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).