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 with mineral nutrition, CO2-enrichment experiments were performed on four British grasses of differing ecology and functional type: Arrhenatherum elatius (L.) Beauv., Festuca ovina L., Festuca rubra L. and Poa annua L. Naturally-lit, glasshouse cabinets were used, with a non-limiting water supply and a daytime mean temperature of 18 degrees C. Two CO2 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 place at planting-out, and at 16, 37 and 58 d thereafter. Fitted curves were used to derive instantaneous values of total dry weight, relative growth 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 close agreement with previous estimates of the CO2 response in the four species. The response, if any, most often arose from an increase in USR being accompanied by a less than proportionate decline in SWF. Responses were sustained throughout the period studied. At the lower nutrient level, all species showed a CO2 response initially, but this declined at a rate which was inversely related to the CO2- responsiveness of the species at the higher nutrient level. The underlying ontogenetic drift appeared to be markedly towards adjustment in SWF and away from that of USR. However, this drift was retarded, suspended or even reversed by low-nutrient conditions and/or by high CO2 responsiveness in the species itself.