SOURCE-SINK RELATIONS IN LOLIUM-PERENNE L. AS REFLECTED BY CARBOHYDRATE CONCENTRATIONS IN LEAVES AND PSEUDO-STEMS DURING REGROWTH IN A FREE-AIR CARBON-DIOXIDE ENRICHMENT (FACE) EXPERIMENT

The effect of an elevated partial pressure of CO2 (P-CO2) on carbohydr ate concentrations in source leaves and pseudostems (stubble) of Loliu m perenne L., (perennial ryegrass) during regrowth was studied in a re gularly defoliated grass sward in the field, The free air carbon dioxi de enrichment (FACE) technology enabled natural environmental conditio ns to be provided, Two levels of nitrogen (N) supply were used to modu late potential plant growth. Carbohydrate concentrations in source lea ves were increased at elevated P-CO2, particularly at low N supply, El evated leaf carbohydrate concentrations were related to an increased s tructural carbon (C) to N ratio and thus reflected an increased C avai lability together with a N-dependent sink limitation, Immediately afte r defoliation, apparent assimilate export rates (differences in the ca rbohydrate concentrations of young source leaves measured in the eveni ng and on the following morning) showed a greater increase at elevated p(CO2) than at ambient p(CO2); however, replenishment of carbohydrate reserves was not accelerated Distinct, treatment-dependent carbohydra te concentrations in pseudo-stems suggested an increasing degree of C- sink limitation from the treatment at ambient p(CO2) with high N suppl y to that at elevated P-CO2 With low N supply. During two growing seas ons, no evidence of a substantial change in the response of the carboh ydrate source in L. perenne to elevated p(CO2) was found, Our results support the view that the response of L. perenne to elevated p(CO2) is restricted by a C-sink limitation, which is particularly severe at lo w N supply.