The effects of sodium erythorbate and ethylenediurea on photosynthetic function of ozone-exposed loblolly pine seedlings
In an open-top chamber study in east Texas, ozone-sensitive loblolly pine (Pinus taeda L.) seedlings were treated with either the antioxidant Ozoban (74.5 percent sodium erythorbate active ingredient (a.i.)) at 0, 1030, or 2060 mg liter- 1 or ethylenediurea (EDU 50 percent a.i.) at 0, 150, or 300 ppm every 2 weeks while being subjected to a range of ozone exposures beginning in April 1994. The ozone exposures included sub-ambient ozone levels (CF), approximate ambient (NF), and 1.5, 2.0, and 2.5 times ambient ozone (1.5% , 2.0% , and 2.5% , respectively). The response variables included net photosynthesis (A), stomatal conductance (g), chloroplast pigment concentration, and total foliar N concentration. Foliar injury due to ozone was observed early in the growing season, but subsided over time. Ozoban did not have any consistent effects throughout the experiment on the response variables, but did cause changes early in the study on gas exchange. Both g and A photosynthesis were greater at elevated ozone levels in seedlings treated with 1030 mg liter- 1 of Ozoban compared to those treated with either 0 or 2060 mg liter- 1 for the first sampling period in June. However, at CF, seedlings treated with 1030 mg liter- 1 of Ozoban showed signs of reduced A compared to the other antioxidant treatments. This indicates that sodium erythorbate may have a negative physiological effect on seedlings in a low ozone environment. For EDU-treated seedlings, no consistent antioxidant treatment effects were observed, but linear regression analysis indicates that EDU shows promise in providing protection from ozone injury. At 150 ppm, EDU may retard stomatal closure in younger pine seedlings. No consistent benefit was afforded to chloroplast pigments for the study by either antioxidant. Ozoban at 2060 mg liter- 1 appeared to have phytotoxic effects with regards to chlorophyll a and total carotenoids in the latter stages of the experiment. No significant Ozoban or EDU effects were observed with respect to total foliar N concentration until the final sampling period in October, where foliage treated with 300 ppm of EDU displayed higher N concentration at all ozone levels except 1.5% ambient ozone. Foliage treated with 150 ppm EDU showed the lowest nitrogen concentration in CF and NF, but the highest in 1.5% . Because of relatively low ozone exposures during the study year and the closeness with which the seedlings were grown, these results may not accurately represent the benefits of sodium erythorbate or EDU in reducing ozone stress in loblolly pine seedlings for field-grown trees.