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

Abstract.

Leaf L-ascorbate content of an ozone (O₃)-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^®-filtered air, and administered one of two O₃ fumigation regimes: chronic exposure (75 nmol O₃ mol^–1 for 7 h day^–1 for 21 days) and acute exposure (180 nmol O₃ mol^–1 for 9 h). Chronic O₃ 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 O₃ on CO₂ assimilation rate observed in control plants (–GalL). Under acute O₃ 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 O₃ 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 O₃ 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 O₃ fumigation, in both the leaf apoplast and symplast. The effect of GalL treatment on ascorbate pools was consistent with the reduction in O₃ damage observed in GalL-fed plants. Attempts to model O₃ interception by the ascorbate pool in the leaf apoplast suggested a greater capacity for O₃ detoxification in GalL-fed plants, which corresponded with the increase in O₃ 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 O₃ fluxes.