| Sagebrush (Artemisia tridentata ssp. tridentata ) constitutively releases methyl jasmonate (MeJA), a compound highly active in inducing a number of physiological responses in plants. Damage increases the absolute quantity of MeJA released as well as the proportion of cis-MeJA, the thermodynamically unstable but putatively biologically active epimer of MeJA. Recently it was demonstrated that tobacco ( Nicotiana attenuata) plants downwind of damaged sagebrush incurred less herbivory as compared to plants growing nearby undamaged sagebrush, a response apparently mediated by an airborne signal. To examine the hypothesis that cis-MeJA is the active cue, we first characterized sagebrush's MeJA release. The increase in MeJA and the cis epimer occurred rapidly after wounding, waned after 4 h, was limited to the damaged tissue, and did not rapidly dissipate. We then developed methods to: (1) entrain sagebrush constituents in water, preserving the epimeric shift; (2) chemically manipulate the trans:cis ratio; and (3) isolate nearly pure cis-MeJA by HPLC. These treatments were applied to a natural population of tobacco plants. However, an outbreak of specialist herbivores consumed all plants and chemical analysis on harvested leaf material was inconclusive. In the laboratory, realistic quantities of trans- and cis-MeJA were applied to tobacco plants using lanolin and aqueous sprays. Defense traits [protease inhibitors, nicotine] and herbivore performance were analyzed but no profound differences were found. The results are not consistent with the hypothesis that cis-MeJA functions as the cue eliciting resistance in N. attenuata. The function of the different MeJA epimers remains to be re-evaluated in natural populations.; Sagebrush also has well-documented allelopathic tendencies that have generally been ascribed to its most abundantly released components, but MeJA is also known to function as both a germination inhibitor and promoter. In field and laboratory studies, we examined the role of MeJA release from sagebrush in affecting N. attenuata germination. Seeds buried next to sagebrush in net bags had reduced germination compared to seeds buried similarly but protected in plastic bags and germination was 60% lower on soils collected from underneath sagebrush than from between plants or outside the sagebrush population. In the laboratory, exposure to sagebrush emissions and realistic levels of volatile and aqueous MeJA exhibited delayed germination. Germination on sagebrush chaff, heated to reduce MeJA, was negatively correlated with the amount of MeJA remaining in the chaff. By supplementing diluted, inactive sagebrush extracts with MeJA, we determined that MeJA could account for 16–60% of the inhibitory activity of sagebrush extract. |
