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

Strawberry (Fragaria x ananassa) plants were grown in field plots at the cu rrent ambient [CO2], and at ambient + 300 and ambient + 600 mu mol mol(-1) [CO2]. Approximately weekly measurements were made of single leaf gas excha nge of upper canopy leaves from early spring through fall of two years, in order to determine the temperature dependence of the stimulation of photosy nthesis by elevated [CO2], whether growth at elevated [CO2] resulted in acc limation of photosynthesis, and whether any photosynthetic acclimation was reduced when fruiting created additional demand for the products of photosy nthesis. Stimulation of photosynthetic CO2 assimilation by short-term incre ases in [CO2] increased strongly with measurement temperature. The stimulat ion exceeded that predicted from the kinetic characteristics of ribulose-1, 5-bisphosphate carboxylase at all temperatures. Acclimation of photosynthes is to growth at elevated [CO2] was evident from early spring through summer , including the fruiting period in early summer, with lower rates under sta ndard measurement conditions in plants grown at elevated [CO2]. The degree of acclimation increased with growth [CO2]. However, there were no signific ant differences between [CO2] treatments in total nitrogen per leaf area, a nd photosynthetic acclimation was reversed one day after switching the [CO2 ] treatments. Tests showed that acclimation did not result from a limitatio n 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 photosynthesi s declined more at ambient than at elevated [CO2]. Acclimation was also not evident during the fall, when plants were vegetative, despite wet conditio ns and continued higher leaf starch content at elevated [CO2]. Stomatal con ductance responded little to short-term changes in [CO2] except during drou ght, and changed in parallel with photosynthetic acclimation through the se asons in response to the long-term [CO2] treatments. The data do not suppor t the hypothesis that source-sink balance controls the seasonal occurrence of photosynthetic acclimation to elevated [CO2] in this species.