Xylem anatomical features and native xylem embolism of several Tamarix spp. species seedlings under different water treatments

Three different Tamarix spp. species (T. ramosissima, T. hispida, T. androssowii) cuttage seedlings were treated with different soil water levels (control, slight drought stress and severe drought stress) for investigating the effects of soil water treatment on the xylem hydraulic features. Xylem hydraulic specific conductivity (K_s(max))and native embolism rates were measured using a XYLEM xylem embolism meter. Xylem vessel and inter-vessel pit features were observed by LM and SEM technique. The relations between xylem hydraulic features with its anatomical characteristics were analyzed. Results showed that:1) the xylem native embolism rates of seedling stem of Tamarix L. species (PLC value, %)had close relations with soil moisture level(r=-0.796, P=0.01), the xylem PLC value increased with the decrease of soil water content; under the optimal water conditions, all Tamarix L. species had higher native PLC values (33%-52%), among them, T. hispida had lower level of PLC value(33%, P < 0.05) than other two species. However, PLC value increased significantly after different drought stress(P < 0.05); stem PLC values of T. androssowii increased to 84% under severe drought stress treatment, and it was significantly higher than other two species(P < 0.05). According to the response of xylem native embolism to different drought stress treatment, the order of stem xylem sensitivity to drought was showed as T. androssowii> T. hispida> T. ramosissima. 2) There were significant inter-plant and inter-specific differences in stem and root xylem vessel and inter-vessel pit quantitative features among 3 Tamarix species(P < 0.05); SEM images indicated that there were no obvious qualitative differences in the inter-vessel pit membrane structures of different Tamarix species. In all species, root and stem xylem intact pit membrane were observed as an uniform deposition of microfibrils across the surface, with no visible pores. 3) Correlation analysis revealed that there was positive significant correlations between xylem K_s(max) value and vessel hydraulic diameter(D_h), vessel surface area(V_a) (r=0.848, P=0.033; r=0.814, P=0.049), and negative relations with solitary vessel index(V_s) (r=-0.925, P=0.008). 4) T. androssowii stem xylem not only had larger vessles, higher vessel grouping index, higher efficiency of hydraulic conductivity and native embolism, which were the xylem characteristics of lower embolism resistant plants, but also had the anatomical features of higher embolism resistant plants such as narrow elliptical pit aperture. Results would deepen the understanding of drought resistant ability of Tamarix species from xylem function and anatomy aspects.