半干旱区凝结水的时空格局及其对植物碳同化的影响和调控机制

Dew is an important potential water resource in arid and semi-arid regions, and is helpful to decrease plant evaporation and increase water content of plants in the case of soil water stress. Dew occurring in soil and plant leaves displayed different functions to plant water use and carbon assimilation with different mechanisms. There are significant differences of the stomatal conductance, carbon assimilation rate and water characteristics among C3, C4 and CAM plants. Through microlysimeter meter,isotope method and fluorescence microscopy in situ experiments in field and controlled experiments, the study aims to investigate the amount and persistence of dew and the dynamic trends of dew in soil and on leaves of dominant C3, C4 and CAM plants at three different habitats, such as fixed sandland, semi-fixed sandland, and lowland. The research will illustrate the effects of dew on the patterns of soil water content in surface and the driving factors of occurrence of dew, such as climatic parameters, vegetation coverage, leave's traits. Further, the research will clarify the pathway of directly absorbing of dew by plants, the amount and the places for transporting of absorbed dew in plants leaves. Moreover, the study will reveal the underlying eco-physiological and morphological mechanisms of plants with different photosynthetic pathway responding to the dew, and then we synthetically evaluate the ecological significance of dew in semi-arid ecosystem. The achievements from the research will be not only helpul for understanding the physiological mechanisms of plants adapt to the drought environment and the ecological functions of dew in semi-arid regions, but also could afford the scientific proofs for the protection of vegetation and restoration of ecology in this region.

凝结水(dew)是干旱和半干旱区重要的潜在水资源之一,其对降低植物蒸腾和帮助干旱胁迫的植物恢复一定水分具有重要作用。发生在土壤和叶片上的凝结水对植物水分利用和碳同化的影响机制不同。C3、C4和CAM光合途径植物的叶片碳同化速率、水分特性和叶片形态特征显著不同。本项目以半干旱区浑善达克沙地三种光合途径的优势植物种为研究对象,通过野外原位实验和控制实验相结合,借助微渗仪、同位素示踪和荧光显微法等手段,阐明固定沙地、半固定沙地和滩地三种生境下凝结水时空变化规律和影响因素; 明确凝结水对表层土壤水分格局的影响及其对植物水分利用效率和碳同化作用的调控机制;探明植物直接吸收利用凝结水的途径、数量和去向;揭示不同光合功能型植物对凝结水响应的形态特征和生理生态学机制;综合评估凝结水的生态效应。研究既有利于深入理解植物对干旱环境的适应机制和凝结水的生态水文意义，而且为半干旱区植被保护和生态恢复提供科学依据。

在未来气候变化影响下，我国北方地区降雨的时空异质性将增大，干旱和极端气候事件将可能增加。因此植物利用凝结水的研究将越来越受到重视。本项目借助微渗仪和同位素的手段，深入研究浑善达克沙地凝结水发生规律、影响因素、及其对植物干旱胁迫的缓减和影响。研究表明:(1) 凝结水通常发生在晴朗的夜间，生长季大约50%的天气有凝结水形成。凝结水通常在夜间20:30分左右开始，至次日早晨7:30。每日凝结水持续时间因大气温度和湿度不同而差异显著，平均持续时间在8小时左右。(2)凝结水量随降雨量的增加而增加，通常在一次降雨之后的几天内，凝结水量较高。2013,2014和2015年日均凝结量分别为0.14 mm d-1, 0.13mm d-1 and 0.15 mm d-1。尽管凝结水的量比较少，但它对由于植物叶片表明水分状况的补充有重要作用。(3)空气温度、大气湿度，风速和云量对凝结水形成影响较大。在阴天或风速大于5m/s，很少有凝结水形成。凝结水发生时的空气湿度都在80%以上。有树冠层覆盖的区域，凝结水量显著低于开阔的草地。但是固定沙地比半固定沙地的凝结水量大，这可能是固定沙地植被蒸腾量大，空气湿度大，造成凝结水量也大。(4)干旱胁迫下的羊草和冰草，随着凝结水发生频率越高，叶片含水量降低的越少，与对照相比，每周发生3次和5次凝结水的冰草叶片相对含水量分别提高了41%和71%；叶片水势也提高，说明冰草叶片吸收了夜间发生的凝结水，凝结水能有效的减缓植物受到的胁迫影响。每周3次和5次凝结水的冰草净光合速率(Pn) 比对照分别提高了0.58倍和1.73倍,。由此可见凝结水有利于植物维持较高的光合速率。每周发生3次凝结水可有效的缓解干旱对植物叶片表面造成的损伤，对叶片表面有一定的保护作用。本研究阐明浑善达克沙地凝结水发生规律和对植物的生态意义，可为半干旱区植被保护和生态恢复提供科学依据。