Seasonal patterns of photosynthetic response and acclimation to elevated carbon dioxide in field-grown strawberry

Strawberry (Fragaria × ananassa) plants were grown in field plots at the current ambient [CO2], and at ambient + 300 and ambient + 600 μmol mol−1 [CO2]. Approximately weekly measurements were made of single leaf gas exchange of upper canopy leaves from early spring through fall of two years, in order to determine the temperature dependence of the stimulation of photosynthesis by elevated [CO2], whether growth at elevated [CO2] resulted in acclimation of photosynthesis, and whether any photosynthetic acclimation was reduced when fruiting created additional demand for the products of photosynthesis. Stimulation of photosynthetic CO2 assimilation by short-term increases in [CO2] increased strongly with measurement temperature. The stimulation exceeded that predicted from the kinetic characteristics of ribulose-1,5-bisphosphate carboxylase at all temperatures. Acclimation of photosynthesis to growth at elevated [CO2] was evident from early spring through summer, including the fruiting period in early summer, with lower rates under standard measurement conditions in plants grown at elevated [CO2]. The degree of acclimation increased with growth [CO2]. However, there were no significant differences between [CO2] treatments in total nitrogen per leaf area, and photosynthetic acclimation was reversed one day after switching the [CO2] treatments. Tests showed that acclimation did not result from a limitation of photosynthesis by triose phosphate utilization rate at elevated [CO2]. Photosynthetic acclimation was not evident during dry periods in midsummer, when the elevated [CO2] treatments conserved soil water and photosynthesis declined more at ambient than at elevated [CO2]. Acclimation was also not evident during the fall, when plants were vegetative, despite wet conditions and continued higher leaf starch content at elevated [CO2]. Stomatal conductance responded little to short-term changes in [CO2] except during drought, and changed in parallel with photosynthetic acclimation through the seasons in response to the long-term [CO2] treatments. The data do not support the hypothesis that source-sink balance controls the seasonal occurrence of photosynthetic acclimation to elevated [CO2] in this species.