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CROP QUALITY & UTILIZATION

Response of Perennial Ryegrass to Free-Air CO₂ Enrichment (FACE) Is Related to the Dynamics of Sward Structure during Regrowth

Institute of Plant Sciences, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland

* Corresponding author (andreas.luescher{at}ipw.agrl.ethz.ch)

At elevated partial pressure of CO₂ (60 Pa pCO₂), field-grown perennial ryegrass (Lolium perenne L.) swards generally show a strong increase in photosynthesis of the youngest fully expanded leaves but a weak yield response. We investigated this discrepancy by determining the effects of elevated pCO₂ on the dynamics of regrowth. During the whole regrowth period, swards at elevated pCO₂ exhibited a greater dry mass of roots (68%), of pseudostems (34%), and of shoot necromass below cutting height (45%). During the first 4 wk of regrowth, when dry mass (DM) and the leaf area index increased strongly, elevated pCO₂ stimulated the yield, total DM, and leaf area by up to 46, 63, and 30%, respectively. Elevated pCO₂ also resulted in an increase of almost 60% in the number of tillers during this period when the transmission of photosynthetically active radiation to the tiller bases was high. Towards the end of regrowth, however, the leaf area, total DM, and yield showed no net increase; under elevated pCO₂, the number of tillers decreased strongly. These results indicate that there was not a strong sink for additionally fixed C during the second half of regrowth. The gap between leaf photosynthesis and yield at elevated pCO₂ is suggested to be related to a loss of tillers when shading of tiller bases was strong and to a change in dry mass allocation.

Abbreviations: DM, dry mass • FACE, Free-Air CO₂ Enrichment • LAI, leaf area index • PAR, photosynthetically active radiation • pCO₂, partial pressure of CO₂ • R/FR ratio, red/far-red ratio