Modification of photosynthesis and growth responses to elevated CO(2) by ozone in two cultivars of winter wheat with different years of release

The beneficial effects of elevated CO(2) on plants are expected to be compromised by the negative effects posed by other global changes. However, little is known about ozone (O(3))-induced modulation of elevated CO(2) response in plants with differential sensitivity to O(3). An old (Triticum aestivum cv. Beijing 6, O(3) tolerant) and a modern (T. aestivum cv. Zhongmai 9, O(3) sensitive) winter wheat cultivar were exposed to elevated CO(2) (714 ppm) and/or O(3) (72 ppb, for 7h d(–1)) in open-topped chambers for 21 d. Plant responses to treatments were assessed by visible leaf symptoms, simultaneous measurements of gas exchange and chlorophyll a fluorescence, in vivo biochemical properties, and growth. It was found that elevated CO(2) resulted in higher growth stimulation in the modern cultivar attributed to a higher energy capture and electron transport rate compared with the old cultivar. Exposure to O(3) caused a greater growth reduction in the modern cultivar due to higher O(3) uptake and a greater loss of photosystem II efficiency (mature leaf) and mesophyll cell activity (young leaf) than in the old cultivar. Elevated CO(2) completely protected both cultivars against the deleterious effects of O(3) under elevated CO(2) and O(3). The modern cultivar showed a greater relative loss of elevated CO(2)-induced growth stimulation due to higher O(3) uptake and greater O(3)-induced photoinhibition than the old cultivar at elevated CO(2) and O(3). Our findings suggest that the elevated CO(2)-induced growth stimulation in the modern cultivar attributed to higher energy capture and electron transport rate can be compromised by its higher O(3) uptake and greater O(3)-induced photoinhibition under elevated CO(2) and O(3) exposure.