Description: This dataset contains leaf water potential measured at predawn and midday of species in the Jena-Ecotron Experiment in 2012. This experiment was conducted in the Montpellier European Ecotron (CNRS, France), an advanced controlled environment facility for ecosystem research, and aimed at understanding the impact of plant species richness (4 vs. 16 species) for ecosystem carbon and water fluxes. The soil monoliths used in this experiment contained plant communities originating from the long- term Jena Experiment (50°57.1' N, 11°37.5' E, 130 m above sea level; mean annual temperature 9.3°C, mean annual precipitation 587 mm) established in May 2002. Twelve plots were selected for the Jena-Ecotron study according to the following criteria: (1) the four functional groups grasses, legumes, small and tall herbs were present, (2) realized species numbers were close to sown species richness, and (3) plots were equally distributed across the experimental field site to account for different soil textures. Large monoliths (2 m² surface area, diameter of 1.6 m, 2 m depth with a weight of 7 to 8 tons) including intact soil and vegetation were excavated in December 2011 and placed in lysimeters. In March 2012, before the start of the vegetation growth, the lysimeters were transported and installed in the Macrocosms platform of the Montpellier European Ecotron. Leaf water potential was measured with a pressure chamber (Pressure Chamber Instruments Model 600, PMS Instrument Company, Albany, Oregon, USA) at predawn (04:00 am - 06:00 am) and midday (01:30 pm - 04:00 pm) for the most abundant species on each plot from 17-19 July 2012. Two to three species were chosen in the 4-species mixtures, and five to eight species in the 16-species mixtures. Measurements were carried out on young, but fully expanded leaves from four individuals per species per plot. Stomatal conductance and leaf greenness were measured on three leaves from different individuals of all available species in each experimental unit. Values of leaf water potential of the individual measurements were averaged per species per plot.