Fig. 4

Histochemical analysis of poplar roots cultivated in hydroponic control conditions for 5 weeks. a Growth in thickness results in a conical three-dimensional geometry of the whole root as well as the central cylinder (n = 12 roots). b Endodermal Casparian band development. Already closely behind the root tip, the first thin Casparian bands were visible in radial endodermal cell walls at 10–20% relative distance. The Casparian bands constantly developed and increased in length even in the basal root segments of 90–100% relative root length. No Casparian bands developed in the hypodermis (no picture shown). c Endodermal suberin lamellae development. At 27.5% relative root length on average, first suberized endodermal cells were visible. Suberization increased in a patchy manner adjacent to the phloem poles to reach an almost full suberization in the basal 90–100% relative distance. No suberin lamellae developed in the hypodermis (no picture shown). Scale bars = 50 µm