Effects of chronic ozone exposure and estimated flux on plant growth and conductance under field conditions
Creators: name: Wright, Elaine Frances
Publisher: name: University of British Columbia
Year: 2009

description: Ozone is a major air pollutant in many parts of the world. In the Lower Mainland of British Columbia, occurrences of levels exceeding the B.C. Level A hourly objective of 50 ppb (parts per billion; nl L⁻¹)are frequent in suburban and rural areas. Growth and yield responses of numerous crop and tree species to ozone at daytime hourly average concentrations up to or exceeding 200 ppb have been described by others. However, this information has largely come from studies using open-top field chambers and using experimental protocols involving the enrichment of the ambient air with ozone that do not simulate true field conditions. The present studies used an open-field zonal air pollution system (ZAPS) to examine the effects of low levels of ozone on the yield and growth dynamics of locally important cultivars of pea (Pisum sativum L. cv. Puget), potato (Solanum tuberosum L. cv. Russet Burbank), bean (Phaseolus vulgaris L. cv. Galamore) and radish (Raphanus sativus L. cvs. Cherry Belle and French Breakfast), and of young Douglas-fir saplings (Pseudotsuga menziesii (Mirb.) Franco), under true field conditions. The experiments were conducted in 1986 and 1988-1990, and involved exposures to 12 randomly assigned treatments characterized by stochastically varying degrees of enrichment of the ambient ozone levels, together with ambient air control plots. In each year, the treatments obtained in the ZAPS had unimodal concentration distributions that fitted the Weibull distribution, and typical season-long diurnal average concentration profiles with mid-afternoon maxima. Since the enrichment levels were proportional to the ambient ozone level, the ranges of concentrations achieved in the treatments varied from day to day and from year to year. For each species and season, the exposure treatments were summarized in terms of seasonal daytime means and various cumulative exposure indices. However, comparisons of linear regressions of different growth variables with different indices led to standardization on the use of the D50 index, defined as the number of days per season or per harvest interval in which an hourly mean ozone concentration of 50 ppb was exceeded in any hour between 0900 and 2100 (PDT). Significant (p &lt;0.05) negative linear regressions of yield with increasing D50 exposure were found for all crops. The effects on final yield were reflected in significant decreases in the dry weights of the total plant, stems and leaves, and in leaf areas, by the time of final harvest. However, during the earlier stages of growth, the effects were less marked and the regressions failed to reach significance, even at p &lt;0.10. Three experiments with radish in 1989 revealed pronounced differences in response, depending on time of year. No significant effects of ozone were observed on either Cherry Belle or French Breakfast cvs. in the first experiment in June, but significant growth reductions were observed in the July-August and August-September plantings. The second planting experienced the highest exposures, but since the exposures during the first and third experiments were comparable, the different responses observed suggest the influence of other environmental factors related to time of year. In general the dynamics of crop growth were adversely affected by increased exposure, reaching significance at the later harvest intervals. Absolute growth rates of all crops and cultivars were significantly reduced (p &lt;0.05), but relative growth rates were more variable and their reductions only reached significance at p &lt;0.10, although in several cases significant increases were observed in the early stages of growth. Overall, adverse effects on the growth of all of the crops were observed even though the daytime hourly average ozone concentrations rarely exceeded 120 ppb. Differences between the responses of the radish cultivars indicated that cv. Cherry Belle is more sensitive than cv. French Breakfast. No consistent significant effects of ozone on the growth of Douglas-fir saplings occurred until the end of the second season of exposure (1989). By then, reductions in second flush growth were observed. These effects were carried over into the following year, as revealed by reduced leader growth in early 1990. Although Douglas-fir has been described as relatively insensitive to ozone, these results suggest that long-term detrimental effects on growth may occur. In all species the relationships between many growth variables and exposure appeared to be non linear. However, although there were several cases in which non-linear Weibull or gamma functions provided improvements in fit compared to simple linear regressions, there was no consistent pattern of improvement. Since indices such as D50 merely define exposure and do not necessarily reflect uptake of ozone by foliage, estimates of ozone flux were made during the experiments with radish and Douglas-fir. Whole plant porometry was used at intervals to determine water vapour conductances, which were then used to estimate ozone fluxes over the preceding time period. With radish, linear regressions of various growth measures using estimated flux (FLUXSUMC) as the dependent variable generally showed poorer fits than regressions based on the D5 0 index. However, with second flush growth of Douglas-fir, better fits were obtained with FLUXSUMC than D50, reaching significance for weight and leaf area at the final harvest in 1989. To avoid the assumption that measured conductances accurately reflected conductances during the preceding interval of days, estimates of ozone flux were also calculated based on multiple linear regressions of conductance and several environmental variables. These estimates of flux (FLUXSUMM) employed the meteorological variables and ozone levels recorded throughout the exposure seasons. Although 75 percent of the comparisons with linear regressions based on the D50 index showed better fits with FLUXSUMM, only the regression for second flush weight at harvest 5 was significant. The present study therefore provides only limited support for the view that the use of flux estimates rather than exposure indices results in improvements in describing growth responses to ozone.