Rate and duration of grain filling in relation to flag leaf senescence andgrain yield in spring wheat (Triticum aestivum) exposed to different concentrations of ozone

Effects of different concentrations of ozone on grain filling, flag leaf se nescence and final grain yield in field-grown spring wheat (Triticum aestiv um L. cv. Dragon) were studied using open-top chambers. The hypothesis test ed was that an ozone-induced reduction in grain yield is mainly related to an enhanced senescence and a shortening of the grain-filling period. The pl ants were exposed to filtered air (F), non-filtered air without extra ozone (NF) or non-filtered air with 3 different levels of ozone added (NF1 +, NF 2 + and NF3 +). The mean daytime (08.00-20.00 h) ozone concentrations durin g the exposure period (31 days) were 7, 20, 34, 48 and 62 nmol mol(-1) in F , NF, NF1 +, NF2 + and NF3 +, respectively. The corresponding ozone doses, expressed as the accumulated exposure over a concentration threshold of 40 nmol mol(-1) (AOT40), were 0, 12, 1989, 5881 and 10375 nmol mol(-1) h, resp ectively, and 884, 2594, 4557, 6188 and 7900 pmol m(-2) respectively, expre ssed as the calculated cumulative flag leaf ozone flux (CFO30). The flag le aves senesced earlier and the grain-filling duration was significantly shor ter at higher ozone exposure compared to F (-5, -13 and -18% in NF1 +, NF2 + and NF3 +, respectively). The relative grain-filling rate did not differ between the treatments. The 1000-grain weights were 10, 28 and 37% lower, a nd the grain yields were 15, 29 and 46% lower than F in NF1 +, NF2 + and NF 3 +, respectively. Ozone exposure had no significant effect on the number o f grains per unit ground area or on straw yield, but significantly reduced the harvest index: and increased the grain protein concentration in NF2 + a nd NF3 + compared to F. The grain yield was negatively correlated with the ozone dose, expressed either as AOT40 or as CFO3 with or without an ozone f lux threshold. The 1000-grain weight,vas positively correlated with the gra in-filling duration (R-2 = 0.998), which in turn was positively correlated with the leaf area duration (R-2 = 0.989).