Changes of Xylem Hydraulic Efficiency and Native Embolism of Tamarix ramosissima Ledeb. Seedlings under Different Drought Stress Conditions and after Rewatering

The impact of drought on the hydraulic traits of plants has drawn increasing attention. Plant responses to drought are species specific, and depend on the plants’ hydraulic strategy. Research has demonstrated that plants can adjust their xylem hydraulic traits of withstanding seasonal drought or acclimatize to drought stress conditions through developing more resistant xylem to drought induced cavitation, reducing root hydraulic conductance, maintaining high hydraulic efficiency in stems and roots, and also by increasing whole plant leaf specific conductivity.

Tamarix ramosissima Ledeb. is one of the dominant desert phreatophyte shrub species native to the Tarim River Basin in northwestern China. It provides remarkable ecological and economic benefits to arid areas in northwestern China, and is also an aggressive alien plant in riparian ecosystems of the southwestern United States. Therefore, many researchers have tried to identify traits that aid in its drought resistance as well as its strong invasive ability. However, their studies focused solely on stems. Xylem water transport capacity of the whole plant under different drought stress conditions has not yet been examined.

In order to determine how hydraulic efficiency and native embolism are different between various organs and affected by short-term drought stress, the response of T. ramosissima seedlings to short-term drought stress and water recovery treatment and how hydraulics and native embolism differ in different organs, including lateral root, main stem and twigs were studied in the Tarim River Basin. These traits included initial hydraulic specific conductivity (K_{s (init)}), maximum hydraulic specific conductivity (K_{s (max)}) and native embolism (PLC, %) of lateral root, main stem and twigs, as well as the xylem anatomical structures of lateral roots and twigs of two-year old T. ramosissima seedlings.

The results showed that drought stress and water recovery treatments had significant impacts on K_{s (max)} value of lateral root, (K_{s (init)}) value and native embolism rate of all different organs (p＜0.01). Severely drought stress treatment induced a significant decrease in K_{s (init)} values of all organs (p＜0.05), however, the values of K_{s (max)} in lateral root increased significantly (p＜0.01). The native embolism rate increased with the intensity of soil drought stress in all different organ and short-term re-watering had only significant impacts on embolism recovery for lateral roots (p＜0.01), embolism recovered 20% as compared to severely water stressed seedlings. Lateral roots had a larger mean vessel diameter (d_mean, d_h) and lower vessel density (VD) compared to the twigs, and their K_{s (max)} values were strongly correlated with xylem vessel diameter.

The main finding has been published on South African Journal of Botany, 2012, 78: 75-82. The paper is also archived at http://www.sciencedirect.com/science/article/pii/S0254629911000664.