Water relations of four perennial plant species at the southern periphery of the Taklimakan desert

The investigations started from the assumption that perennial plants in the foreland of river oases in the Taklimakan desert are able to take up water from soils of low water content and low water potential during long drought periods. In order to check this assumption, P-V curves were established and selected water relation parameters were derived. Alhagi sparsifolia, Calligonum caput-medusae, Tamarix ramosissima, and Populus euphratica, growing in the foreland of Qira oasis were investigated. Contrary to the expectation, the osmotic potentials both at plant water saturation and at incipient plasmolysis are high in comparison with other desert plants and reveal that the investigated plants do not need to take up water against a low soil water potential. The osmotic potential at incipient plasmolysis is moderately decreasing during the growing season in three of the species, but this seems to be an endogenous process that is not caused by drought stress. Small differences between osmotic potentials at incipient plasmolysis and the minimum plant water potentials that are reached during daily courses point to a transpiration control that accepts very low turgor potentials. A plasmolysis risk is possibly diminished by an increase of cell wall elasticity during the growing season. The results support the conclusion that all investigated species have permanent access to groundwater and live as phreatophytes in the foreland of Qira oasis.