Populus trichocarpa
Cuttings of the Populus trichocarpa 101-74 genotype (INRA, Orl*ans, France) were planted in 20 L pots containing a mixture of peat and sand (50/50, v/v) amended with magnesian chalk (60 g/100 L) and fertilized with a slow releasing fertilizer (Nutricote T100, 13/13/13/2, N/P/K/Mg   trace elements). All cuttings were placed in a greenhouse where temperature was maintained in the range 15–27*C and exposed to natural daylight (ranging from 350 to 900 µmol m-2 s-1). Plants were manually watered every second day during 5 weeks. Shoot apices from 20 individuals were then harvested, cleared from differentiated tissues to isolate the shoot apical meristem, frozen in liquid nitrogen and conserved at -80*C.