Source‐sink relations in <i>Lolium perenne</i> L. as reflected by carbohydrate concentrations in leaves and pseudo‐stems during regrowth in a free air carbon dioxide enrichment (FACE) experiment^*

Abstract

ABSTRACT The effect of an elevated partial pressure of CO 2 ( p CO2 ) on carbohydrate concentrations in source leaves and pseudo‐stems (stubble) of Lolium perenne L. (perennial ryegrass) during regrowth was studied in a regularly defoliated grass sward in the field. The free air carbon dioxide enrichment (FACE) technology enabled natural environmental conditions to be provided. Two levels of nitrogen (N) supply were used to modulate potential plant growth. Carbohydrate concentrations in source leaves were increased at elevated p CO2 , particularly at low N supply. Elevated leaf carbohydrate concentrations were related to an increased structural carbon (C) to N ratio and thus reflected an increased C availability together with a N‐dependent sink limitation. Immediately after defoliation, apparent assimilate export rates (differences in the carbohydrate concentrations of young source leaves measured in the evening 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 carbohydrate concentrations in pseudo‐stems suggested an increasing degree of C‐sink limitation from the treatment at ambient p CO2 with high N supply to that at elevated p CO2 with low N supply. During two growing seasons, no evidence of a substantial change in the response of the carbohydrate 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 low N supply.