Exposure of spring wheat (Triticum aestivum) to ozone in open‐top chambers. Effects on acyl lipid composition and chlorophyll content of flag leaves

Spring wheat (Triticum aestivum L. cv. Drabant) was exposed to different concentrations of ozone in open-top chambers in a field of commercially grown wheat. The following treatments were used: charcoal-filtered air (CF), non-filtered air (NF) and non-filtered air supplemented with 25 nl l(-1) ozone (NF+) and 35 nl l(-1) ozone (NF++) for 7 h d(-1). Flag leaves were analysed for chlorophyll content, total fatty acid composition and fatty acid compositions and contents of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The chlorophyll content decreased significantly with time and ozone concentration. After 4 wk of exposure, the content of MGDG was significantly lower in flag leaves from the NF++ treatments than in those from the other treatments. The contents of DGDG and PC also decreased while that of PE increased, although these changes were not statistically significant. In the total lipid extract the proportion of 18:3 decreased with time and ozone concentration. Similar changes occurred in MGDG 18:3 at a later stage. Using the concept of accumulated exposure over threshold (AOT), it was evident that the parameters studied differed in ozone sensitivity. After 4 wk of exposure, the best linear dose-response relationships between accumulated exposure and chlorophyll as well as MGDG contents were obtained with a threshold ozone concentration of 30 nl l(-1). For total lipid 18:3 and MGDG 18:3 the corresponding thresholds were 40 and 50 nl l(-1), respectively. Furthermore, the sensitivity to ozone, expressed as loss of chlorophyll, increased with increasing flag leaf age. In leaves exposed for 7, 13 and 20 d, the best linear fits were obtained with AOT50, AOT40 and AOT30, respectively. In conclusion, ozone sensitivity increased with flag leaf age and different membrane constituents were differently sensitive to ozone. The results support previous suggestions that ozone causes premature senescence in wheat flag leaves.