Acclimation of photosynthesis to elevated CO2 and temperature in five British native species of contrasting functional type

Acclimation of photosynthesis to growth at elevated CO₂ concentration varies markedly between species. Species functionally classified as stress-tolerators (S) and ruderals (R), are thought to be incapable, or the least capable, of responding positively in terms of growth to elevated CO₂]. Is this pattern of response also apparent in leaf photosynthesis of wild S- and R-strategists? Acclimatory loss of a photosynthetic and growth response to elevated CO₂] is assumed to reflect limitation on capacity to utilize additional photosynthate. The doubling of pre-industrial global CO₂] is expected to coincide with a 3 °C increase in mean temperature which could stimulate growth; will photosynthetic capacity at elevated CO₂] be greater when the concurrent temperature increase is simulated? Five species from natural grassland of NW Europe and of contrasting ecological strategy were grown in hemispherical greenhouses, environmentally controlled to track the external microclimate. Within a replicated design, plants were grown at (i) current ambient CO₂] and temperature, (ii) elevated CO₂] (ambient + 340 μmol mol^–1) and ambient temperature, (iii) ambient CO₂] and elevated temperature (ambient + 3 °C), or (iv) elevated CO₂] and elevated temperature. After 75–104 days, the CO₂ response of light-saturated rates of photosynthesis (A_sat) was analysed in controlled-environment cuvettes in a field laboratory. There was no acclimatory loss of photosynthetic capacity with growth in elevated CO₂] or elevated temperature over this period in Poa alpina (S), Bellis perennis (R) or Plantago lanceolata (mixed C-S-R strategist), and a significant (P ? ? bl 0.05) increase in capacity in Helianthemum nummularium (S) and Poa annua (R). Photosynthetic rates of leaves grown and measured in elevated CO₂] were therefore significantly higher than rates for leaves grown and measured in ambient CO₂], for all species. With the exception of Poa alpina, stomatal conductance and stomatal limitation on A_sat showed no acclimatory response to growth in elevated CO₂]. Carboxylation efficiency, determined from the initial slope of the response of A_sat to intercellular CO₂ concentration was significantly increased by elevated CO₂] and elevated temperature in H.nummularium, implying a possible increase in in vivo RubisCO activity. Increased carboxylation efficiency of this species was also reflected by an increase in the CO₂- and light-saturated rates of photosynthesis, indicating an increased capacity for regeneration of the primary CO₂ acceptor in photosynthesis. The results show that R-strategists and slow-growing S-strategists, are inherently capable of large increases in leaf photosynthetic capacity with growth in elevated CO₂] in contrast to expectations from growth studies. With the exception of P.annua, where there was a significant negative interaction between CO₂ and temperature, concurrent increase in growth temperature had little effect on this pattern of response.