Response of perennial ryegrass to free-air CO2 enrichment (FACE) is related to the dynamics of sward structure during regrowth

At elevated partial pressure Of CO2 (60 Pa pCO(2)), field-grown perennial r yegrass (Lolium perenne L.) swards generally show a strong increase in phot osynthesis of the youngest fully expanded leaves but a weak yield response. We investigated this discrepancy by determining the effects of elevated pC O(2) on the dynamics of regrowth. During the whole regrowth period, swards at elevated pCO(2) exhibited a greater dry mass of roots (68%), of pseudost ems (34%), and of shoot necromass below cutting height (45%). During the fi rst 4 wk of regrowth, when dry mass (DM) and the leaf area index increased strongly, elevated pCO(2) stimulated the yield, total DM, and leaf area by up to 46, 63, and 30%, respectively. Elevated pCO(2) also resulted in an in crease of almost 60% in the number of tillers during this period when the t ransmission of photosynthetically active radiation to the tiller bases was high. Towards the end of regrowth, however, the leaf area, total DM, and yi eld showed no net increase; under elevated pCO(2), the number of tillers de creased strongly. These results indicate that there was not a strong sink f or additionally fixed C during the second half of regrowth. The gap between leaf photosynthesis and yield at elevated pCO(2) is suggested to be relate d to a loss of tillers when shading of tiller bases was strong and to a cha nge in dry mass allocation.