Effects of different ozone exposure regimes on photosynthesis, assimilates and thousand grain weight in spring wheat

The aim of the study was to compare two ozone exposure patterns with different peak concentrations under equal dose conditions. Spring wheat (Triticum aesthivum cv. 'Nandu') was 'pre-cultivated' under glasshouse conditions and exposed to two different ozone exposure scenarios (A: 1700-0900 h: 20 ppb, 0900-1700 h: 65 ppb; B: 1600-1200 h: 20 ppb, 1200-1600 h: 110 ppb) in large fumigation chambers for two weeks during anthesis. Control plants received charcoal filtered ambient air (<5 ppb ozone 24 h/day) (CF). Chlorophyll content as well as chlorophyll fluorescence (Fv:Fm ratio) and carbohydrate composition (hexoses, sucrose, starch) were monitored in flag leaves and in the case of carbohydrate content also in the penultimate internode during and after ozone exposure. Thousand grain weight (TGW) was determined for estimating yield effects, photosynthesis decreased markedly in both treatments expressed as Fv :Fm ratio. in treatment B chlorophyll content (a + b) of leaves was reduced by 40% after 7 and 14 days of exposure (p less than or equal to 0.001). Carbohydrate content decreased significantly as compared with CF-plants to 20% (p less than or equal to 0.001) in the case of sucrase and starch. Effects were especially pronounced in treatment B (110 ppb for 4 h) and could be observed even seven days after exposure under CF conditions. After 14 days of exposure during anthesis, plants were incapable of recovery as shown by a significant reduction in thousand grain weight of ozone treated plants compared with CF-plants (treatment B: 22% at p less than or equal to 0.001). Effects on photosynthesis and carbon content even under moderate daily ozone peaks as in treatment A induced a significant yield loss of 11% (p less than or equal to 0.001) as well, when applied during anthesis. It can be concluded that under equal dose conditions ozone treatments with comparatively high peak concentration induce more pronounced effects than ozone patterns with moderate peaks. (C) 2000 Elsevier Science B.V. All rights reserved.