Effect of flooding of soil on growth and subsequent responses of Taxodium distichum seedlings to SO₂

Unflooded and flooded, twelve-week-old Taxodium distichum (L.) Rich seedlings were grown for eight weeks in a growth chamber and then exposed to 0, 0·5, 1·10 or 1·5 μl litre^-1 SO₂ for 48 h. The seedlings were grown in the glasshouse for an additional six weeks. Flooding with stagnant water decreased the rate of height growth, needle initiation and expansion, and dry weight increment of leaves and roots, but not stems. Flooding of unfumigated seedlings for an additional six weeks also significantly reduced dry weight increment of stems. The greater reduction in root growth over shoot growth in flooded seedlings will tend to create leaf water deficits and predispose seedlings to drought injury when the flood waters recede. Exposure to SO₂ at 1·0 μl litre^-1 or higher concentration reduced growth in both previously unflooded or flooded seedlings, but growth was reduced more in the unflooded seedlings which absorbed more S. Growth of seedlings was reduced more by flooding than by SO₂. A significant flooding-SO₂ interaction on dry weight increment of leaves, but not roots or stems, was shown. The effects of flooding and SO₂ on growth differed when based on analysis of dry weight increment or relative growth rates of seedlings. The need for greater standardisation of methods of determining plant responses to pollution is emphasized.