YIELD RESPONSE OF WINTER-WHEAT TO CHRONIC DOSES OF OZONE

The relative sensitivity of 11 soft red winter wheat, Triticum aestivum L. cultivars, exposed as young plants to ambient levels of ozone (O3) was determined. On the basis of the shoot dry weight response, the cultivar Holly was determined to be significantly more sensitive than Oasis or Coker 47-27: Blueboy II showed intermediate sensitivity. Plants of these 4 cultivars, grown in pots or in the ground, were exposed for 54 days in open-top field chambers to different O3 concentrations added to existing levels of ambient oxidants for 7 h/day. The effects of O3 on foliage injury, growth, and yield were determined. For the 4 cultivars combined, the threshold O3 concentration (7h/day seasonal mean) for significant injury and decreased growth and yield was between 0.06 and 0.10 ppm. For potted plants exposed to 0.10 and 0.13 ppm O3, seed weight yields were 10 and 27% less, respectively, than for those grown in 'charcoal-filtered air' chambers (0.03 ppm O3). For plants in the ground exposed to 0.10 and 0.13 ppm O3, the yields were 16 and 33% less, respectively, than for those at 0.03 ppm O3. The relative sensitivity of cultivars to O3 as young plants could not be used to predict O3 effects on seed yield.