Hydroponically cultivated radish fed L-galactono-1,4-lactone exhibit increased tolerance to ozone

Leaf L-ascorbate content of an ozone (O-3)sensitive radish genotype (Raphanus sativus L. cv. Cherry Belle) was increased 2-fold by feeding hydroponically cultivated plants L-galactono-1,4-lactone (GalL). Plants were grown in controlled-environment chambers ventilated with charcoal/Purafil((R))-filtered air, and administered one Of two 03 fumigation regimes: chronic exposure (75 nmol O-3 mol(-1) for 7 h day(-1) for 21 days) and acute exposure (180 nmol 03 mol(-1) for 9 h). Chronic 03 exposure decreased root growth by 11% in plants maintained in pure nutrient solution (-GalL), but resulted in no change in root growth in GalL-fed plants (+GalL). Similarly, GalL-feeding counteracted the negative effects Of O-3 on CO2 assimilation rate observed in control plants (-GalL). Under acute 03 exposure, GalL-fed plants showed none of the visible symptoms of injury, which were extensive in plants not fed GalL. Leaf CO2 assimilation rate was decreased by acute 03 exposure in both GalL treatments, but the extent of the decline was less marked in GalL-fed plants. No significant changes in stomatal conductance resulted from GalL treatment, SO 03 uptake into leaves was equivalent in +GalL and -GalL plants. Feeding GalL, on the other hand, enhanced the level of ascorbate, and resulted in the maintenance of the redox state of ascorbate under acute 03 fumigation, in both the leaf apoplast and symplast. The effect of GalL treatment on ascorbate pools was consistent with the reduction in 03 damage observed in GalL-fed plants. Attempts to model 03 interception by the ascorbate pool in the leaf apoplast suggested a greater capacity for 03 detoxification in GalL-fed plants, which corresponded with the increase in 03 tolerance observed. However, modelled data for GalL-fed plants suggested that additional constituents of the leaf apoplast may play an important role in the attenuation of environmentally relevant 103 fluxes.