Although no large effect on leaf physiology was observed, the results are consistent with methanol both impairing photochemistry of the light reactions via formaldehyde toxicity and stimulating photosynthesis and dark respiration through formate oxidation to CO2. The results demonstrate the large physiological and biochemical impacts of high growth temperature on poplar seedlings and highlight the enhancement of the optimum temperature of ETR as a rapid thermal acclimation mechanism. A small number of extracted leaf tissue metabolites (55 out of 10 015) showed significantly altered abundances under LGT and methanol treatments relative to water controls, and this increased in compound number (222) at the HGT. However, we were unable to determine if soil methanol was sufficiently taken up by the plant to have a direct effect on foliar processes. Methanol application did not significantly alter leaf gas exchange but slightly reduced the suppression of Rd and Rp by the high growth temperature while slightly impairing ETR, Fv′/Fm′, and qp. A large number of metabolites (867) were induced under HGT, many implicated in flavonoid biosynthesis highlighting a potentially protective role for these compounds. In contrast, HGT plants showed an upregulation of nonphotochemical quenching (NPQt), the optimum temperature for ETR, and leaf isoprene emissions at 40 ☌. Relative to plants grown under the low growth temperature (LGT), high growth temperature (HGT) plants showed a suppression of leaf water use and carbon cycling including transpiration (E), net photosynthesis (Pn), an estimate of photorespiration (Rp), and dark respiration (Rd), attributed to reductions in stomatal conductance and direct negative effects on gas exchange and photosynthetic machinery. Here, we characterized temperature acclimation at 35 ☌ of leaf gas exchange, chlorophyll fluorescence, and extractable metabolites of potted Populus trichocarpa saplings and examined potential influences of mM concentrations of methanol added during soil watering over a two-month period. Foliar and root applications of methanol have been implicated in plant acclimation to high temperature via the C1 pathway. High temperatures alter the thermal sensitivities of numerous physiological and biochemical processes that impact tree growth and productivity.
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