The morphological and physiological responses of Tamarix ramosissima seedling to different irrigation methods in the extremely arid area

Abstract

Water shortages are not only the main limiting factor of plant survival and growth in many habitats, but also are the direct cause for vegetation degradation in arid regions. The lower reaches of Tarim River are located in an extremely arid climate in northwestern China. The vegetation structure along the river banks of the lower reaches of the Tarim River is relatively simple, with only a few plant species present. Major plant species include such trees as Populus euphratica and shrubs such as Tamarix ramosissima, and Tamarix hispida. During the past 50 years, intense human activities effects in this region have led to the nearly complete interception of water in 321 km of the watercourse in the lower reaches of the Tarim River; as a result, the groundwater table has dropped considerably and natural vegetation which depends on that groundwater has become severely degraded. To preserve the endangered desert river bank forest vegetation and restore the damaged ecological system, an Ecological Water Conveyance Project was initiated in 2000. Artificially planting native dominant plant species including Tamarix spp. to initiate rapid rehabilitation of the plant community has become an important research subject in this area. However, the increased river flow from this project has produced very limited results in increasing both the amount of intermittent water flow in the river and in raising the downstream ground water table. As a result, the water demands of the T. ramosissima seedlings in the early growth stage could not be met. Therefore, irrigating field transplanted seedlings during the early stage of growth has been very important. T. ramosissima was once widely distributed in the desert plains and in the sandy and alkaline lands in northwestern China. It was also a dominant shrub in the desert riparian forest in the lower reaches of the Tarim River and played a key role in ecological rehabilitation and maintenance of the stability of the riparian forest in this area. The objective of this study was to test the growth and physiological responses of T. ramosissima seedling to different irrigation methods, i.e. layered side irrigation (LSI) and aboveground irrigation (AGI), using three water levels, i.e. high (W1, 50 L plant-1 repetition-1), medium (W2, 25 L plant-1 repetition-1) and low (W3, 12.5 L plant-1 repetition-1) water levels. The results showed that LSI increased basal diameter, stem length, crown width and growth rate of the seedling; LSI also increased soil moisture under the same water level conditions when compared with the AGI method. LSI significantly increased belowground biomass, total biomass and the root shoot ratio (R / S) of the seedling under W1 (P < 0.05). LSI significantly increased stem water potential under W1 and W2 (P < 0.05). The findings show that LSI can be used to increase water use efficiency of the seedlings. LSI increased photochemical quantum yield of PS II in the light (Yield) of the seedlings under W1 and W2, and this increase was statistically significant under W1 (P < 0.05). Our results suggest that LSI caused rapid and large amounts of growth and biomass production in T. ramosissima seedlings, as well as had a valuable positive effect on water potential and photosynthesis which benefited the seedling's survival rates during the early stage of growth.