Effects of increased atmospheric CO2 on S. fuscum photosynthesis
The net photosynthesis rate ofS. fuscum , as characterized by the D6S /D6R ratio, was significantly higher at the higher (current) atmospheric CO2 level than at the lower (pre-industrial) level at low WT (Fig. 2A). D6S /D6R ratios were lower at the high CO2 level when the WT was low (representing typical growth conditions for S. fuscum ), thus indicating suppression of photorespiration relative to C assimilation during the 20th century. In contrast, when theSphagnum plants were water-saturated, increasing atmospheric CO2 from 280 to 400 ppm had no significant detected effects on their photosynthesis, i.e. on the D6S /D6R ratio. The observed shift in the D6S /D6R ratio of 0.03 units at low WT (Fig. 2A) is consistent with results of a previous comparison of contemporary samples with >100-year-old herbarium samples of S. fuscum (Ehlers et al ., 2015).
Of all tested environmental variables, the CO2concentration had the largest effect, explaining 53% of the variance of D6S /D6R under typical hydrological (low WT) conditions for S. fuscum (Table S1). Thus, our experimental data confirm that the detected suppression of photorespiration in herbarium samples (Ehlers et al ., 2015) was due to increases in atmospheric CO2 during the last century. Estimates ofc c/c a revealed an increase under contemporary CO2 levels at low WT, and a high negative correlation with the D6S /D6R ratio. This implies that the suppression of photorespiration derives from the higher intercellular CO2 concentration suppressing Rubisco oxygenation. In line with the increase in c c, the CO2 diffusion gradient (ca-cc ) was higher at the high CO2 level than at the low CO2 level when the WT was low, supporting the increase in CO2assimilation at the 400 ppm CO2 level (Fig. S3).