The determination of an adequate container volume that maximize vegetative growth and the adoption of an efficient irrigation system in soilless culture, which improve water use efficiency without affecting crop performance, have become a priority in ornamental industry. A greenhouse experiment was conducted aiming to assess the effects of two container volumes (1 dm³ or 3 dm³) and two irrigation systems (closed drip-irrigation or subirrigation) on growth parameters, ornamental quality, SPAD index, leaf gas exchange, agronomical and physiological water use efficiency (WUE_A and WUE_P) of containerized Euphorbia × lomi Rauh. There were no significant differences in terms of plant growth parameters between the two irrigation systems. The subirrigation system was more efficient in terms of water use than the drip-irrigation system since it could save on average 27% of water. The WUE_A recorded with subirrigation in 3 dm³ and 1 dm³ containers were significantly higher by 43% and 81% compared with those recorded with drip-irrigation. The plant height, leaf number, leaf area, root length and shoot dry biomass were significantly lower by 45.7%, 39.5%, 45.5%, 35.1% and 43.1%, respectively when the Euphorbia × lomi plants were cultivated in the 1 dm³ containers. The best crop performance recorded in the 3 dm³ containers was related to a higher photosynthetic activity and higher leaf chlorophyll content (i.e. SPAD index) with respect to the plants grown in the 1 dm³ containers.

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