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Effects of low levels of ozone on plant populations Bennett, James Peter


Recent evidence has shown that low levels of many air pollutants can now be measured almost anywhere on the earth's surface and that the concept of a "pollutant" is only relative to the dose received by the organism and/or the population. Although the range of sensitivity to air pollutants in plants is very broad, it is possible to hypothesize that a majority of plant species are able to adapt phenotypically and genotypically to these previously overlooked components of the atmosphere. To test this general hypothesis, a set of experiments was carried out to investigate adaptive mechanisms in plant form, efficiency, and competitive ability in annual ryegrass and crimson clover populations exposed to filtered air, .03 and .09 ppm O₃ daily during the vegetative phase of growth. Allometric analysis showed that the root systems of these plants were the organs most sensitive to the O₃ treatments, possibly due to impaired translocation of growth substances from the stressed aerial portion of the plant. In addition, the strategy of using new photosynthetic products for expanding the assimilatory apparatus or directing those products to other metabolic or reproductive activities was not affected by the O₃ treatments used in these experiments. Growth analysis of the aerial portion of these plants revealed that overall efficiency (Relative Growth Rate and Crop Growth Rate) was not impaired by the low O₃ treatments at either the individual plant or crop stand levels because inverse compensating adjustments occurred between the relative size of the assimilatory system (Leaf Area Ratio or Leaf Area Index, which consistently decreased) and the system's photosynthetic efficiency (Unit Leaf Rate, which often increased in O₃). In plant competition experiments using deWit replacement series at a constant density (mixtures) and in monocultural spacing experiments, ryegrass displayed an innate ability to occupy more space than clover under O₃ stress and this effect (an increase of about one tiller/plant) was enhanced by the presence of clover in the mixtures. Although fewer tillers/ plant were produced in the mixtures, the increase in tillering due to O₃ was greater than in the monocultures. This unexpected result was corroborated by a close agreement between predicted and observed ryegrass crowding coefficients in the high O₃ treatment, good agreement between two separate runs of the same experiment, and agreement of the observed equilibrium ratio in high O₃ (2 clover : 1 ryegrass) with the field seeding ratio. It is concluded that these pasture species during their vegetative growth phase, exposed daily to low ambient levels of O₃, do not show seriously impaired growth or qualitative changes in mixture, but that plant form and the relative size of the plant system is decreased, suggesting that phenotypic and genotypic adaptations take place more at the morpho-genetic level of plant organization than at the population or community level.

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