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Nodule Activity in Soybean Cultivars Exposed to Ozone and Sulfur Dioxide¹

ABSTRACT

In an effort to obtain a greater understanding of the mechanisms of action for air pollutants on yield-limiting processes in crop plants, studies were conducted in the field and greenhouse in 1979 to investigate the effects of O₃ and SO₂ exposures on nodule activities in roots and chlorophylls a and b contents in leaves of soybeans [Glycine max (L.) Merr.]. Six cultivars of soybeans were exposed 4 h d^–1 for 5 successive days during flowering and pod development to the combination of 6.7 µmol m^–3 O₃ and 11.1 µmol m^–3 SO₂ in activated charcoal-filtered air. Acetylene reduction procedures were used to assay nodule activities.

Significant differences (P < 0.1) were observed in total nodule activity (TNA) per plant for treatments and cultivars in combined analysis of variance (ANOVAs) for two greenhouse studies and two exposure dates in the field; however, treatment x cultivar interactions were nonsignificant in all cases. Compared with filtered air controls, the air pollution treatment reduced TNA by an average of 16.4 and 27.4% in the greenhouse and field studies, respectively. Specific nodule activities (SNA) were unaffected by the treatments in both studies. In the greenhouse studies, the O₄ and SO₂ treatment lowered the chlorophyll a content by 14.1%, but no significant differences were observed in the field study. Chlorophyll b contents were unaffected by treatments in both investigations. In these studies, the sensitivity of changes in chlorophyll contents in soybean leaves following exposures to O₃ and SO₂ at concentrations found in the atmosphere were found to be too low to be of practical value for screening plants for tolerance to air pollutants.

Key Words: air pollution • nitrogen fixation • roots • chlorophyll a • chlorophyll b

¹ Scientific Article A3703. Contribution no. 6679 of the Maryland Agric. Exp. Stn., Dep. of Agron., College Park, MD 20742. (Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. degree.)

² Graduate assistant and associate professors, respectively, Dep. of Agron., Univ. of Maryland, College Park, MD 20742.

Received for publication November 3, 1983.