PHOTOSYNTHETIC ACCLIMATION TO ELEVATED CO2 IS MODIFIED BY SOURCE-SINKBALANCE IN 3 COMPONENT SPECIES OF CHALK GRASSLAND SWARDS GROWN IN A FREE-AIR CARBON-DIOXIDE ENRICHMENT (FACE) EXPERIMENT

Artificial chalk grassland swards were exposed to either ambient air o r air enriched to 600 mu mol mol(-1) CO2, using free-air CO2 enrichmen t technology, and subjected to an 8 week simulated grazing regime. Aft er 14 months of treatment, ribulose-1,5-bisphosphate carboxylase (Rubi sco) activity (V-c,V-max) and electron transport mediated ribulose-1,5 -bisphosphate (RuBP) regeneration capacity (J(max)), estimated from le af gas exchange, were significantly lower in fully expanded leaves of Anthyllis vulneraria L. (a legume) and Sanguisorba minor Scop, grown i n elevated CO2. After a change in source:sink balance brought about by defoliation, photosynthetic capacity was fully restored in A. vulnera ria and S. minor, but acclimation continued in the grass Bromopsis ere cta (Hudson) Fourr. Changes in net photosynthesis (P-n) with growth at elevated CO2 ranged from a 1.6% reduction in precut leaves of A. vuln eraria to a 47.1% stimulation in postcut leaves of S. minor. Stomatal acclimation was observed in leaves of A. vulneraria (reduced stomatal density) and B. erecta (reduced stomatal conductance). The results are discussed in terms of whole-plant resource-use optimization and chalk grassland community competitive interactions at elevated CO2.