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Characterization of Chloroplasts Isolated from Triazine-Susceptible and Triazine-Resistant Biotypes of Brassica campestris L. ¹

United States Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh, North Carolina 27650, Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27650, Department of Botany, North Carolina State University, Raleigh, North Carolina 27650, Department of Botany, Arizona State University, Tempe, Arizona 85281, Department of Microbiology, Arizona State University, Tempe, Arizona 85281

Chloroplasts isolated from triazine-susceptible and triazine-resistant biotypes of Brassica campestris L. were analyzed for lipid composition, ultrastructure, and relative quantum requirements of photosynthesis. In general, phospholipids, but not glycolipids in chloroplasts from the triazine-resistant biotype had a higher linolenic acid concentration and lower levels of oleic and linoleic fatty acids, than chloroplasts from triazine-susceptible plants. Chloroplasts from the triazine-resistant biotype had a 1.6-fold higher concentration of t-3-hexadecenoic acid with a concomitantly lower palmitic acid concentration in phosphatidylglycerol. Phosphatidylglycerol previously has been hypothesized to be a boundary lipid for photosystem II. Chloroplasts from the triazine-resistant biotype had a lower chlorophyll a/b ratio and exhibited increased grana stacking. Light-saturation curves revealed that the relative quantum requirement for whole chain electron transport at limiting light intensities was lower for the susceptible biotype than for the triazine-resistant biotype. Although the level of the chlorophyll a/b light-harvesting complex associated with photosystem II was greater in resistant biotypes, the increased levels of the light-harvesting complex did not increase the photosynthetic efficiency enough to overcome the rate limitation that is inherited concomitantly with the modification of the Striazine binding site.