Introduction
Terrestrial ecosystem dynamics are dramatically changing in response to trends in surface warming and drought (Palut and Canziani, 2007). Increased magnitude, frequency, and spatial distribution of abiotic stress anomalies threaten the ability of natural and managed ecosystems to produce sustainable food, wood, biofuels, and other bioproducts as well as to mitigate increased atmospheric CO2 by photosynthetic conversion to biomass. Driven by increasing leaf-atmosphere vapor pressure deficits and soil moisture limitations (Eamus et al., 2013), partial stomatal closure in response to high temperature and drought stress reduces leaf gas exchange including net photosynthesis and transpiration fluxes (Dewhirst et al., 2021a). This phenomenon is well documented on diurnal time scales in numerous ecosystems, where a mid-day depression of net photosynthesis, transpiration, and stomatal conductance are associated with high vapor pressure deficits and leaf temperatures above their optimal values (Pathre et al., 1998).
Prolonged excessive water loss via transpiration not replaced by water uptake from the soil can result in drought-induced tissue senescence and mortality, thereby converting individual plants and ecosystems from net sinks of CO2 to net sources (McDowell et al., 2008; Jardine et al., 2015; Liu et al., 2021). In northern China, trees of the fast growing genus poplar, which are actively being investigated for afforestation efforts and as renewable sources of biofuels and bioproducts globally, have experienced large-scale dieback and mortality in recent years (Ji et al., 2020). An estimated 79.5% of the area of the poplar forests have experienced severe degradation with an observed trend of narrower tree-ring widths of intact trees together with reduced soil moisture. These observations highlight the need to understand the mechanisms of poplar forest growth suppression and die-back in response to drought stress (Ji et al., 2020).
A common thread among many of the biochemical and physiological processes that determine ecosystem dynamic responses to climate change variables are alterations in plant cell wall chemical composition, structure, and function (Dewhirst et al., 2020a). A large proportion of the plant cell wall polymers can be heavily modified with methyl and O -acetyl ester groups which may play important roles in cell growth and tissue development (Peaucelle et al., 2012), proper xylem (Yuan et al., 2016) and stomatal functioning (Amsbury et al., 2016), central carbon and energy metabolism (Jardine et al., 2017), and stress communication and signaling (Novaković et al., 2018). For example, wood of hybrid poplar trees, one of the fastest growing temperate trees in the world, is composed of lignin (22%), cellulose (40%), hemicellulose (20%) dominated by theO -acetylated polysaccharide glucuronoxylan, and other polysaccharides such as pectins (18%), which can be both heavilyO -acetylated and methyl-esterified (Sannigrahi et al., 2010). The two main components of the plant primary cell wall, the pectin matrix and the cellulose/xyloglucan network, are constantly being remodeled to support dynamic morphological and physiological processes from daily growth and stress response patterns, to developmental changes over longer time scales (Chebli and Geitmann, 2017). This remodeling is regulated, in part, by a number of loosening and stiffening agents including pectin and xylan methyl and acetyl esterases which catalyze the hydrolysis of cell wall esters on the wall. The hydrolysis of methyl and O -acetyl esters leads to rapid physicochemical changes in the cell wall and the release of methanol (Fall, 2003) and acetic acid (Scheller, 2017). Given that cell wall methyl and O -acetyl esters are known to modify cell wall elasticity/rigidity (Peaucelle et al., 2011), and previous observations have shown links between bulk cell wall elasticity and water relations (Roig-Oliver et al., 2020), they may play important roles in the response to drought (Ganie and Ahammed, 2021). However, how the degree of cell wall esterification varies with abiotic stress is largely unknown (Pauly and Keegstra, 2010; Gille and Pauly, 2012).
One evolutionarily conserved, but poorly understood survival strategy in plants is drought-induced activation of aerobic fermentation, resulting in the formation of acetate (Kim et al., 2017). The degree of acetate accumulation in plants predicts survivability, in part by mediating protein acetylation and jasmonate defense signaling. In this study, we hypothesize that due to hydraulic limitations to growth during drought, cell wall-derived MeOH production is inhibited. Moreover, although cell wall-derived AA may be an important source of foliar AA emission to the atmosphere (Dewhirst et al., 2020b), we hypothesize that aerobic fermentation becomes the dominant source of leaf AA emissions during drought, as a central component of the plant drought response. We aimed to define real-time patterns in MeOH and AA emissions together with the fermentation volatiles acetaldehyde, ethanol, and acetone in parallel with leaf gas exchange (net photosynthesis, transpiration, stomatal conductance) and leaf water potential during experimental drought stress in 2-year old potted California poplar (Populus trichocarpa ) trees and complementary MeOH and AA gas exchange studies on leaf bulk cell wall preparations and physiologically active leaves, branches and whole ecosystems.
We define the AA/MeOH emission ratio as a potentially sensitive atmospheric indicator of environmental and biological conditions that favor rapid plant growth versus reduced growth and defense activation. Using detached branches and whole plant xylem delivery of a 10 mM13C2-acetate solution via the transpiration stream followed by analysis of13C2-acetate content of leaf cell walls preparations, we evaluate the hypothesis that aerobic fermentation signaling can impact the acetylation of numerous biopolymers including cell wall carbohydrates.