ROLE FOR THE PHOTOSYNTHATE DEMAND OF ECTOMYCORRHIZAS IN THE RESPONSE OF DOUGLAS-FIR SEEDLINGS TO DRYING SOIL

Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings were inoculated with different species of ectomycorrhizal fungi, Rhizopogon vinicolor FSL788-5, Laccaria laccata S238-A, or Hebeloma crustuliniforme HeCr2, to determine how different fungi affect the response of photosynthesis and water relations of seedlings to drying soil. Potted seedlings were grown in a greenhouse for 6 months under well-watered conditions, then transferred to a growth chamber where measurements were made as the soil dried. Rhizopogon enhanced both net photosynthesis rate and stomatal conductance compared to non-mycorrhizal controls (P < 0.01) over the soil water potential range of -0.05 to -0.50 MPa, despite 0.2 to 0.3 MPa lower leaf water potential. Hebeloma tended to enhance, while Laccaria decreased net photosynthesis rate and stomatal conductance of host seedlings over this range of soil water potential, but neither fungus affected leaf potential. Our observations for Rhizopogon and Laccaria could not be explained by existing hypotheses based on mycorrhizal effects on plant size, nutrition, osmotic adjustment, or water uptake characteristics. Nutrition may have been a factor for Hebeloma. We propose that in the absence of nutritional and water uptake effects, net photosynthesis rate and stomatal conductance are correlated with rate of export of photosynthate to the mycorrhizal fungus. Strong mycorrhizal demand for photosynthate stimulates photosynthesis, to which stomata respond by opening, notwithstanding water stress. Our results for Rhizopogon are consistent with this hypothesis.