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Plant morphology and architecture are essential characteristics for all plants, but perhaps most importantly for agricultural species because economic traits are linked to simple features such as blade length and plant height. Key morphological traits likely respond to CO2 concentration ([CO2]), and the degree of this response could be influenced by water availability; however, this has received comparatively little research attention. This study aimed to determine the impacts of [CO2] on gross morphology of perennial ryegrass (Lolium perenne L.), the most widespread temperate pasture species, and whether these impacts are influenced by water availability. Perennial ryegrass cv. Base AR37 was grown in a well-fertilised FACE (free-air carbon dioxide enrichment) experiment in southern Tasmania. Plants were exposed to three CO2 concentrations (400 (ambient), 475 and 550 mu mol mol(-1)) at three watering-treatment levels (adequate, limited and excess). Shoot dry weight, height, total leaf area, leaf-blade separation, leaf size, relative water content and specific leaf area were determined, as well as shoot density per unit area as a measure of tillering. Plant morphology responded dramatically to elevated [CO2], plants being smaller with shorter leaf-blade separation lengths and smaller leaves than in ambient (control) plots. Elevated [CO2] increased tillering but did not substantially affect relative water content or specific leaf area. Water supply did not affect any measured trait or the response to elevated [CO2]. Observed impacts of elevated [CO2] on the morphology of a globally important forage crop could have profound implications for pasture productivity. The reductions in plant and leaf size were consistent across a range of soil-water availability, indicating that they are likely to be uniform. Elucidating the mechanisms driving these responses will be essential to improving predictability of these changes and may assist in breeding varieties suited to future conditions.