Effects of salinity and water stress on the physiological and ecological processes and plasticity of Tamarix ramosissima seedlings

Abstract

Tamarix ramosissima grows in extremely arid areas and is a dominant species of desert riparian forests in the lower reaches of the Tarim River. Using a single factor control test with indoor potted plants, T. ramosissima seedlings were treated with either salt or water stress, and compared to a control treatment. For salt stress, plants were treated with one of five salinity gradients: a control group with no salt stress (S0); light salt stress (S1); moderate salt stress (S2); middle, and high salt stress (S3); and high salt stress (S4). In addition, five moisture gradients were employed: groups D0, D1, D2, D3, and D5 (flooding), for which soil water contents were 75%, 55%, 35%, 15% field water capacity, and flooding, respectively. Combined treatments with both salt and water stress were not performed. The goal of the present study was to analyze the effects of salt and water stress on T. ramosissima seedlings by measuring photosynthetic characteristics, anatomical structure, and morphological plasticity characteristics, such as height, crown breadth, epidermis, and changes to the cortex, vascular cylinder etc. Because structure provides the basis for function, the anatomical structure of desert plants can be indicative of their photochemical efficiency. The results showed that: 1) salt and water stress resulted in relatively lower growth rates, biomass, and smaller plant height and crown width of T. ramosissima seedlings. In addition, the root to shoot biomass ratio initially increased and then decreased rapidly. Salt stress adversely affected seedling growth to a greater extent than water stress did (P<0.05). 2) No significant differences were observed in the S1, D1, and S2 treatments. Fluorescence parameters decreased significantly (P<0.05), and photochemical efficiency and photosynthetic activity were inhibited in the S3 and D2 treatments. With increasing stress, the chlorophyll and the leaf water contents decreased suggesting that increased stress is not conducive to normal development. 3) Salt and water stress both affected the xylem conduits of T. ramosissima seedlings. Under salt stress, the diameters of the root xylem vessels were significantly smaller than that of the control group, and vessel densities were significantly higher than that of control group. The opposite trends were observed under water stress. The percentage of total diameters of epidermal and palisade tissue, and the cortical thickness of seedlings increased under water stress, all adaptations that are favorable for water retention and improved photosynthetic efficiency. 4) The height growth rate, chlorophyll content, leaf cuticle thickness, assimilating branches, and the root plasticity index were greatest under conditions of either salt or water stress. Seedlings were more tolerant to water stress than to salt stress. The mean plasticity indexes could be classified as root>assimilating shoots>leaf under salt stress, and leaf>root>assimilating branches under water stress. In summary, T. ramosissima seedlings were better adapted to low levels of stress as observed in the S0 and D1 treatments. Compared to water stress, salt stress is a more critical ecological factor that is important to inhibition of the growth of T. ramosissima seedlings.