EFFECT OF GROWTH STAGE ON SOYBEAN RESPONSE TO CHRONIC OZONE EXPOSURE

Four cultivars of soybean [Glycine max (L.) Merr., cv. Forrest, Davis, Bragg, and Ransom] were exposed to chronic doses of O3 in open-top field chambers to determine if a relationship exists between growth stage and yield response to O3. The season was divided into four 31-d quarters (Q1-Q4) in which plants were exposed either to charcoal-filtered air (C) or to nonfiltered air with O3 added (N). Sixteen different quarterly sequences represented all possible combinations of the C and N treatments. The mean 31-d O3 concentration (7 h d-1 mean) in the N treatment was almost identical at 96 to 98 nL L-1 (ppb) for each quarter. In the sequence with N during all four quarters (NNNN), plants were exposed from emergence to physiological maturity. Mean yields in the NNNN treatment, expressed as a percentage of yield in the CCCC treatment ranged from 44% for Forrest (56% loss) to 70% for Bragg (30% loss). Ozone exposure during mid- to late-growth stages generally caused a greater yield decrease than exposure during early growth stages. Models to estimate O3-yield relationships were developed separately for each cultivar using seasonal and quarterly O3 statistics. A statistical comparison for relative goodness-of-fit between these two models was not possible. However, the models based on the quarterly O3 values provided estimates with less absolute deviation from the observed data than models based on seasonal O3 values. Models based on quarterly O3 values generally confirmed that the relative effect of O3 during a given quarter was related to when the pod-fill stage occurred. For example, with the early-maturing Forrest, pod fill occurred mainly during Q3 so O3 stress during Q3 had the greatest effect. For the later-maturing Bragg, pod fill occurred mainly during Q4 so O3 stress during Q4 had the greatest effect.