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The anthropogenic increase of atmospheric CO2 concentration (ca) is impacting carbon (C), water and nitrogen (N) cycles in grassland and other terrestrial biomes. Plant canopy stomatal conductance is a key player in these coupled cycles: it is a physiological control of vegetation water-use efficiency (the ratio of C gain by photosynthesis to water loss by transpiration), and it responds to photosynthetic activity, which is influenced by vegetation N status. It is unknown if the ca-increase and climate change over the last century have already affected canopy stomatal conductance and its links with C and N processes in grassland. Results: Here, we assessed two independent proxies of (growing season-integrating canopy-scale) stomatal conductance changes over the last century: trends of δ18O in cellulose (δ18Ocellulose) in archived herbage from a wide range of grassland communities on the Park Grass Experiment at Rothamsted (U.K.), and changes of the ratio of yields to the CO2 concentration gradient between the atmosphere and the leaf internal gas space (ca – ci). The two proxies correlated closely (R2=0.70), in agreement with the hypothesis. In addition, the sensitivity of δ18Ocellulose-changes to estimated stomatal conductance changes agreed broadly with published sensitivities across a range of contemporary field and controlled environment studies, further supporting the utility of δ18Ocellulose-changes for historical reconstruction of stomatal conductance changes at Park Grass. Trends of δ18Ocellulose differed strongly between plots and indicated much greater reductions of stomatal conductance in grass-rich than dicot-rich communities. Reductions of stomatal conductance were connected with reductions of yield trends, nitrogen acquisition and nitrogen nutrition index. Although all plots were nitrogen-limited or phosphorus- and nitrogen-co-limited to different degrees, long-term reductions of stomatal conductance were largely independent of fertilizer regimes and soil pH, except for nitrogen fertilizer supply which promoted the abundance of grasses.

Mächler, F., Lehnherr, B., Schnyder, H. and Nösberger, J. 1985. A CO2 concentrating system in leaves of higher C3-plants predicted by a model based on RuBP carboxylase/oxygenase kinetics and 14CO2/12CO2 exchange.−J. exp. Bot. 36: 1542-1550. A model is presented which compares the ratio of the two activities of the enzyme nbulose bisphosphate carboxylase/oxygenase as determined in vitro with the ratio of photosynthesis to photorespiration in leaves as determined from differential 14CO2/12CO2 uptake or from CO2 compensation concentration. Discrepancies between measurements made in vitro and in vivo are attributed to the effect of a CO2 concentrating system in the leaf cells. Interference from dark respiration is discussed. A CO2 concentrating system is postulated which is efficient mainly at low temperature and low CO2 concentration

Detached leaves of white clover (Trifolium repens L.) were kept for 1 h under various conditions of temperature, oxygen concentration and light intensity. Rates of photosynthesis were measured where appropriate and then ribulosebisphosphate carboxylase oxygenase (RuBPCO) was extracted rapidly and its initial activity measured immediately. The extracted activity increased with increased intensity of illumination of the leaves. Where leaves were pretreated at low light intensity, the lower the temperature of the leaves the higher the extracted activity of RuBPCO. At high light intensity temperature did not affect the activity of subsequently extracted RuBPCO but the light intensity which was necessary for maximum activity increased with temperature. Activity of RuBPCO from leaves pretreated in the dark was least when CO2 was low and temperature high. Leaves, pretreated at low temperatures and high light intensity in 20% O2, yielded higher activity in extracts than leaves pretreated under similar conditions but in 2% O2. A relatively weak temperature response of photosynthesis at low irradiances was associated with a decrease in extractable RuBPCO activity with increasing temperature. A strong temperature dependence of the oxygen inhibition of photosynthesis was associated with lower extractable RuBPCO activity in leaves pretreated at low oxygen concentration at low temperatures. With leaves from plants grown at low temperatures prior to treatment of leaves, oxygen inhibition of photosynthesis was less temperature dependent and activity of RuBPCO in extracts was not decreased by low O2 at low temperatures. Differences in the activation of RuBPCO appear to influence photosynthesis and account for an absence of oxygen inhibition of photosynthesis at low temperatures in plants grown in warm conditions