Conclusions and prospects
Although plants are known to activate growth suppression and defense signaling during abiotic stress, the biochemical, physiological, and ecological mechanisms involved are under intense investigation. Of particular importance is the development of non-destructive in situ methods that are able to characterize the onset of reversible and irreversible phases of drought stress including alterations in growth and defense balances and their associated changes in leaf CO2 and H2O gas exchange fluxes. Moreover, although cell walls constitute the majority of plant biomass, little information exists on the impact of abiotic stress on the degree of cell wall methylation and O -acetylation, which can change biomass properties and value as a renewable source of biofuel or bioproducts. Therefore, understanding the extent to which these changes occur in woody tissue as it develops could help to understand recalcitrance of biomass when harvested, and the extent to which this is driven by exposure of trees to abiotic stress.
In this study, we show that foliar AA/MeOH emissions ratios are a sensitive indicator of the balance between plant growth and defense during drought which can be studied in real-time from individual leaves to whole ecosystems. We identified the active growth phase associated with rapid biomass accumulation and high rates of leaf gas exchange as highly enriched in MeOH emissions relative to AA. Temperature sensitivity studies of MeOH and AA emissions from isolated leaf cell wall preparations (AIR) showed that these volatiles can be released at high rates directly from hydrated AIR with AA/MeOH ratios and their temperature sensitivities similar to emission observations from physiologically active leaves, branches, and ecosystems under non-drought stressed conditions.
However, drought stress was discovered to activate the reversible suppression of MeOH emissions, potentially linked directly to the inhibition of pectin demethylation and suppression of cell wall expansion and growth. Continued exposure to drought conditions lead to numerous coordinated leaf physiological and biochemical changes including reduced stomatal conductance, the suppression of net photosynthesis and transpiration, and the activation of aerobic fermentation. The simultaneous suppression of MeOH emissions and increase in AA emissions during drought resulted in large increases in AA/MeOH foliar emissions ratios from 400-3,500%. While current methods require destructive sampling, our observations suggest that real-time AA emissions (together with other fermentation volatiles like acetaldehyde, ethanol, and acetate) may represent a new highly sensitive technique to evaluate aerobic fermentation-linked defensive processes during abiotic stress. Moreover, results from13C2-acetate labeling of the transpiration stream followed by 1H-NMR analysis of the acetate content of leaf cell wall preparations demonstrates that leaf cell wall O -acetylation is highly dynamic may derive from aerobic fermentation via acetate activation to acetyl-CoA. Critical to the understanding of the roles of cell wall esters in plant abiotic stress responses is to elucidate their functional role(s) during drought including impacts on growth, hydraulics, defense signaling, and carbon and energy metabolism. Rapid changes to cell wall methyl andO -acetyl ester content during drought may allow plants to quickly respond to environmental signals potentially critical for survival during climate extremes.