The Effect of Ozone on Photosynthesis, Vegetative Growth and Productivity of Plum Trees (Prunus Salicina, cv. Casselman) in the San Joaquin Valley of California

Principal Investigator: Dr. L. E. Williams
University of California, Davis
1 November 1993

ARB Contract No. A133-137 (Full Report)

ABSTRACT

Nursery stock of plum (Prunus salicina Lindel . , cv. "Casselman") were planted 1 April 1988 in an experimental orchard at the University of California Kearney Agricultural Center near Fresno, California. The trees were enclosed in open-top fumigation chambers on 1 May 1989, and were exposed to three different ozone concentration treatments (charcoal filtered air, ambient air, and ambient air + ozone) during the 1989 through 1992 growing seasons (typically 1 April to 1 November). A no-chamber treatment plot was utilized to assess chamber effects on tree performance. This final report details the results of the exposures during the initial commercial bearing period (1991 through 1993) in this orchard. The mean 12-h (0800-2000 h Pacific Daylight Time [PDT]) ozone concentrations during the experimental periods in the charcoal filtered, ambient, ambient + ozone, and no-chamber treatments were 0.034, 0.050, 0.0945, and 0.058 ppm in 1991, and 0.027, 0.045, 0.087, and 0.054 ppm in 1992, respectively. Leaf net CO, assimilation rate of "Casselman" plum decreased with increasing ozone concentrations from the charcoal filtered to ambient + ozone treatment. There was no difference in plum leaf net CO, assimilation rate between the ambient chamber and no-chamber plots. Trees in the ambient + ozone treatment generally had greater leaf-fall earlier in the growing season than those of the other treatments. Trunk cross-sectional area growth was increased by increased ozone concentration in 1992. Fruit number per tree decreased as ozone concentration increased from the charcoal filtered to ambient + ozone treatment, significantly affecting yield. Yield of plum trees was 19.8, 15.9, 6.8 and 15.8 kg tree-1 in 1991 and 27.4, 23.7, 20.5 and 19.9 kg tree-1 in 1992 in the charcoal filtered, ambient, ambient + ozone and no-chamber treatments, respectively.

The last year of the study determined possible carryover effects of the previous year's ozone environment on the current season's yield of trees grown under ambient conditions. Four of five treatment plots (redesignated as the post-chamber (PC) treatments;) were not enclosed or exposed to different ozone concentrations during the 1993 growing season. The mean 12-h (0800-2000 h PDT) ozone concentrations in 1993 in the exposed, charcoal filtered, ambient, ambient + ozone and no-chamber treatments were 0.036, 0.047, 0.105 and 0.055 ppm, respectively. The mean 12-h ozone concentration in the PC treatment was 0.052 ppm. Yield of plum trees exposed to the different ozone treatments in 1993 was 19.1, 14.5, 8.8 and 20.1 kg tree-1 for the charcoal filtered, ambient, and ambient + ozone, and no-chamber treatments, respectively. Yield of trees in the post-chamber treatments in 1993 (previously exposed to different ozone concentrations from 1989 through 1992) was 16.7, .17.9 and 16.0 kg tree-1 in the previous charcoal filtered, ambient and ambient + ozone treatments, respectively. The similarity in yield of the post-chamber treatments indicates that a decrease or increase in air quality in the current growing season can affect yield of "Casselman" plum trees. The results indicate that the previous year's seasonal ozone concentration will affect the number of flowers formed for next season's crop and the current year's exposure to ozone will affect fruit abscission after fruit set. The exact mechanisms leading to reduced fruit numbers due to ozone exposure remain to be determined.

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