Fig. 1

Experimental setup of hydroponic poplar cultivation. Plants cultivated in axenic tissue culture for 6 to 8 weeks were transferred into soil and grown for another 8 to 10 weeks (including 2 weeks of acclimatization with gradually decreased relative humidity) before use in hydroponic experiments (a). To initiate a hydroponic experiment, the matured plants were dissected into stem cuttings and fixed to grey, lightproof PVC disks using foam material. The leaf surface area was reduced to 4 cm² to minimize evaporation capacity. During an up to 2-week-long rooting phase in pots filled with non-aerated tap water, rooted stem cuttings were transferred to a new PVC disk located on pots filled with aerated ½ Hoagland solution [49] to grow for further 2 weeks before an optional 1-week-long stress phase in the fifth week (b). Five rooted stem cuttings of a single plant yielded one biological replicate. Images in a are not to scale