Poplar root anatomy after exposure to elevated O 3 in combination with nitrogen and phosphorus

Elevated O3, particularly in interaction with N and/or P levels, induced tissue- and cell type-specific changes in the anatomical structure of poplar roots. Ozone (O3) sensitive poplar clone Populus maximowiczii Henry × berolinensis Dippel was subjected to two levels of O3 (ambient and 2 × ambient) in combination with two levels of nitrogen (N) (0 and 80 kg ha−1) and three levels of phosphorus (P) (0, 40 and 80 kg ha−1) in an open-air pot experiment. Effects of O3 in combination with N and P on anatomical structure of xylem, bark and primary tissues and consequences for theoretical hydraulic properties were investigated after one growing season in 2-mm roots under the microscope. Effect of O3 as single factor was observed as increased primary xylem area and number of protoxylem poles, increased secondary xylem area and accumulated potential hydraulic conductivity. Vessel density, but not vessel size was negatively affected by O3. Stronger correlation between tangential vessel diameter and vessel density was encountered under ambient O3, indicating a slight dysregulation of vessel formation under elevated O3. Increasing P resulted in increased number of protoxylem poles, while N mostly acted in interaction with O3 or P. There was a strong interaction between O3 × N × P on vessel grouping index and mean vessel group size. At elevated O3, effects of both nutrients on vessel grouping and mean vessel group size were repressed. Under ambient O3, application of N resulted in increased fibre cell thickness, which was not the case under elevated O3, indicating carbon-saving mechanisms.