Ectomycorrhiza and hydrogel protect hybrid poplar from water deficit and unravel plastic responses of xylem anatomy

To investigate whether the performance of Populus x canescens, a water-spending species, can be improved by Paxillus involutus ectomycorrhizas (EM) and soil amendment with water-retaining polymers (hydrogel), the pre-dawn water potential, growth performance and the hydraulic architecture of well-irrigated and drought-stressed plants were studied. Roots of well-irrigated plants were about 75% colonized with P. involutus regardless the presence of hydrogel. Drought stress resulted in an about 10% loss of EM. Non-inoculated plants showed marginal EM colonization (3%). EM poplar in hydrogel-amended soil maintained the highest pre-dawn water potential under drought stress, showed lowest biomass loss and kept almost full photosynthetic activity, whereas non-EM plants in soil without hydrogel exhibited severe leaf loss under these conditions and did not show net CO2 assimilation. Non-EM plants in hydrogel-amended soil and EM plants without hydrogel showed intermediate stress symptoms. The soluble carbohydrate content of roots was correlated with the pre-dawn potential indicating osmotic regulation under drought stress. EM plants displayed larger vessel cross-sectional areas and increased radial stern growth compared with non-EM plants. EM plants contained the lowest numbers of vessels (mm(-2)) but the largest vessel diameters, whereas non-EM drought-stressed plants in soil without hydrogel showed the smallest vessel diameters and highest vessel numbers. Vessel diameter and vessel numbers were strictly inversely correlated suggesting strong developmental coordination between these two parameters. The finding that neither vessel diameters nor vessel numbers were correlated with the pre-dawn water potential suggests that the coordination between these wood features may be achieved by signals acting independent of acute water stress. Overall, EM and hydrogel, especially in combination, help to avoid drought stress. Thereby, these soil amendments improve plant performance and may be useful for the establishment of plantations on marginal sites. (C) 2010 Elsevier B.V. All rights reserved.