Ectomycorrhizas and water relations of trees: a review

Abstract

There is plenty of evidence for improved nutrient acquisition by ectomycorrhizas in trees; however, their role in water uptake is much less clear. In addition to experiments showing improved performance during drought by mycorrhizal plants, there are several studies showing reduced root hydraulic conductivity and reduced water uptake in mycorrhizal roots. The clearest direct mechanism for increased water uptake is the increased extension growth and absorbing surface area, particularly in fungal species with external mycelium of the long-distance exploration type. Some studies have found increased aquaporin function and, consequently, increased root hydraulic conductivity in ectomycorrhizal plants while other studies showed no effect of ectomycorrhizal associations on root water flow properties. The aquaporin function of the fungal hyphae is also likely to be important for the uptake of water by the ectomycorrhizal plant, but more work needs to be done in this area. The best-known indirect mechanism for mycorrhizal effects on water relations is improved nutrient status of the host. Others include altered carbohydrate assimilation via stomatal function, possibly mediated by changes in growth regulator balance; increased sink strength in mycorrhizal roots; antioxidant metabolism; and changes in osmotic adjustment. None of these possibilities has been sufficiently explored. The mycorrhizal structure may also reduce water movement because of different fine root architecture (thickness), cell wall hydrophobicity or the larger number of membranes that water has to cross on the way from the soil to the xylem. In future studies, pot experiments comparing mycorrhizal and nonmycorrhizal plants will still be useful in studying well-defined physiological details. However, the quantitative importance of ectomycorrhizas for tree water uptake and water relations can only be assessed by field studies using innovative approaches. Hydraulic redistribution can support nutrient uptake during prolonged dry periods. In large trees with deep root systems, it may turn out that the most important function of mycorrhizas during drought is to facilitate nutrient acquisition.

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…Hyphal length density can vary between 1 and > 100 m g −1 of soil (Smith et al, 2010). Thus, mycorrhizal hyphae may access water not available to plant roots, presumably because fungal hyphae can penetrate small water-filled pores to a greater extent than the larger roots (Bornyasz et al, 2005;Allen, 2007;Graham et al, 2010;Lehto and Zwiazek, 2011). Thus, mycorrhizae may be a factor that facilitates plant access to rock moisture and matrix waters that would otherwise be inaccessible to roots.…”

Nutrient addition effects on tropical dry forests: a mini-review from microbial to ecosystem scales

“…Theory predicts that N deposition may lead to increased mycorrhizal biomass and more parasitic phenotypes under Nlimitation and decreased biomass/more mutualistic phenotypes under P-limitation (Johnson et al, 2015). Finally, because mycorrhizae may enhance plant water uptake to different degrees depending on nutrient availability and mycorrhizal type (Auge, 2001(Auge, , 2004Lehto and Zwiazek, 2011), any change in mycorrhizal colonization or composition related to nutrient deposition may affect plant water relations during drought, potentially leading to differential feedbacks on evergreen and deciduous species in TDFs.…”

Plant genetic effects on soils under climate change

“…Genetically-based soil microbial interactions may buffer plants against climatic change and allow species persistence (through differential survival of genotypes; Van der Putten et al 2013). For example, microbial communities may enhance water relations and thus plant persistence through periods of drought (Lehto and Zwiazek 2011;Augé 2004;Lau and Lennon 2012;case study below). Similarly, plant-microbe interactions may regulate nutrient cycling with effects on plant performance, enhancing persistence in the face of climate change.…”