EFFECTS OF LOW OZONE EXPOSURE OF SPRING WHEAT ON NET CO2 UPTAKE, RUBISCO, LEAF SENESCENCE AND GRAIN FILLING

Effects of ozone on spring wheat (Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons ( 1992-1993) at Jokioinen in south-west Finland. The wheat was exposed t o filtered air (CF), non-filtered air (NF), non-filtered air+35 nl l(- 1) ozone for 8 h d(-1) (NF+) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone trea tment (NF+, 45 nl l(-1) 1000-1800 hours, seasonal mean) the net CO2 up take of wheat flag leaves was decreased by c. 40% relative to CF and N F treatments, both initial and total activity of Rubisco and the quant ity of protein-bound SH groups were decreased significantly. Added ozo ne also significantly accelerated flag leaf senescence recorded as a d ecrease in chloroplast size. The effect was significant 2 wk after ant hesis, and senescence was complete after 4 wk. In the CF and NF treatm ents senescence was complete 5 wk after anthesis. The significant effe ct of ozone on the chloroplasts and net CO2 uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filli ng period was shorter for ozone fumigated plants, kernels were smaller . The decrease in 1000-grain weight explained most of the yield reduct ion in the plants under NF+ treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activ ity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.