TEMPORAL AND NUTRITIONAL INFLUENCES ON THE RESPONSE TO ELEVATED CO2 IN SELECTED BRITISH GRASSES

To investigate the duration of the CO2 response and its interaction wi th mineral nutrition, CO2-enrichment experiments were performed on fou r British grasses of differing ecology and functional type: Arrhenathe rum elatius (L.) Beauv., Festuca ovina L., Festuca rubra L. and Poa an nua L. Naturally-lit, glasshouse cabinets were used, with a non-limiti ng water supply and a daytime mean temperature of 18 degrees C. Two CO 2 treatments were maintained at nominal concentrations of 350 and 700 vpm and were combined factorially with two levels of balanced mineral nutrition at conductivities of 0.1 and 1 mS cm(-1). Harvests took plac e at planting-out, and at 16, 37 and 58 d thereafter. Fitted curves we re used to derive instantaneous values of total dry weight, relative g rowth rate (RGR), shoot weight fraction (SWF) and unit shoot rate (USR ) for all combinations of species, CO2 level, nutrient level and time of harvesting. At the higher nutrient level there was a reasonably clo se agreement with previous estimates of the CO2 response in the four s pecies. The response, if any, most often arose from an increase in USR being accompanied by a less than proportionate decline in SWF. Respon ses were sustained throughout the period studied. At the lower nutrien t level, all species showed a CO2 response initially, but this decline d at a rate which was inversely related to the CO2- responsiveness of the species at the higher nutrient level. The underlying ontogenetic d rift appeared to be markedly towards adjustment in SWF and away from t hat of USR. However, this drift was retarded, suspended or even revers ed by low-nutrient conditions and/or by high CO2 responsiveness in the species itself.