Is Decreased Xylem Sap Surface Tension Associated With Embolism and Loss of Xylem Hydraulic Conductivity in Pathogen-Infected Norway Spruce Saplings?

Teemu Paljakka; Kaisa Rissanen; Anni Vanhatalo; Yann Salmon; Tuula Jyske; Nønne L Prisle; Riikka Linnakoski; Jack J Lin; Tapio Laakso; Risto Kasanen; Jaana Bäck; Teemu Hölttä

doi:10.3389/fpls.2020.01090

Is Decreased Xylem Sap Surface Tension Associated With Embolism and Loss of Xylem Hydraulic Conductivity in Pathogen-Infected Norway Spruce Saplings?

Front Plant Sci. 2020 Jul 16:11:1090. doi: 10.3389/fpls.2020.01090. eCollection 2020.

Authors

Teemu Paljakka¹, Kaisa Rissanen¹, Anni Vanhatalo¹, Yann Salmon^{1

2}, Tuula Jyske³, Nønne L Prisle⁴, Riikka Linnakoski⁵, Jack J Lin⁴, Tapio Laakso³, Risto Kasanen⁶, Jaana Bäck^{1

6}, Teemu Hölttä^{1

6}

Affiliations

¹ Faculty of Agriculture and Forestry, Institute for Atmospheric and Earth System Research/Forest Sciences, University of Helsinki, Helsinki, Finland.
² Faculty of Science, Institute for Atmospheric and Earth System Research/Physics, University of Helsinki, Helsinki, Finland.
³ Natural Resources Institute Finland (Luke), Espoo, Finland.
⁴ Nano and Molecular Systems Research Unit, University of Oulu, Oulu, Finland.
⁵ Natural Resources Institute Finland (Luke), Helsinki, Finland.
⁶ Forest Sciences/Faculty of Agriculture and Forestry, University of Helsinki, Helsinki, Finland.

Abstract

Increased abiotic stress along with increasing temperatures, dry periods and forest disturbances may favor biotic stressors such as simultaneous invasion of bark beetle and ophiostomatoid fungi. It is not fully understood how tree desiccation is associated with colonization of sapwood by fungi. A decrease in xylem sap surface tension (σ_xylem) as a result of infection has been hypothesized to cause xylem embolism by lowering the threshold for air-seeding at the pits between conduits and disruptions in tree water transport. However, this hypothesis has not yet been tested. We investigated tree water relations by measuring the stem xylem hydraulic conductivity (K_stem), σ_xylem, stem relative water content (RWC_stem), and water potential (Ψ_stem), and canopy conductance (g_canopy), as well as the compound composition in xylem sap in Norway spruce (Picea abies) saplings. We conducted our measurements at the later stage of Endoconidiophora polonica infection when visible symptoms had occurred in xylem. Saplings of two clones (44 trees altogether) were allocated to treatments of inoculated, wounded control and intact control trees in a greenhouse. The saplings were destructively sampled every second week during summer 2016. σ_xylem, K_stem and RWC_stem decreased following the inoculation, which may indicate that decreased σ_xylem resulted in increased embolism. g_canopy did not differ between treatments indicating that stomata responded to Ψ_stem rather than to embolism formation. Concentrations of quinic acid, myo-inositol, sucrose and alkylphenol increased in the xylem sap of inoculated trees. Myo-inositol concentrations also correlated negatively with σ_xylem and K_stem. Our study is a preliminary investigation of the role of σ_xylem in E. polonica infected trees based on previous hypotheses. The results suggest that E. polonica infection can lead to a simultaneous decrease in xylem sap surface tension and a decline in tree hydraulic conductivity, thus hampering tree water transport.

Keywords: Endoconidiophora polonica; Picea abies (Norway spruce); embolism; hydraulic conductivity; plant-pathogen interactions; surface tension; xylem sap composition; xylem transport.