Gas exchange and photosynthetic acclimation over subambient to elevated CO2 in a C3-C4 grassland

Atmospheric CO2 (C-a) has risen dramatically since preglacial times and is projected to double in the next century. As part of a 4-year study, we examined leaf gas exchange and photosynthetic acclimation in C-3 and C-4 plants using unique chambers that maintained a continuous C-a gradient from 200 to 550 mu mol mol(-1) in a natural grassland. Our goals were to characterize linear, nonlinear and threshold responses to increasing C-a from past to future C-a levels. Photosynthesis (A), stomatal conductance (g(s)), leaf water-use efficiency (A/g(s)) and leaf N content were measured in three common species: Bothriochloa ischaemum, a C-4 perennial grass, Bromus japonicus, a C-3 annual grass, and Solanum dimidiatum, a C-3 perennial forb. Assimilation responses to internal CO2 concentrations (A/C-i curves) and photosynthetically active radiation (A/PAR curves) were also assessed, and acclimation parameters estimated from these data. Photosynthesis increased linearly with C-a in all species (P < 0.05). S. dimidiatum and B. ischaemum had greater carboxylation rates for Rubisco and PEP carboxylase, respectively, at subambient than superambient C-a (P < 0.05). To our knowledge, this is the first published evidence of A up-regulation at subambient Ca in the field. No species showed downregulation at superambient C-a. Stomatal conductance generally showed curvilinear decreases with C-a in the perennial species (P < 0.05), with steeper declines over subambient Ca than superambient, suggesting that plant water relations have already changed significantly with past Ca increases. Resource-use efficiency (A/g(s) and A/leaf N) in all species increased linearly with C-a. As both C-3 and C-4 plants had significant responses in A, g(s), A/g(s) and A/leaf N to C-a enrichment, future Ca increases in this grassland may not favour C3 species as much as originally thought. Non-linear responses and acclimation to low Ca should be incorporated into mechanistic models to better predict the effects of past and present rising Ca on grassland ecosystems.