Developing xylem and mature xylem were collected separately during the harvest and shortly frozen in liquid nitrogen. Plants were multiplied by in vitro propagation and kept on 1/2 MS-medium for four weeks. Afterwards, plants were grown in Long Ashton nutrient solution for 8 weeks until the treatment started. Poplars were kept in a culture room (air temperature: 24 °C, relative humidity: 25%, light regime: 16 h light/8 h dark, photosynthetic active radiation (PAR): 150 μmol photons m-2 s-1). Three treatments were applied for 30 days: control treatment (-ABA), continuous ABA treatment (+ABA), and discontinuous ABA treatment (±ABA). No ABA was fed to control plants. ABA was added to the Long Ashton solution to achieve the specified concentration. ABA (100 µM) was fed to +ABA plants for all 30 days of the treatment. For ±ABA plants, ABA was fed only at the beginning (0 to 2 days), the middle (14 to 16 days) and the end (28 to 30 days) of the treatment. Nucleic acid was extracted according to CTAB extraction protocol (Chang et al., 1993). TruSeq mRNA Sample Prep kit v2 (Illumina, San Diego, CA, USA) according to manufacturer's instructions.
Experimental Factor: sampling site
mature xylem
Experimental Factor: stimulus
discontinuous abscisic acid