Contrasting effects of simazine on the photosynthetic physiology and leaf morphology of two Populus clones

Differential effects of simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) on the physiology of two Populus clones were investigated in a greenhouse study. Additions of 5 mg/pot simazine to young plants had no deleterious morphological or physiological effects on clone NC 5328 (P. x euramericana cv. I 45/51; Section Aigeiros), but reduced the rate of CO2 fixation, increased CO2 compensation concentrations and lowered the specific leaf weight of clone NE 388 (P. maximowiczu x P. trichocarpa cv. Kingston; section Tacamahaca). Abaxial leaf conductance to water vapor was not affected in NE 388. Deleterious effects of simazine on NE 388 were detected ca 48 h after exposure of plants to simazine and generally became more pronounced thereafter. Visual symptoms of injury were evident at ca 2 weeks after simazine application. Toxic responses to simazine in clone NE 388 varied in different portions of the crown. Inhibition of photosynthesis and increased CO2 compensation concentrations were more pronounced in the region of recently matured leaves, but were somewhat less in the region of expanding leaves. Older mature leaves in the lower crown region showed no visual symptoms of injury and the rate of photosynthesis and CO2 compensation concentrations were largely unaffected.