HYDRAULIC LIFT - CONSEQUENCES OF WATER EFFLUX FROM THE ROOTS OF PLANTS

Hydraulic lift is the passive movement of water from roots into soil l ayers with lower water potential, while other parts of the root system in moister soil layers, usually at depth, are absorbing water. Here, we review the brief history of laboratory and field evidence supportin g this phenomenon and discuss some of the consequences of this below-g round behavior for the ecology of plants. Hydraulic lift has been show n in a relatively small number of species (27 species of herbs, grasse s, shrubs, and trees), but there is no fundamental reason why it shoul d not be more common as long as active root systems are spanning a gra dient in soil water potential (Psi(s)) and that the resistance to wate r loss from roots is low. While the majority of documented cases of hy draulic lift in the field are for semiarid and arid land species inhab iting desert and steppe environments, recent studies indicate that hyd raulic lift is not restricted to these species or regions. Large quant ities of water, amounting to an appreciable fraction of daily transpir ation, are lifted at night. This temporary partial rehydration of uppe r soil layers provides a source of water, along with soil moisture dee per in the profile, for transpiration the following day and, under con ditions of high atmospheric demand, can substantially facilitate water movement through the soil-plant-atmosphere system. Release of water i nto the upper soil layers has been shown to afford the opportunity for neighboring; plants to utilize this source of water. Also, because so ils tend to dry from the surface downward and nutrients are usually mo st plentiful in the upper soil layers, lifted water may provide moistu re that facilitates favorable biogeochemical conditions for enhancing mineral nutrient availability, microbial processes, and the acquisitio n of nutrients by roots. Hydraulic lift may also prolong or enhance fi ne-root activity by keeping them hydrated. Such indirect benefits of h ydraulic lift may have been the primary selective force in the evoluti on of this process. Alternatively, hydraulic lift may simply be the co nsequence of roots not possessing true rectifying properties (i.e., ro ots are leaky to water). Finally, the direction of water movement may also be downward or horizontal if the prevailing Psi(s) gradient so di ctates, i.e., inverse, or lateral, hydraulic lift. Such downward movem ent through the root system may allow growth of roots in otherwise dry soil at depth, permitting the establishment of many phreatophytic spe cies.