The fraction of expanding to expanded leaves determines the biomass response of Populus to elevated CO2

We examined whether the effects of elevated CO2 on growth of 1-year old Populus deltoides saplings was a function of the assimilation responses of particular leaf developmental stages. Saplings were grown for; 100 days at ambient (approximately 350 ppm) and elevated (ambient + 200 ppm) CO2 in forced-air greenhouses. Biomass, biomass distribution, growth rates, and leaf initiation and expansion rates were unaffected by elevated CO2. Leaf nitrogen (N), the leaf C:N ratio, and leaf lignin concentrations were also unaffected. Carbon gain was significantly greater in expanding leaves of saplings grown at elevated compared to ambient CO2. The Rubisco content in expanding leaves was not affected by CO2 concentration. Carbon gain and Rubisco content were significantly lower in fully expanded leaves of saplings grown at elevated compared to ambient CO2, indicating CO2-induced down-regulation in fully expanded leaves. Elevated CO2 likely had no overall effect on bio mass accumulation due to the more rapid decline in carbon gain as leaves matured in saplings grown at elevated compared to ambient CO2. This decline in carbon gain has been documented in other species and shown to be related to a balance between sink/source balance and acclimation. Our data suggest that variation in growth responses to elevated CO2 can result from differences in leaf assimilation responses in expanding versus expanded leaves as they develop under elevated CO2.