干旱区生态环境知识资源中心

Ephedra alata subsp. alenda is the most important pioneer plant of the moving and semi-stable sand dunes in the deserts and steppes of south Tunisia and occurs naturally in the Grand Erg Oriental, one of the most extreme habitats for plant growth on the planet. A new analysis of physiological performance of this medically important and internationally threatened xerophytic shrub was conducted to assess possible mechanisms of drought tolerance and how these relate to its ecological success. Five-month old plants, grown under controlled climatic conditions, were subjected to a well-watered control treatment or progressive drought by withholding water for 14d with subsequent recovery for 7 d. Soil water depletion significantly reduced stem relative water content (RWC) water potential (Psi(w)) and osmotic potential (Psi(pi)). Ephedra displayed more negative Psi(w) and Psi(pi) values of ca. -3.5 and -4.1 MPa, respectively, at the end of the drought treatment, and were associated with turgor loss. Low stem Psi(w) reduced stomatal conductance (g(s)), photosynthetic CO2 assimilation rates (A(CO2)), transpiration (E) and internal CO2 concentration (C-i). However, instantaneous (WUE; A(CO2) E-1) and intrinsic (WUEi; A(CO2) g(s)(-1)) water use efficiency (WUE) increased gradually as water deficit was intensified. Stomatal closure therefore only exerted limited control against dehydration and could not compensate for decreases in soil water status, typical of anisohydric behavior. Drought-stressed stems accumulated high levels of praline up to 480% of control values, highlighting a pivotal role in osmotic adjustment during intense water deficit. In contrast, the osmotic adaptation to soluble sugars was limited. Drought-stressed plants increased A(CO2), E, gs and C-i and decreased WUE and WUEi during the first 48 h after re-watering, such that they reached similar values to those of control plants by the end of the experiment. Stem proline levels of drought-stressed plants returned to near control values with re-watering. Overall, rapid recovery of photosynthesis following drought-breaking moisture appears to be a critical mechanism allowing E. alata to withstand and survive dry environments. (C) 2014 Elsevier B.V. All rights reserved.