Comparative physiology of radish populations with differential sensitivity to O₃ and SO₂

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Virginia Polytechnic Institute and State University

Radish plants (Raphanus sativus L. cv Cherry Belle) were exposed to 0.10 μl l⁻¹ ozone (O₃) or 0.50 μl l⁻¹ sulfur dioxide (SO2) for 4 h d⁻¹, 3 d wk⁻¹ for 3 weeks. From these fumigated plants, individuals were selected that were resistant or sensitive to these pollutants. The selected plants were used as parental material in a breeding program to produce lines differing in resistance to O₃ and SO₂. Non-selected (NS) plants from the original populations served as controls.

F₁ populations were raised and exposed to O₃ or SO₂ with the same fumigation regime used for the parents. The plants were harvested 30 days after emergence and dry weights were determined. Plants selected for O₃ resistance (O3R) weighed significantly more than either plants selected for sensitivity to O₃ (O₃S) or NS plants when exposed to either O₃ or SO₂. The hypocotyl was most affected by pollutant exposure, leading to reduced root/shoot ratios. Plants selected for resistance or sensitivity to SO₂ generally had biomass production similar to that of NS plants. Growth analysis at early stages of growth indicated that both O₃R and O₃S plants had less growth under O₃ fumigated conditions; however, by maturity O₃R plants had similar amounts of growth under fumigated or non-fumigated conditions. Ozone fumigations tended to decrease free sugar concentrations in leaves at early stages of growth in both O₃R and O₃S plants, and caused some accumulations of carbohydrates during late stages of growth in O₃S plants. Allocation of Mc was significantly lower to hypocotyls and roots of O₃ fumigated O₃S plants. Allocation to hypocotyls of O₃R plants was not affected, although both O₃R and O₃S plant groups had lower photosynthetic rates due to O₃ fumigation. Ozone did not significantly affect chlorophyll concentrations in leaves of either sensitive or resistant plants, nor was the time of new leaf production affected by fumigation.

These experiments demonstrated the potential of O₃ to influence the composition of sensitive plant populations. However, SO₂ was a much less powerful influence on the composition of these populations.