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Water deficit and high temperature often occur simultaneously, but their effects on plants are usually investigated separately. The aim of this study was to test how interactions between water stress and nocturnal warming affect carbon allocation in the perennial grass,Leymus chinensis. Plant biomass, dry mass allocation,14C partitioning and carbon isotope composition (δ13C) were measured. Severe and extreme water stress during nocturnal warming decreased the allocation of dry mass and14C partitioning below ground to the roots, but moderate water stress significantly increased the below-ground allocation of dry mass and 14C, especially at the lower night temperature. The δ13C values were more positive at day/night temperatures of 30/20°C than at 30/25°C, and greater in the roots than in the leaves. By plotting the δ13C values of the leaves against the δ13C values of the roots, the slopes of regressions were steeper at low than at high night temperature, also indicating that nocturnal warming reduces carbon allocation below ground to the roots. The results suggest that nocturnal warming may weaken acclimation during water stress in this species by regulating carbon allocation between source and sink organs. [ABSTRACT FROM AUTHOR]