Hydraulic redistribution of soil water by roots and fungal symbionts

Background/Question/Methods Hydraulic redistribution (HR) is the process of water movement from moister to drier portions of the soil profile using plant root systems as a conduit. HR maximizes the amount of water available to plants during dry periods, thus increasing plant transpiration, carbon assimilation and water use efficiency. Water efflux from the roots of plants into dry soil layers has potentially important consequences for root growth and performance, plant nutrient acquisition, biogeochemical nutrient cycling, rhizosphere microbial activity and plant-plant interactions. In recent years, a growing body of research has begun to examine the relevance and implications of HR for plant and ecosystem functioning. Results/Conclusions Mesocosm studies have shown that internal HR of water within root systems can prolong the lifespan of fine roots in dry soil, and that water efflux from roots can enhance nutrient uptake from dry topsoil layers. Direct water transfer from host plants conducting HR to their fungal symbionts has been shown to maintain the integrity and functionality of mycorrhizal roots and extramatrical hyphae in very dry soil during mesocosm experiments. HR has also been shown to buffer mycorrhizal roots and rhizosphere hyphae in topsoil layers against the negative effects of seasonal drought in an oak woodland ecosystem. Efflux of hydraulically redistributed water from mycorrhizal hyphal tips has been detected with the aid of fluorescent tracers in mesocosm studies. Since the mycorrhizal hyphosphere spreads beyond the rhizosphere, water transport through (and leakage from) extramatrical mycorrhizal mycelia (EMM) may significantly increase the volume of soil influenced by HR, particularly in ectomycorrhizal systems. A number of greenhouse and field studies now indicate that EMM provide a pathway for the transfer of hydraulically redistributed water among neighbouring plants which are connected by common mycorrhizal networks. This is a previously undescribed facilitative mechanism which could significantly enhance seedling survival and growth in drought-prone ecosystems. However, determining the relative importance of the direct mycorrhizal hyphal pathway versus the extramycelial root-soil pathway during HR has proved difficult so far. Whereas these two pathways are not mutually exclusive, and likely occur simultaneously, the implications of hydraulic redistribution through both root systems and EMM are expected to differ in subtle but potentially important ways from those of water efflux from roots only. Current gaps in knowledge, emerging hypothesis and future research directions will be summarized and discussed in the light of recent advances in our understanding of HR.