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

Abstract

Chloroplasts isolated from triazine-susceptible and triazine-resistant biotypes of Brassica campestris L. were analyzed for Upid composition, ultrastructure, and relative quantum requirements of photosynthesis. In general, phospholipids, but not glycolipids in chloroplasts from the triazineresistant 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-A-hexadecenoic acid with a concomitantly lower palmitic acid concentration in phosphatidylglycerol. Phosphatidylglycerol previously has been hypothesized to be a boundary lipid for photosystem 11. 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. Althougb 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.Variability in the sensitivity of photosynthesis to S-triazine herbicides has been demonstrated in 10 different weed species (14). The mechanism for resistance to triazine has resulted from specific genetic mutations or subtle modifications of the biochemistry in the photosynthetic apparatus.Radosevich and Appleby (19) have shown that the differences in resistance found in common groundsel (Senecio vulgaris L.) biotypes were not attributed to variation in germination time, rooting depth, or morphology resulting in differences in exposure to the herbicide. They also demonstrated that uptake and eventual metabolism of the triazines was identical for the susceptible and resistant groundsel biotypes (18) (14,15). In those studies, [4C]azido-atrazine was shown to bind to a 32 kd polypeptide of susceptible, but not herbicideresistant, chloroplast membranes. Analysis of the atrazine-binding protein by SDS-polyacrylamide gel electrophoresis, however, has failed to demonstrate any protein modifications between biotypes that might account for the mechanism of this type of triazine resistance.The 32 kd polypeptide is an integral component of the PSII structural complex within the membrane. Hence, the environment provided by neighboring polypeptides and lipids surrounding the binding site on the 32 kd protein could affect the binding of triazine molecules. In an attempt to explore these possibilities, we have analyzed the lipid composition, ultrastructure, and photosynthetic efficiency of chloroplasts from triazine-susceptible and resistant Brassica biotypes to determine both the nature of the lipoidal compl...

Similar Photosynthetic Performance in Low Light-Grown Isonuclear Triazine-Resistant and -Susceptible Brassica napus L

“…,imilar small difference in photon yield may Further evidence that the resistance alteration causes slow nt in low PFD growth chamber-grown plants Qa to Qb electron transfer in low PFD-grown plants comes from the oxygen flash yield data. As seen in Table II 6,20) and in resistant chloroplasts (5,20 (12), high PFD exposure does indeed cause a differential lowering of photon yield in resistant leaf discs. Our evidence suggests that the decrease in photon yield in resistant plants is due to increased sensitivity of Dl to photoinhibitory damage (12).…”

The Biology of Canadian Weeds. 137. Brassica napus L. and B. rapa L.

“…Carbon exchange rates were lower (up to 28%) in triazine-resistant domesticated B. napus and B. rapa genotypes compared with triazine-susceptible genotypes, despite similar chlorophyll a and b contents (Hobbs 1987). Burke et al (1982) observed differences in chlorophyll a:b ratios and chloroplast fatty acid composition among triazine-resistant and triazine-susceptible biotypes of weedy B. rapa in the United States. An interaction between temperature and light levels in the competitive outcome between triazine-tolerant and triazine-susceptible rapid-cycling domesticated B. rapa was observed.…”

Triazine Resistance in Senecio vulgaris Parental and Nearly Isonuclear Backcrossed Biotypes Is Correlated with Reduced Productivity

“…The photosynthetic rate and biomass accumulation of greenhouse grown susceptible and resistant S. vulgaris biotypes were measured 28, 35, 42, 50, 57, and (17). PSII efficiency in the use of separated charge for oxygen evolution and electron transport is lower in resistant biotypes (12, 17) which explains the lower quantum yields characteristic of triazine resistant biotypes (5,13,15,17,19).As expected from the reduced photosynthetic potential of triazine resistant biotypes, several studies have shown that in the absence of triazine herbicides, resistant biotypes produce less biomass than susceptible biotypes. In Amaranthus spp., B. napus, Chenopodium album, and Senecio vulgaris, resistant biotypes accumulated less biomass and produced less seed than susceptible biotypes in competitive and noncompetitive conditons (7,8,11,29).…”