Responses of two Populus clones to elevated atmospheric CO2 concentration in the field

Two poplar clones, hybrid Populus deltoides Bartr. Ex Marsh x Populus nigra L. (Populus x euramericana), clone I-214, and Populus deltoides, clone Lux , were grown from clonal hardwood cuttings for one growing season in either ambient (360 mu mol mol(-1)) or elevated (560 mu mol mol(-1)) [CO2] in FAC E-system rings at Rapolano Terme (Siena, Italy). Both clones I-214 and Lux exhibited a higher above-ground biomass, photosynthesis at light saturation and instantaneous transpiration efficiency (ITE) in CO2-enriched air. The elevated [CO2]-induced responses of clone I-214 included increased investme nt in branch and leaf biomass, and enhanced stem volume. The elevated [CO2] -induced responses of clone Lux included an increase in the number of branc hes and leaf area (which might result in a higher leaf area index, LAI). Ph otosynthetic acclimation under elevated [CO2] was found only during the ear ly morning and only in clone I-214. Stomatal conductance and transpiration ton a leaf area basis) decreased under elevated [CO2] particularly in clone Lux and at the end of the experiment. The effects of elevated [CO2] on lea f osmotic potential were limited, at least in conditions of non-limiting wa ter availability. Clonal differences in response to elevated [CO2] should b e taken in account when planning future poplar plantations in the forecast warmer and drier Mediterranean sites. ((C) Inra/Elsevier, Paris.).