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, 2x[O-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 a-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 mu mol g(-1) when SWC dropped from 25 to 20%, and alpha-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 2x[O-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.