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

Isonuclear triazine-susceptible and triazine-resistant Senecio vulgaris L. biotypes were developed by making reciprocal crosses between susceptible and resistant biotypes to obtain F(1) hybrids and backcrossing the hybrids to the appropriate pollen parent. The electrophoretic isozyme patterns of the enzyme aconitase obtained from leaf extracts of triazine-susceptible parental (S) and backcrossed (SxR(BC6)) biotypes, and triazine-resistant parental (R) and backcrossed (RxS(BC6)) biotypes verified that the biotypes had the expected nuclear genomes. Atrazine inhibition of chloroplast whole chain electron transport from water to methyl viologen was measured to verify susceptibility or resistance to triazine herbicides. The photosynthetic rate and biomass accumulation of greenhouse grown susceptible and resistant S. vulgaris biotypes were measured 28, 35, 42, 50, 57, and 64 days after planting to determine the effect of altered chloroplast function. S and SxR(BC6) biotypes had CO(2) assimilation rates of 16.2 and 16.6 micromoles CO(2) per square meter per second, respectively, and I(50) values (herbicide concentration producing 50% inhibition) of about 0.49 micromolar atrazine. The corresponding values for the R and RxS(BC6) biotypes were 14.7 and 14.6 micromoles CO(2) per square meter per second with I(50) values of 65.0 micromolar atrazine. The S biotype was larger and more productive than the R biotype at all harvests. At the harvest 57 days after planting, mean shoot dry weight was 33.2 and 8.7 grams for the S and R biotypes, respectively. The growth effect associated with chloroplast differences was shown in comparisons of the S biotype with the RxS(BC6) biotype and of the SxR(BC6) biotype with the R biotype. The RxS(BC6) biotype had 72% of the shoot dry weight of the S biotype while the R biotype had 55% of the shoot dry weight of the SxR(BC6) biotype. The RxS(BC6) and R biotypes produced about 73 and 62% of the leaf area of the S and SxR(BC6) biotypes, respectively. Relative growth rate was similar in biotypes with the same nuclear genome; however, instantaneous unit leaf rate was higher in the S compared to the RxS(BC6) biotype and in the SxR(BC6) compared to the R biotype. At 57 days after planting, the cumulative leaf area duration (i.e. photosynthetic opportunity) of the RxS(BC6) and R biotypes was 86 and 66% of that of the S and SxR(BC6) biotypes, respectively. Our data indicate that impaired chloroplast function in triazine resistant S. vulgaris biotypes limits growth and productivity at the whole plant level.