Discussion
Fast growing poplar trees are increasingly being utilized as a
sustainable source of bioproducts and biofuels as well as carbon
farming, urban greening, hillslope stabilization, and marginal land
restoration and re-forestation
(Ragauskaset al., 2006; Furtado et al., 2014). Field observations
have consistently shown that non-water limited poplar plantations have
high growth and productivity rates, but are highly sensitive to drought
(Ji
et al., 2020). Understanding the biological mechanisms and
environmental thresholds that determine plant responses to drought
stress is critical for predicting how the structure and function of
managed ecosystems will respond to environmental change
(McDowellet al., 2008). Previous studies have characterized the sequence
of plant hydraulic, physiological, biochemical, and structural changes
associated with reversible and irreversible responses to drought stress.
For example, leaf dehydration responses of ten angiosperm species showed
stomatal closure and a decrease in xylem conductance occurring first as
a reversible response
(Truebaet al., 2019). This was followed by reaching the turgor loss
point, xylem embolism, and the cessation of transpiration as a critical
irreversible threshold following which further irreversible damage
occurred including to the membranes, pigments, and other components of
the photochemical system in the chloroplast
(Truebaet al., 2019). While ecosystem response to water deficit can be
detected by current remote sensing methods such as solar induced
fluorescence (SIF)
(Sunet al., 2015), and various normalized vegetation indices such as
the Normalized Difference Vegetation Index (NDVI)
(Peterset al., 2002) and Enhanced Vegetation Index (EVI)
(Auliaet al., 2016), these generally only identify extreme drought and
the associated irreversible loss of major leaf function such as
transpiration and net carbon assimilation. For example, in 2-yr oldPopulus deltoides individuals, while strong responses of net
photosynthesis and stomatal conductance to initial water stress were
observed at the leaf level, SIF showed relatively minimal changes
(Helmet al., 2020). It was concluded that the value of SIF as an
accurate estimator of net photosynthesis may decrease during mild stress
events of short duration, especially when the response is primarily
stomatal and not fully coupled with the degradation of photosynthetic
capacity. This highlights the need for new methods to better understand
the biochemical, physiological, and ecological mechanisms in situassociated with the onset of drought stress including processes that
alter plant growth and defense balances and their associated changes in
leaf CO2 and H2O gas exchange fluxes.
In this study, we present foliar AA/MeOH emission ratios as non-invasive
gas-phase chemical observational method providing insights into the
dynamics of growth and defense processes during abiotic stress from cell
wall leaf isolations, individual leaves, branches, and whole ecosystems.
We observed 4 potentially distinct phases during the drought response in
poplar trees, with unique CO2/H2O gas
exchange, volatile emissions (MeOH and AA), and cell wall
characteristics including:
1) Growth Phase: Well hydrated leaves characterized by high
rates of net photosynthesis (Anet), transpiration
(E) and stomatal conductance (gs), and
less negative leaf water potential (LWP) values. MeOH and AA
emissions increase with temperature, with AA/MeOH emission ratios low
with values below 30%.
2) Methanol Suppression Phase: Characterized by high rates ofA, E and gs. Leaf MeOH emissions
are suppressed and AA emissions remain low. The ratio of AA/MeOH may
increase relative to the growth phase but remain below 30%. Drought
effects reversible.
3) Aerobic fermentation Phase Reduction inAnet, E, gs, and
MeOH emissions, with high and sustained emissions of the fermentation
volatiles acetaldehyde, acetic acid (AA), ethanol, and acetone. Cell
wall O-acetylation increases and the ratio of AA/MeOH greatly
increases to 400-3,500%. Drought effects may or may not be reversible.
4) Senescence Phase: Anet, E,gs close to zero indicating a loss of functional
photosynthetic capacity and irreversible degradation of cellular
structural integrity. The ratio of AA/MeOH emissions begins to decline
from the aerobic fermentation phase, but remains high (e.g. 50-100%).