Photorespiration response of S. majus
The observed CO2-response of the D6S /D6R ratio ofS. fuscum at high WT suggests that lawns and hollows, which generally experience relatively high WT levels, do not suppress photorespiration at high CO2 levels. To test this, we analyzed D6S /D6Rratios of S. majus grown at different CO2 and WT levels (under high temperature and low light conditions). Increasing CO2 from 280 to 400 ppm did not have any significant effect on the D6S /D6Rratio (range: 0.90-0.98) of S. majus at both low (-7 cm) and high WT (~0 cm, R2=0.01, P =0.153, Table 2, Fig. 4A). In contrast, raising the WT resulted in a significant increase in the D6S /D6R ratio (0.05-0.06), indicating increased photorespiration/photosynthesis ratios at high WT levels (R2=0.94, P <0.001, Table 2). Biomass production did not show any significant difference in response to increasing atmospheric CO2, but raising the WT reduced biomass production by ~40% (R2=0.49, P =0.01, Table 2, Fig. 4B). Elevated atmospheric CO2 concentrations increased biomass density by 2-fold at low WT (R2=0.18, P =0.048, Table 2, Fig. 4C). Concomitantly, height increment significantly decreased at low WT by 55% (R2=0.5, P =0.012, Table 2, Fig. 4D, Fig. S2). The water content ranged between 9.1 and 11.9 g g-1 at low WT and 10.8 and 17 g g-1at high WT, thus increasing the WT shifts the water content from optimal to suboptimal conditions for photosynthesis (Schipperges & Rydin, 1998). Altogether, these data indicate that despite optimal moisture conditions for photosynthesis, increasing atmospheric CO2 does not affect C assimilation of S. majus .