Abstract:

The southern rim of the Taklimakan Desert is the most arid region in central of Asia where mean annual precipitation is less than 40 mm and the potential evaporation is about 2 600 mm. The Qira (Cele) oasis is located in this region, and the perennial tree, Tamarix ramosissima is the dominant and most important indigenous species that grows along the margins of the Qira oasis. Besides sheltering the oasis against wind blown sand, this species is used by local people as construction material for houses and fences and as a fuel source. In view of large population increases in Qira, the maintenance of the vegetation belt by T. ramosissima is of high ecological and economical importance. Generally, water is the limiting factor that restricts vegetation regeneration and restoration in desert areas, and investigations into the water relations of plants can provide valuable information on the capability of plants to cope with the harsh conditions. Moreover, it can also provide information on how to restore and regenerate vegetation in the arid lands surrounding the oases. Therefore, the objectives of our investigation were as follows: 1) To assess the status of drought stress in T. ramosissima during the growing season; 2) To assess the importance of physiological and morphological features in acclimatization and adaptation to this extreme environment; and 3) To investigate the response of the water status in T. ramosissima to a single irrigation event in the summer that imitated natural flooding. In order to achieve these goals, water-relation parameters of twigs were derived from pressure-volume curves (PV curves) measured each month from April to October using a pressure chamber. In addition, the daily course of twig water potentials, including predawn potentials, were measured using a pressure chamber. The effect of flooding on soil water content was monitored using Time Domain Reflectometry (TDR) with sensors at five different soil depths. An automatic weather station was installed and climatic variables were recoreded continuously. The results showed that T. ramosissima was able to balance water deficits that developed during the day and maintained nearly constant and rather high predawn water potentials during the growing season. Except for one measurement, the water and osmotic potentials decreased only moderately during the growing season, and this seemed to be an endogenous process that was not caused by drought stress. T. ramosissima did not show any physiological adaptations to drought stress; the rapid decreases in osmotic potential and increased apoplastic water, incipient plasmolysis at relatively high water content and very small differences in osmotic potential are not characteristic drought responses. T. ramosissima depends on having a constant water supply to cope with the extremely high water pressure deficits and the relatively high evaporative demand of its environment which it obtains by maintaining contanct with deep water sources through the production of an extensive root system. Hence, a single artificial flooding event had no effect on plant water relations. Since T. ramosissima depends on ground water availability, the long-term survival of this species at the Qira oasis will require that the ground water table is maintained at a depth that can be accessed by the roots of T. ramosissima. The water and osmotic potentials of T. ramosissima were higher than those of typical desert plants and were more similar to mesophytic plants. The primary mechanism of T. ramosissima to cope with arid environments is through drought avoidance.