Abstract.The effect of growth temperature on biomass production and photosynthesis of nearly‐isonuclear triazine‐resistant and ‐susceptibleSenecio vulgarisL. bio‐types was investigated. Plants were grown in growth chambers with day/night temperatures of 13/8, 20/15 and 30/25°C, and were harvested 35, 42, 49 and 56d after planting (DAP). The S biotype produced more shoot dry weight than the R × SBC6biotype, and the S × RBC6biotype produced more shoot dry weight than the R biotype at all DAP and growth temperature combinations. The S and S × RBC6biotypes had greater photosynthetic rates than the R X SBC6and R biotypes, respectively. Thus, plants containing the susceptible chloroplast genome produced more biomass and had greater photosynthetic rates than those with the resistant chloroplast genome, when in association with the same nuclear genome. There was no differential temperature effect on biomass production of isonuclear plants possessing resistant or susceptible chloroplast genomes. However, there was a large differential temperature effect on the amount of biomass produced by plants containing different nuclear genomes (R or S) in association with the same chloroplast genome. The R nuclear genome appeared to be better adapted to cooler growth temperatures while the S nuclear genome was better adapted to warmer growth