This dataset contains information on how variations on temperature and photoperiod affects poplar seedling growth in Eco chambers. Plants were propagated on sterile media (1/2MS, 0.7% Agar, 1mg/L IAA) by cutting the shoot tip and 3-4 leaves and placing them in a weck jar on new medium. Once the explants rooted after incubation at 200 mmol light, 21C day/18C night, long day conditions (16:8) and ambient humidity, they were transplanted onto soil (6:2:1 Peat-free: sand-vermiculite, fertilizer: 4 g/L Osmocote exact standard 15-9-12) and grown first under protective covers and then open in 24C, long day conditions (LD), 65% humidity, 350mmol light (experiments R1-3) or transferred into the Eco chamber (temperature as indicated in experiment, 80-98% humidity, 350mmol LED light, long day conditions). After 3-4 weeks, plants of R1-R3 were transferred in the conditions described in the experiment (R1: 24C SD vs 15C LD; R2: 15C SD vs 15C LD; R3: 15C LD vs 24C LD) and grown for a month before moving them into 5C low light conditions which resulted in senescence and leaf drop. Eco chamber plants were grown in the Eco chamber for about four weeks and after not showing senescence, harvested directly after the experiment without cold incubation. All plants were measured in height and diameter (6cm above soil), leaf number and area regularly and gas exchanged was measured with a TARGAS-1 instrument (Hansatech) at 500mmol light intensity. After the experiment, cross-sections were taken and embedded in resin for further microscopic analysis (data not ready and not included). Full details about this nonGeographicDataset can be found at https://doi.org/10.5285/46c23be6-63ba-400d-859b-050dcb207898