Effects of drought and canopy ozone exposure on antioxidants in fine roots of mature European beech (Fagus sylvatica)

We quantified ascorbate, glutathione and alpha-tocopherol in fine roots of mature Fagus sylvatica L. under free-air canopy ozone (O(3)) exposure (twice ambient O(3) concentration, 2xO(3)]) during two growing seasons that differed in the extent of summer drought (exceptional drought year 2003, average year 2004). This design allowed us to test whether O(3) exposure or drought, or both, affected root antioxidants during the growing season. In both years, root ascorbate and alpha-tocopherol showed a similar relationship with volumetric soil water content (SWC): ascorbate concentrations on a root dry mass basis increased from about 6 to 12 micromol g(-1) when SWC dropped from 25 to 20%, and a-tocopherol increased from 100 to 150 nmol g(-1) at SWC values below 20%. Root glutathione showed no relationship with SWC or differences between the dry and the average year, but it was significantly and consistently diminished by 2xO(3)]. Our results were inconclusive as to whether shoot-root translocation of glutathione or glutathione production in the roots was diminished. Phloem glutathione concentrations in the canopy remained constant, but reduced transport velocity in the phloem and, as a consequence, reduced mass flow of glutathione cannot be ruled out.