The effect of ozone on horticultural crops important in the Fraser Valley of British Columbia

Field: Agricultural and Biological Sciences (Life Sciences)

An analysis of air quality data from British Columbia has identified the Lower Mainland and surrounding rural areas as one of the regions in Canada where the Canadian Maximum Acceptable Air Quality Objective of 0.082 ppm ozone for one hour is frequently exceeded. Ozone at this level has the potential for affecting crops in the Fraser Valley. Field experiments were undertaken to attempt to evaluate the effect of randomly fluctuating levels of ozone on the yield of two cultivars each of Brassica oleracea L. (broccoli), Phaseolus vulgaris L. (bean), Pisum sativum L. (pea), Daucus carota L. (carrot) in 1985 and on one cultivar each of Solanum tuberosum L. (potato) and Pisum sativum L. (pea) in 1986, using a zonal air pollution system. As there is no current consensus regarding the most appropriate numerical expression of pollutant exposure to use in vegetation response studies, a comparison of various exposure terms was also undertaken. Ozone was added in various proportions to ambient levels between 0700 and 2100 hours (PDT) throughout the growing season. Three levels of ozone addition were used in 1985 and 12 in 1986. In 1985, treatments were assigned to three blocks over which ozone was released. Each block was supplied with different total amounts of supplementary ozone, a fourth block serving as an ambient air control. In 1986, ozone treatments were randomly assigned to four sub-plots on each of the three blocks over which ozone was released, with each block receiving the same total amount of supplementary ozone. Different treatments were achieved by each sub-plot being subject to different rates of release and degrees of mixing. For both years the ozone concentration distributions achieved over the season were approximately log-normal. Additional analysis of the air quality data from the ambient air plot found other types of skewed distributions such as the three parameter Weibull, three parameter gamma and Johnsons SB (four parameter log-normal) provided better descriptions of the data. The distribution providing the best fit depended on the concentration averaging time, the daily time span over which the ozone concentrations are analyzed and the selection criterion used. In 1985, field observations indicated that there were numerous plot to plot differences for disease and soil factors, which were confounded with the ozone treatments applied. Without true replication of the treatments, differentiation between the effects due to ozone and those from abiotic and biotic causes was not possible, and hence no clear conclusions concerning ozone response could be drawn. In 1986, without the confounding of ozone and plot location, significant linear reductions in yield were found for pea and pod fresh weight using the number of days on which the concentration exceeded 25 ppb, during the vegetative growing period (D25²). A significant linear reduction in fresh potato tuber weight was found using the geometric mean of all geometric mean ozone concentrations computed between 1200 and 1259-h for the season (GH12) as the exposure statistic. A significant multiple linear regression was found for pea fresh weight using the D25¹, statistic together with the number of occurrences in which the concentration exceeded 25 ppb for two, three and four consecutive hours (2C25, 3C25 and 4C25 respectively) in an episode; and for pod fresh weight using D25, 2C25 and 3C25 as independent variates. From the results presented it seems clear that ozone at the concentrations dispensed here would have a significant negative impact on crops grown in the Fraser Valley. Based on the 1986 experiment the best case estimate indicates that yield reductions of 28% could be expected for peas and potatoes at 37 ppb ozone (expressed as the season-long 7-h mean, M7). The exposure statistics used in the present study in comparison with the season-long 7 and 12 hour means provided good fits with the data. They are easily calculated from ambient air quality data and present attractive alternatives to those exposure statistics currently in use, for assessing the potential impact of ozone on crops in the Fraser Valley and for use in the setting of air quality standards.