Title:
Differential drought-induced modulation of ozone tolerance in winter wheat species.

Abstract:
Recent reports challenge the widely accepted idea that drought may offer protection against ozone (O(3)) damage in plants. However, little is known about the impact of drought on the magnitude of O(3) tolerance in winter wheat species. Two winter wheat species with contrasting sensitivity to O(3) (O(3) tolerant, primitive wheat, T. turgidum ssp. durum; O(3) sensitive, modern wheat, T. aestivum L. cv. Xiaoyan 22) were exposed to O(3) (83ppb O(3), 7h d(-1)) and/or drought (42% soil water capacity) from flowering to grain maturity to assess drought-induced modulation of O(3) tolerance. Plant responses to stress treatments were assessed by determining in vivo biochemical parameters, gas exchange, chlorophyll a fluorescence, and grain yield. The primitive wheat demonstrated higher O(3) tolerance than the modern species, with the latter exhibiting higher drought tolerance than the former. This suggested that there was no cross-tolerance of the two stresses when applied separately in these species/cultivars of winter wheat. The primitive wheat lost O(3) tolerance, while the modern species showed improved tolerance to O(3) under combined drought and O(3) exposure. This indicated the existence of differential behaviour of the two wheat species between a single stress and the combination of the two stresses. The observed O(3) tolerance in the two wheat species was related to their magnitude of drought tolerance under a combination of drought and O(3) exposure. The results clearly demonstrate that O(3) tolerance of a drought-sensitive winter wheat species can be completely lost under combined drought and O(3) exposure.