Abstract
The effect of NaCl on growth, biomass and ion relations of two salt-tolerant isolates of Paxillus involutus, MAJ and NAU were investigated. The two Paxillus strains were exposed to the following concentrations of NaCl: 0, 100, 200 and 500 mmol·L−1. Growth of MAJ and NAU was enhanced by 100 mmol·L−1 NaCl but severely inhibited at the concentration of 500 mmol·L−1. NAU exhibited a greater capacity to exclude Na+ and Cl− under all salinity levels, whereas the salt-includer MAJ had a higher capacity in nutrient uptake under salt stress. The ratios Na+/K+, Na+/Ca2+ and Na+/Mg2+ in NaCl-treated MAJ and NAU did not increase at levels of 100 and 200 mmol·L−1 NaCl but markedly increased at 500 mmol·L−1. This suggests that the two strains, especially MAJ, enhanced nutrient uptake corresponding to the increased Na+ influx at moderate salinity. We conclude that both MAJ and NAU are able to tolerate 500 mmol·L−1 NaCl but there are species-specific differences in retaining ionic homeostasis in the two Paxillus strains. NAU is a salt-excluder, MAJ is a salt-includer but retains a high capacity in nutrient selectivity under saline conditions. Their definite mechanisms to enhance salt tolerance of mycorrhizal hosts need further study.
Highlights
Most plants in temperate forest form mutualistic arbor associations with ectomycorrhiza (EcM) fungi
Non-salt control strains of the two P. involutus exhibited a vigorous growth after transformation onto new medium and maintained at a higher growth rate until day 18 (Fig. 1)
Growth rates of the two strains were reduced by 200 mmol·L–1 NaCl after 12 days of salt treatment, in which stressed MAJ returned to control levels and salinized NAU were lower than controls (Fig. 1)
Summary
Most plants in temperate forest form mutualistic arbor associations with ectomycorrhiza (EcM) fungi. It is estimated that at least 30% of the microbial biomass in boreal forest soils is the extraradical hyphae of EcM fungi (Högberg and Högberg, 2002). The positive effects of EcM fungi on plant production and in terrestrial ecosystem maintenance have been widely recognized. EcM are the dominant nutrient-gathering organ in temperate ecosystems and these structures provide host with nutrition, especially nitrogen and phosphorus (Pella, 1990; Bücking et al, 2002; Tibbett and Sanders, 2002). It is suggested that over 50% of CO2 released from boreal forest soils is accounted for by the respiration of tree roots and their associated EcM fungi (Högberg et al, 2001). 20%–30% of current assimilate from seedling hosts is allocated to EcM fungi (Söderström, 2002)
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