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item Bunce, James
item Sicher, Richard

Submitted to: Photosynthesis Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: An increase in the rate of photosynthesis is anticipated in many plants as the concentration of carbon dioxide in the atmosphere continues to rise. The increase in photosynthesis leads to increased growth. However, in some cases the stimulation in photosynthesis by elevated carbon dioxide is reduced over time by a process termed acclimation. Reduction in the stimulation of photosynthesis also reduces the stimulation of crop growth. The existence of photosynthetic acclimation in soybean is controversial. While testing for the existence of photosynthetic acclimation in field-grown soybean, we observed that evidence for acclimation varied from day to day, and was related to weather. On days with either dry soil or dry air, acclimation was apparent. On days shortly after precipitation events and on humid days there was no evidence of acclimation. In neither case was there any significant difference between carbon dioxide treatments in the amounts of enzymes usually associated with acclimation of photosynthesis to elevated carbon dioxide. The data indicated that evidence of photosynthetic acclimation to elevated carbon dioxide in soybeans results from greater susceptibility to water stress in plants grown at elevated carbon dioxide. This work will be of use to scientists predicting responses of crop plants to increasing atmospheric carbon dioxide.

Technical Abstract: The occurrence of photosynthetic acclimation to elevated concentrations of carbon dioxide in soybean remains controversial. In this study, we made midday measurements of single leaf gas exchange rates of upper canopy leaves of soybeans grown in the field at 350 and 700 ppm carbon dioxide in opened topped chambers. Results sometimes indicated up to 50% higher photosynthetic rates measured at 700 ppm in plants grown at the ambient compared to the elevated carbon dioxide concentration. On other days, mean photosynthetic rates were nearly identical. Even on days with significantly lower photosynthetic rates in the plants grown at elevated carbon dioxide, there was no reduction in Rubisco, chlorophyll, or soluble protein content per unit of leaf area. Over three years, gas exchange evidence of acclimation occured on days when either soil was dry or the water vapor pressure deficit was high (n = 12 days) and did not occur on days after rain or on days with low water vapor pressure deficit (n = 9 days). On days when photosynthetic acclimation was evident, midday leaf water potentials were consistently 0.2 to 0.3 MPa lower for the plants grown at elevated than at ambient carbon dioxide. This suggested that greater susceptibility to water stress in plants grown at elevated carbon dioxide caused the apparent photosynthetic acclimation. The day-to-day variation in evidence of photosynthetic acclimation observed here may account for some of the conflicting results in the literature concerning the existence of acclimation to elevated dioxide in field-grown soybean.