Scholz Fabian G, Bucci Sandra J, Goldstein Guillermo, Meinzer Frederick C, Franco Augusto C
Laboratorio de Ecologia Funcional, Departamento de Biologia, FCEN, Universidad de Buenos Aires, Ciudad Universitaria, Nunez, Buenos Aires, Argentina.
Tree Physiol. 2002 Jun;22(9):603-12. doi: 10.1093/treephys/22.9.603.
The magnitude and direction of water transport by the roots of eight dominant Brazilian savanna (Cerrado) woody species were determined with a heat pulse system that allowed bidirectional measurements of sap flow. The patterns of sap flow observed during the dry season in species with dimorphic root systems were consistent with the occurrence of hydraulic redistribution of soil water, the movement of water from moist to drier regions of the soil profile via plant roots. In these species, shallow roots exhibited positive sap flow (from the soil into the plant) during the day and negative sap flow (from the plant into the soil) during the night. Sap flow in the taproots was positive throughout the 24-h period. Diel fluctuations in soil water potential, with maximum values occurring at night, provided evidence for partial rewetting of upper soil layers by water released from shallow roots. In other species, shallow roots exhibited negative sap flow during both the day and night, indicating that hydraulic redistribution was occurring continuously. A third sap flow pattern was observed at the end of the dry season after a heavy rainfall event when sap flow became negative in the taproot, and positive in the small roots, indicating movement of water from upper soil layers into shallow roots, and then into taproots and deeper soil layers. Experimental manipulations employed to evaluate the response of hydraulic redistribution to changes in plant and environmental conditions included watering the soil surface above shallow roots, decreasing transpiration by covering the plant and cutting roots where probes were inserted. Natural and manipulated patterns of sap flow in roots and stems were consistent with passive movement of water toward competing sinks in the soil and plant. Because dry shallow soil layers were often a stronger sink than the shoot, we suggest that the presence of a dimorphic root system in deciduous species may play a role in facilitating leaf expansion near the end of the dry season when the soil surrounding shallow lateral roots is still dry.
利用热脉冲系统测定了巴西稀树草原（塞拉多）8种优势木本植物根系水分运输的大小和方向，该系统可对液流进行双向测量。在旱季观察到的具有二型根系的物种的液流模式与土壤水分水力再分配现象一致，即水分通过植物根系从土壤湿润区域向干燥区域移动。在这些物种中，浅根在白天表现出正向液流（从土壤进入植物），在夜间表现出负向液流（从植物进入土壤）。主根中的液流在24小时内均为正向。土壤水势的昼夜波动在夜间达到最大值，这为浅根释放的水分使上层土壤部分再湿润提供了证据。在其他物种中，浅根在白天和夜间均表现出负向液流，表明水力再分配在持续发生。在一次强降雨事件后的旱季末期，观察到了第三种液流模式，此时主根中的液流变为负向，小根中的液流变为正向，这表明水分从上层土壤层进入浅根，然后进入主根和更深的土壤层。用于评估水力再分配对植物和环境条件变化响应的实验操作包括给浅根上方的土壤表面浇水、通过覆盖植物减少蒸腾作用以及在插入探针的位置切断根系。根和茎中自然和人为控制的液流模式与水分向土壤和植物中竞争汇的被动移动一致。由于干燥的浅层土壤通常比地上部分是更强的汇，我们认为落叶物种中存在二型根系可能在旱季末期当浅侧根周围的土壤仍然干燥时促进叶片扩展方面发挥作用。
