhibitedonly at very high CO 2 levels. Overall, our results confirm anisoprene-analogous behavior of monoterpene emissions from holm oak.Emissions exhibit a non-linear response curve to CO 2 similar to thatcurrently used for isoprene emission in the MEGAN model, with no differencebetween major individual monoterpene species and plant chemotype.Simulations estimating the annual VOC releases from holm oak leaves atdouble atmospheric CO 2 indicate that the observed high-CO 2 inhibition is unlikely to offset the increase in emissions due to thepredicted warming.

[学科分类] 大气科学