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 .