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Principal Investigator: David M. Olszyk
University of California, Riverside
April 1988
ARB Contract No. A5-151-33 (Full Report)
ABSTRACT
The Statewide Air Pollution Research Center has a continuing mission to investigate the effects of air pollutants
on vegetation, and to determine the amount of losses being caused by these pollutants. The Department of Botany
and Plant Sciences has a continuing mission to investigate basic and applied aspects of plant science research.
To further this mission we jointly conducted the pilot project "The Effects of Ozone on Primary Determinants
of Plant Productivity." The study evaluated the relationships among ozone exposure, gas exchange. chlorophyll
fluorescence. and whole plant productivity responses in depth for a baseline species: spinach (Spinacia oleracea);
and for four species differing in ozone sensitivity: lettuce (Lactuca sativa). corn (Zea mays), squash (Cucumis
pepo), and radish (Raphanus sativus). The study was conducted in a greenhouse using plants grown under conditions
which allow for harvesting of both shoots and roots: i.e. a hydroponic system for spinach and a loose artificial
media for the four other species. Spinach was exposed twice weekly for four weeks. Two patterns of exposure were
used: a square wave (0.15 ppm continuously for four hours). or a triangle (0 to peak of 0.30 to 0 ppm over four
hours for mean of 0.15 ppm). The four other species were exposed only to a triangle pattern with a peak of
0.24" ppm and a mean of 0.12 ppm ozone. The spinach experiment was repeated with four groups of plants.
and the four species experiment was repeated with two groups. Gas exchange was measured in terms of net photosynthesis,
stomatal conductance and transpiration using a portable photosynthesis system with associated computer. Productivity
was measured in terms of fresh and dry weights for shoots and roots. Gas exchange was compared to productivity
in terms of a relative growth rate calculation (RGR. a measure of total dry weight gain over time as a function
of initial weight).
Both plant gas exchange and productivity were affected by the relatively mild ozone stress used in this study.
Spinach showed statistically significant decreases in both stomatal conductance and transpiration due to ozone
for nearly all groups of plants, with the triangle pattern producing greater effects than the square wave pattern
of exposure. Net photosynthesis was not affected by ozone to the same extent as the other gas exchange parameters.
with a statistically significant decrease found only for one group of plants. Spinach productivity was reduced
by ozone, primarily when measured as leaf or shoot fresh weight. Spinach dry weights were reduced by ozone only
for one of the four plant groups. The four species varied in 'their responses to ozone. Corn and radish showed
decreases in stomatal conductance, radish showed a decrease in photosynthesis, and lettuce actually showed an increase
in photosynthesis in response to ozone exposure. However, these responses did not necessarily occur for both groups
of plants. Transpiration was not affected by ozone for any species. Radish was the only species showing consistent
ozone effects on productivity, with decreases found in both shoot, root and total fresh or dry weights. Gas exchange
was not consistently correlated with productivity (as indicated by RGR). Photosynthesis was correlated with RGR
only for control plants. This occurred for two of the four groups of spinach plants and for one group each of
radish and squash plants. Stomatal conductance was not correlated with RGR for any group of spinach plants, and
only for the second group of squash plants (for both for control ozone exposed and second group of lettuce
plants [ozone exposed]). The poor correlations between gas exchange and productivity were likely associated with
the high degree of variability in plant responses, especially with ozone exposure.
The leaf sampling method for chlorophyll fluorescence determinations yielded stable, reproducible signals from
control plants. However, alterations in fluorescence induced by low levels of ozone exposure were not as easily
observed. The subtle ozone effects included a 10-20% lowering of the peak fluorescence and a slight distortion
of secondary peaks. No changes could be seen in the initial (Fo) level of fluorescence (the so-called "dead"
or baseline fluorescence unaffected by photochemistry). This study clearly indicated that even though changes in
gas exchange and plant productivity occur with ozone exposure, that the relationships between them are difficult
to detect. The correlations are highly dependent on species, gas exchange parameter, and group of plants. Recommendations
for further research include: more intensive gas exchange and RGR measurements to determine correlations, measurement
of gas exchange and productivity in the field, conduct additional chloophyll fluorescence research in the laboratory,
and use triangle exposure patterns in future research.
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