Water acquisition in rainfed lowland rice evaluated through the isotope analysis of xylem water

这项研究的目的是建立一种方法，以确定从较深的土层中获取水分的作物种类或品种。我们重点研究了土壤和木质部汁液中氢同位素比值（dD）的天然丰度。在赞比亚的实地考察中，我们分析了降水和地下水中的dD，发现地下水在试验期间dD值保持恒定，而降水中dD值出现了较大的波动。对田间作物木质部汁液的进一步分析也显示出dD值的波动，表明降水的影响。但不同物种间的dD波动存在显著差异；与其他物种相比，特定物种的dD值总是更接近地下水。结果表明，不同物种对地下水水源的依赖程度存在差异。为了解释具体的差异，对根进行了解剖研究。结果表明，对地下水依赖程度高的种属完全不发育通气组织，而对地下水依赖程度低的种属发育良好。此外，还发现作物物种发生水力抬升，即通过植物根系的水运动。这是液压举升在农业田间研究中的首次报道。我们进一步研究了液压升降是否可以增加某些农业实践，发现遮阳是有效的手段。假设根系较深的植物可以通过调节液压升力来发挥类似喷头的作用。

The aim of the research was to establish a methodology to identify crop species or varieties that acquire water from deeper soil layers. We have focused on the natural abundance of hydrogen isotope ratio (dD) in water of both soil and xylem sap. In the field investigation at Zambia, we have analyzed dD in precipitation and the groundwater, and found that while the groundwater had a constant dD values through the experimental period the precipitation showed a heavy fluctuation in dD values. Further analysis of the xylem sap of crops grown in the field has shown also the fluctuated dD values, suggesting the effect of the precipitation. However, there were significant differences in the dD fluctuation between species; a particular species had dD values always closer to the groundwater comparing with other species. The results imply the species differences in the dependency on the groundwater as water source. To explain the specific differences, the anatomical investigations of the roots were conducted. As a result, it was found that the species with high dependency on the groundwater did not develop aerenchyma at all but the species with low dependency developed well. Moreover, it was found that a crop species occurs hydraulic lift that is a water movement through a plant root system. This was the first report of hydraulic lift in agricultural field study. We have further examined if hydraulic lift can increase with certain agricultural practices, and found that shading is an effective means. It is assumed that plants with deeper root system may be utilized as a sprinkler-like function by regulating hydraulic lift.