Genotypic variation in physiological and growth responses of Populus tremuloides to elevated atmospheric CO2 concentration

Physiological and biomass responses of six genotypes of Populus tremuloides Michx., grown in ambient (357 mu mol mol(-1)) or twice ambient (707 mu mol mol(-1)) CO2 concentration ([CO2]) and in low-N or high-N soils, were stud ied in 1995 and 1996 in northern Lower Michigan, USA. There was a significa nt CO2 x genotype interaction in photosynthetic responses. Net CO2 assimila tion (A) was significantly enhanced by elevated [CO2] for five genotypes in high-N soil and for four genotypes in low-N soil. Enhancement of A by elev ated [CO2] ranged from 14 to 68%. Genotypes also differed in their biomass responses to elevated [CO2], but biomass responses were poorly correlated w ith A responses. There was a correlation between magnitude of A enhancement by elevated [CO2] and stomatal sensitivity to CO2. Genotypes with low stom atal sensitivity to CO2 had a significantly higher A at elevated [CO2] than at ambient [CO2], but elevated [CO2] did not affect the ratio of intercell ular [CO2] to leaf surface [CO2]. Stomatal conductance and A of different g enotypes responded differentially to recovery from drought stress. Photosyn thetic quantum yield and light compensation point were unaffected by elevat ed [CO2]. We conclude that P. tremuloides genotypes will respond differenti ally to rising atmospheric [CO2], with the degree of response dependent on other abiotic factors, such as soil N and water availability The observed g enotypic variation in growth could result in altered genotypic representati on within natural populations and could affect the composition and structur e of plant communities in a higher [CO2] environment in the future.