Abstract
Wheat ( Triticum aestivum L.) cv. Minaret was grown in open‐top chambers (OTCs) in 1995 and 1996 under three carbon dioxide (CO 2 ) and two ozone (O 3 ) levels. Plants were harvested regularly between anthesis and maturity to examine the rate of grain growth (dG/dt; mg d –1 ) and the rate of increase in harvest index (dHI/dt;% d –1 ). The duration of grain filling was not affected by elevated CO 2 or O 3 , but was 12 days shorter in 1995, when the daily mean temperature was over 3 °C higher than in 1996. Season‐long exposure to elevated CO 2 (680 μmol mol –1 ) significantly increased the rate of grain growth in both years and mean grain weight at maturity (MGW) was up to 11% higher than in the chambered ambient air control (chAA; 383 μmol mol –1 ). However, the increase in final yield obtained under elevated CO 2 relative to the chAA control in 1996 resulted primarily from a 27% increase in grain number per unit ground area. dG/dt was significantly reduced by elevated O 3 under ambient CO 2 conditions in 1995, but final grain yield was not affected because of a concurrent increase in grain number. Neither dG/dt nor dHI/dt were affected by the higher mean O 3 concentrations applied in 1996 (77 vs. 66 nmol mol –1 ); the differing effects of O 3 on grain growth in 1995 and 1996 observed in both the ambient and elevated CO 2 treatments may reflect the contrasting temperature environments experienced. Grain yield was nevetheless reduced under elevated O 3 in 1996, primarily because of a substantial decrease in grain number. The data obtained show that, although exposure to elevated CO 2 and O 3 individually or in combination may affect both dG/dt and dHI/dt, the presence of elevated CO 2 does not protect against substantial O 3 ‐induced yield losses resulting from its direct deleterious impact on reproductive processes. The implications of these results for food production under future climatic conditions are considered.