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Tylosema esculentum (Burch) A. Schreib is an underutilized drought-tolerant legume, native to the arid and semi-arid regions of southern Africa, which produces protein- and oil-rich seed and tubers with potential for use as human food and animal fodder. Laboratory experiments were conducted, in order to investigate the nature of the characteristic leaflet movements of this species and the potential effects of potassium on them. The rhythm of leaflet movements was maintained in constant dark conditions and at several temperature regimes and it is also synchronized with the light cycles. Substantially, this study confirms the endogenous nature of T esculentum leaflet movements, since they have all the main criteria of circadian rhythms. The crucial role of potassium on these movements and consequently on the total water economy of the species is also indicated, as long as there was an undoubted absence of leaflet movements in the potassium-deficient plants. This effect has to be taken seriously into account for any future attempts of the cultivation of the plant in arid and semi-arid environments. (C) 2008 Elsevier Ltd. All rights reserved.