- 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.