Leaf Extracellular Ascorbate Metabolism in Relation to Ozone Tolerance in Two Soybean Cultivars

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Title: Leaf Extracellular Ascorbate Metabolism in Relation to Ozone Tolerance in Two Soybean Cultivars
Author: Cheng, Fang-yi
Advisors: Fitzgerald L. Booker, Committee Member
Kent O. Burkey, Committee Chair
Marcia L. Gumpertz, Committee Member
Abstract: Antioxidants in the leaf apoplast have the potential to detoxify ozone (O3) and active oxygen species (AOS), and thus may play a role in preventing plant injury. To investigate this possibility, two experiments were conducted to compare antioxidant metabolism in the leaf extracellular spaces of O3-tolerant (Essex) and O3-sensitive (Forrest) cultivars of soybean [Glycine max (L.) Merr.]. In the first experiment, a canopy profile consisting of the 2nd, 4th and 6th main stem trifoliates was sampled from plants grown for four weeks in a greenhouse supplied with charcoal-filtered air (CF) to assess genetic and leaf age effects under low stress conditions. In general, apoplastic ascorbate (AA) levels were low (< 30 nmol AA g-1 FW) in both cultivars. Apoplastic dehydroascorbate (DHA) levels were higher (> 50 nmol DHA g-1 FW) relative to AA, particularly in younger leaves (250-450 nmol DHA g-1 FW), resulting in a low ascorbate redox state. Ascorbate oxidase (AO) and ascorbate peroxidase (APX) activity were found in leaf apoplast samples, suggesting that the enzymes played a role in maintaining ascorbate primarily in the oxidized state. In the second experiment, the second main stem trifoliates of four week-old plants were compared in the two genotypes following exposure to CF or elevated O3 conditions. Following a six-day treatment period with 77 ppb O3 for 7 h d-1, foliar injury was greater in O3-treated Forrest than in Essex, and total leaf guaiacol peroxidase activity was correlated with the greater O3 sensitivity of Forrest. Under both CF and elevated O3 conditions, there was a significantly higher apoplastic total antioxidant capacity in Essex than in Forrest correlating with greater O3 tolerance. Although reduced AA concentration was greater in apoplasts sample from O3-treated Essex than Forrest, the difference (6.5 nmol g-1 FW) was unlikely to play a major role in the differential O3 sensitivity of these two cultivars. Apoplast AA was generally less than 30% of the total antioxidant capacity found in apoplast samples, suggesting that other antioxidants might be involved in the detoxification of O3 in the extracellular space in these soybean cultivars.
Date: 2004-11-11
Degree: MS
Discipline: Crop Science
URI: http://www.lib.ncsu.edu/resolver/1840.16/1204

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