The influence of ectomyorrhizae on drought tolerance characteristics of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) seedlings Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9g54xm03j

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  • Douglas-fir seedlings were inoculated with different species of ectomycorrhizae-forming fungi in order to test the concept that ectomycorrhizae enhance the drought tolerance of seedlings and to investigate the mechanisms responsible for this effect. Seedlings were transplanted at age 6 to 8 weeks into pots containing pasteurized loam soil and inoculated with either Rhizopogon vinicolor (Rv), Laccaria laccata (L1), or Hebeloma crustuliniforme (Hc), or left uninoculated. Rv and He colonization produced abundant hyphal growth, while Ll produced much less hyphae. After 4 months under well-watered greenhouse conditions, neither Rv- or L1- colonized seedlings had significantly different dry mass and leaf N, P, K, and Ca concentrations compared to nonmycorrhizal controls. Higher nutrient concentrations of Hc-colonized seedlings resulted from suppressed growth, since total amounts of these nutrients were equal to or less than for nonmycorrhizal controls. Seedlings were transferred to a growth room where photosynthesis, stomatal conductance, and plant water potential components were measured under well-watered and soil water-limiting conditions. Drought tolerance, as evaluated by net photosynthesis rate over the soil water potential range of -0.05 to -0.6 MPa, was clearly enhanced by Rv, somewhat enhanced by Hc, and decreased by Ll compared to nonmycorrhizal controls. Stomatal conductances closely followed net photosynthesis rates. Compared to control seedlings, leaf water potentials of mycorrhizal seedlings were lower (Rv by 0.2 to 0.3 MPa) or similar (L1 and Hc) over the entire range of soil water potential. Significantly reduced root lengths (Rv 65% of control; Hc 70% of control; Ll 90% of control) may have counteracted a mycorrhizal benefit of efficient water absorption. It is hypothesized that higher net photosynthesis rate and stomatal conductance despite lower leaf water potential, as observed for Rv-colonized seedlings, can arise from an ectomycorrhizae-altered carbon economy of the plants. According to this hypothesis, net photosynthesis rate and stomatal conductance are correlated with photosynthate sink demand, which here would be increased by export to the mycorrhizal fungus. Strong mycorrhizal demand, which occurs at some cost to plant growth, stimulates photosynthesis, to which the stomata respond by opening in spite of water stress. The degree to which this effect was observed in this study correlated with the visual abundance of hyphal growth which each fungal species developed.
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