The water relations of the tropical tuber-producing legume marama bean [ Tylosema esculentum (Burch) A. Schreib] which produces both tubers and beans for human consumption in the dry regions of southern Africa, were studied in field and greenhouse experiments in Athens, Greece. Two water treatments were imposed by means of different irrigation frequencies on plants grown from seed and tuber in the field experiment, whereas four water treatments were set up in the greenhouse on plants grown from seed for two cultivation periods by applying different amounts of water. Leaf water potential (Psi l) and relative water content (RWC), the solute potential at zero turgor (psi so), leaf stomatal conductance (g s), the angle between the two leaflets, and soil water content were recorded throughout the experiments, both on seasonal and diurnal basis. The values of Psi l were systematically lower in the drier treatments but never fell below -1.15 MPa. The plants grown from tubers exhibited consistently higher values of Psi l and RWC in comparison to those grown from seeds, indicating higher levels of hydration in the former. Progressive osmotic adjustment, more intense in plants grown from tubers, was detected as plants grew older. An exponential relation between g s and leaflet angle was also detected. Our results are discussed in conjunction with field observations taken in Botswana and indicate that marama is endowed with several drought avoidance mechanisms, which, in parallel with osmotic adjustment, enable it to survive under very harsh conditions.
