Experiments were conducted to test the relative ability of mycorrhizal and non-mycorrhizal Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings to tolerate and recover from drought conditions, using reduction in CO2 fixation as an overall indicator of plant moisture stress. Seedlings were watered daily or conditioned to cyclic drying and re-wetting of the soil. Net photosynthetic rates of mycorrhizal and non-mycorrhizal seedlings watered daily did not differ significantly; however, drought-stressed mycorrhizal seedlings fixed CO20 at a rate ten times that of non-mycorrhizal seedlings. Total leaf water potentials of mycorrhizal plants were lower (more negative) than those of non-mycorrhizal plants but they recovered more rapidly.
Non-mycorrhizal seedlings and seedlings inoculated with four ectomycorrhizal fungus species were allowed to become desiccated, then were rewatered and compared for their ability to tolerate and recover from drought. Seedlings inoculated with Rhizapogon vintcolor were less affected by drought than any of the other mycorrhizal or non-mycorrhizal treatments. Net photosynthetic rate of Rhizopogon-inoculated seedlings 24 h following re-watering was seven times that of non-mycorrhizal seedlings. The transpiration rate of Rhizopogon-inoculated seedlings was low before desiccation, declined rapidly during the drought period and, after re-watering, quickly resumed a rate higher than that for other treatments.