新疆古尔班通古特沙漠南缘多枝柽柳光合作用及水分利用的生态适应性

Ecologicaladaptability of photosynthesis and water use for Tamarix ramosissima in the southern periphery of Gurbantunggut Desert,Xinjiang

以新疆古尔班通古特沙漠南缘荒漠绿洲过渡带的建群种多枝柽柳(Tamarix ramosissima)为研究对象,分别在自然状态,模拟降水、遮光处理以及降水和遮光双因子处理下,利用LI-6400光合作用系统和Model 3500植物水分压力室分别测定光合作用和枝条水势,研究多枝柽柳枝条水势对浅层土壤水分和光照变化的响应,以及在不同条件下的水分利用效率(WUE)和光能利用效率(LUE)的日进程。结果表明:这4种处理方式下,枝条水势随土壤水分的变化没有明显的差异,趋势表现为黎明前枝条水势最高(-1.2 MPa),正午枝条水势最低(-3.2 MPa),太阳落山后枝条水势逐渐升高的趋势;WUE和LUE日变化呈现出近似双峰型抛物线,WUE日变化曲线在11:00和16:00有两个不明显的波峰,LUE日变化曲线在10:00和16:00出现两个明显波峰。当光照发生变化时,遮光处理下的WUE和LUE日变化都较自然状态下的日变化低,分别降低了1.5μmol CO2/mmol H2O的WUE和0.20×10-2μmol CO2/μmol APAR的LUE。研究表明,作为深根系植物的多枝柽柳,生存主要依靠地下水来补给水分,所以浅层土壤水分条件变化没有明显引起光合和蒸腾的响应。而午间光强、高温是构成多枝柽柳光抑制现象的主要原因,这是多枝柽柳长期在荒漠绿洲过渡带形成的对抗环境胁迫的一种表现。

Photosynthesis and water use are the most important factors limiting photosynthetic assimilation of carbon dioxide and growth of individual plant in terrestrial ecosystems. To understand the role of photosynthesis and water use in plant of the desert-oasis ecotone, this study took Tamarix ramosissima-a native dominant desert shrub-as a subject to investigate the plant adaptability to light and precipitation under different controlled conditions. The experiment was conducted within the desert-oasis ecotone in the southern periphery of Gurbantunggut Desert with four treatments. The treatments are A： natural state without any disturbance, B： simulated precipitation with 12 mm which is equivalent to the average of the maximum precipitation event within September last five years taking place in target desert-oasis ecotone, C ： shading, i.e. the whole plant covered by opaque black cloth and D： a combination of treatments B and C. Using the LI-6400 system by LI-COR, Inc and Model 3005 plant water pressure chamber, the authors studied the responses of photosynthesis and the variation of water potential of branches to the simulated precipitation and light variation, and investigated the diurnal course of water use efficiency （WUE） and light use efficiency （LUE） of T. ramosissima. Results showed that the treatment B resulted in richwater content ranging from 15 % to 43% in shallow soil layer within the depth of O-40cm, depending on the evaporation, but water content in the soil layer between 40cm and lOOcm indicates that water from precipitation can only infiltrate to does not exit significant different from in natural state, which the depth of 40cm of the soil under the condition with the maximum precipitation event. There is slight difference in variations of the water potential in branches with changes of surface soil moisture under the four treatments. The highest and lowest water potential in branch presented in predawn （-1.2 MPa） and midday （-3.2 MPa） respectively, and then the water potential in branch gradually increased after the sunset. The diurnal variations of WUE and LUE in T. ramosissima showed bimodal pattern. The diurnal courses of WUE and LUE under shading conditions was lower 1.5 μmol CO2/mmol H20 and 0. 20 ×10^-2 μmol CO2/μmol APAR than those under natural conditions with the variation of light, respectively. It is demonstrated that even the shallow soil is under an extreme drought condition, water potential and photosynthesis of T. ramosissima branches are still able to stay at a normal level. A lower water potential indicates that T. ramosissima is able to survive under extreme drought stress. Physiological activity and biomass accumulation of T. ramosissima rely on the stable groundwater, and variations of soil moisture in the upper soil layers has slight influence on its photosynthesis and evapotranspiration. The ＂ photosynthetic descent＂ phenomenon of T. ramosissima is caused by the high light and temperature in midday, which reflected the ecological adaptability of T. ramosissima in the course of cooperative evolution with the environment. Furthermore, the sensitive regulation of stomata was the adaptability mechanism in response of T. ramosissima to the variation of light and droughty habitats. These phenomena are significantly related to the water use strategy, leaf morphology, root function and eco- physiological adaptation of T. ramosissima. However, the groundwater table decreased in some transition regions between deserts and oasis over the past 50 years. As a result, T. ramosissima is difficult to obtain underground water and can not effectively adjust the individual ecological structure and water use strategies within certain period to adapt to changes in water conditions. They generate negative effect on phreatophyte species- T. ramosissima, and even threatens its survival in the future. Therefore, it is suggested to prevent overexploitation of groundwater and over-harvesting T. ramosissima.