Species-specific metabolic response to combined heat+drought
stress
The differences observed in metabolic response of young white spruce and
paper birch trees to the combination of heatwave and drought stress
versus the response to either independent stress is indicative of the
problem of generalizing plant responses across species to multiple
stressors acting simultaneously in the growth environment. White spruce
showed many unique metabolic responses to the combined stress, while
paper birch displayed few. The HD birch tended to share responses
with the D plants (e.g. total leaf N, Spd, Spm, and
several AAs) or the response was between that of the D andH plants (e.g. soluble sugars). These findings partially
support our first hypothesis that plant responses to the combination of
heatwave and drought stress will be unique from either independent
stress. Furthermore, the unique responses in spruce did not remain
constant over time, but instead were mainly limited to the first year of
heatwave exposure. A potential explanation for this is that “ecological
stress memory” is responsible for the adjustments between the first and
second year of treatment (Walter, Beierkuhnlein, Jentsch & Kreyling
2013). The idea behind ecological stress memory is that individual
plants will respond to a stress event differently if they have
previously been exposed to that stress.
The spruce exhibited many distinctive metabolic responses to the
combined stress, especially in the first season (e.g. AAs, PAs,
TSP, chlorophyll a +b ) whereas the birch did not. These
findings do not support our second or third hypotheses that N and C
metabolite pools in birch will show a greater response to heat+drought
stress than the spruce, and that the changes observed in N and C
metabolism after one year of stress will be carried over into the
following growing season. In the first season of prolonged HDstress, relatively more metabolic adjustment occurred through the
accumulation of several AAs and TSP whereas in the second season ofHD stress, adjustment occurred through changes in PAs and
fructose. The major adjustments to individual AAs from the HDtreatment in spruce show how carbon and nitrogen assimilation can become
completely disrupted under multiple stressors. For example, Ser, Gly,
and Gln are involved in the recycling of carbon from photorespiration
(Blackwell, Murray & Lea 1990). Photorespiration is known to increase
during both drought and high temperatures (Jordan & Ogren 1984; Wingleret al. 1999) and it is very likely that the activity of this
pathway was elevated in the HD spruce, as indicated by the 5-fold
increase in Ser and 2-fold increase in Gln. Unfortunately, during the
analysis Gly did not separate from Arg and Thr, therefore changes in Gly
content in 2016 could not be assessed. Furthermore, the elevated levels
of Phe and Trp suggest a greater proportion of carbon may have been
allocated to protective compounds, e.g. phenolics. Such response
was observed in Eucalyptus exposed to heat+drought triggering a
novel accumulation of cinnamate, a substrate central to phenylpropanoid
biosynthesis derived from Phe (Correia et al. 2018). Proline is a
well-known indicator of osmotic stress and both Pro and GABA have been
shown to provide protection from a number of environmental stress
factors (Bouché & Fromm 2004; Liang, Zhang, Natarajan & Becker 2013b).
The substantial increases in Pro and GABA indicate the combination of
heat and drought may have drastically increased osmotic stress in these
plants that was not experienced under either independent stress. Two
decades ago, it was suggested that until recent climate warming, drought
may have been the only factor limiting growth of white spruce (Barber,
Juday & Finney 2000). If elevated temperatures had not been a selective
pressure in white spruce’s recent life history (i.e. no
transgenerational epigenetic inheritance), these plants may have
responded by overcompensating in AA accumulation when experiencing both
heat and drought stress for the first time. But by the second season,
the plants had already experienced the combined stress and therefore did
not have the same over-compensatory response due to ecological stress
memory. The first year may have primed their stress-response systems for
a more prepared response the following year (Hilker & Schmülling 2019).
This may also explain why no carry-over effect was observed at the start
of the second season. Our findings highlight the importance of not
inferring plant responses to multiple stressors based on the responses
to either independent stress for these data show that the addition of
high temperatures during a prolonged drought can have major consequences
on primary metabolism in spruce that are not experienced when either
stress is applied independently.
The only unique responses in the HD birch plants from either of
the individual stressor was the accumulation of Trp and Phe in late
August 2016. Phenylalanine and Trp are aromatic AAs that are precursors
to a wide variety of secondary metabolites, the phenylpropanoids derived
from Phe, and auxin, phytoalexins, glucosinolates, and alkaloids derived
from Trp (Radwanski & Last 1995; Tzin & Galili 2010). All of these
compounds have significant roles in protection against abiotic and
biotic stress and their production is stimulated by stress (Dixon &
Paiva 1995). The novel accumulation of Phe and Trp in the HDplants suggests there were likely accumulations of other phenolic
compounds involved in the catabolism of Phe and Trp. It was surprising
that the HD birch did not accumulate sucrose in either year in
response to the combined stress. These plants also did not accumulate
Pro in either year which is why sucrose may may accumulated since it
often replaces Pro as the dominant osmolyte under the combination of
high temperatures and drought (Rizhsky et al. 2004b). Also, to
our surprise, the combined stress did not uniquely alter soluble
inorganic ion concentration or other soluble sugar concentrations (aside
from xylose+arabinose in late 2017).