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The effects of the interaction between high growth temperatures and water stress on gas-exchange properties of Populus nigra saplings were investigated. Water stress was expressed as a function of soil water content (SWC) or fraction of transpirable soil water (FTSW). Isoprene emission and photosynthesis (A) did not acclimate in response to elevated temperature, whereas dark (R_n) and light (R_d) respiration underwent thermal acclimation. R_d was ∼30% lower than R_n irrespective of growth temperature and water stress level. Water stress induced a sharp decline, but not a complete inhibition, of both R_n and R_d. There was no significant effect of high growth temperature on the responses of A, stomatal conductance (g_s), isoprene emission, R_n or R_d to FTSW. High growth temperature resulted in a significant increase in the SWC endpoint. Photosynthesis was limited mainly by CO₂ acquisition in water …