Growth suppression and defense signaling are simultaneous strategies that plants invoke to respond to abiotic stress. Here, we show that the drought stress response of poplar trees ( Populus trichocarpa) is initiated by a suppression in cell wall derived methanol (MeOH) emissions and activation of acetic acid (AA) fermentation defenses. Temperature sensitive emissions dominated by MeOH (AA/MeOH < 30%) were observed from physiologically active leaves, branches, detached stems, leaf cell wall isolations, and whole ecosystems. In contrast, drought treatment resulted in a suppression of MeOH emissions and strong enhancement in AA emissions together with fermentation volatiles acetaldehyde, ethanol, and acetone. These drought-induced changes coincided with a reduction in stomatal conductance, photosynthesis, transpiration, and leaf water potential. The strong enhancement in AA/MeOH emission ratios during drought (400-3,500%) was associated with an increase in acetate content of whole leaf cell walls, which became significantly 13C 2-labeled following the delivery of 13C 2-acetate via the transpiration stream. The results are consistent with MeOH and AA production at high temperature in hydrated tissues associated with accelerated primary cell wall growth processes, which are downregulated during drought. Our observations are consistent with drought-induced activation of aerobic fermentation driving high rates of foliar AA emissions and enhancements in leaf cell wall O-acetylation. We suggest that atmospheric AA/MeOH emission ratios could be useful as a highly sensitive signal in studies investigating environmental and biological factors influencing growth-defense trade-offs in plants and ecosystems.

Growth suppression and defense signaling are simultaneous strategies that plants invoke to respond to abiotic stress. Here, we show that the drought stress response of poplar trees ( Populus trichocarpa) is initiated by a suppression in cell wall derived methanol (meOH) emissions and activation of acetic acid (AA) fermentation defenses. Temperature sensitive emissions dominated by meOH (AA/meOH < 30%) were observed from physiologically active branches, detached stems, leaf cell wall isolations, and whole ecosystems. In contrast, drought treatment resulted in a suppression of meOH emissions and strong enhancement in AA emissions together with fermentation volatiles acetaldehyde, ethanol, and acetone. These drought-induced changes coincided with a reduction in stomatal conductance, photosynthesis, transpiration, and leaf water potential. The strong enhancement in AA/meOH emission ratios during drought (400-3,500%) was associated with an increase in acetate content of whole leaf cell walls, which became significantly 13C 1,2-labeled following the delivery of 13C 1,2-acetate via the transpiration stream. The results are consistent with central roles of acetate fermentation in regulating plant defense and metabolic responses to drought, and suggest that cell wall O-acetylation may be reversible allowing plants to rapidly respond to drought stresses by down-regulating methyl ester hydrolysis and growth processes while enhancing O-acetylation. We suggest that AA/meOH emission ratios could be used as a highly sensitive non-destructive sensor to discriminate between thresholds of rapid plant growth and drought stress responses.