Abstract
Drought stress is one of the most important abiotic stressors of land plants. Consequently, there is a rich body of literature concerned with the impact of drought on plants and plant strategies for responding to drought. However, the impact of other organisms, especially rhizosphere biota, has only recently been taken into account when investigating the impact of drought stress on plants. This broadened research focus suggests that mycorrhizal symbionts may have an important unrecognized role in maintaining plant hydraulic function under drought stress. A key aspect to the role of mycorrhizal fungi in improved plant hydraulic function is the capacity of those fungi to form a common mycorrhizal network (CMN), an assemblage of mycorrhizal fungi that associate with multiple plant partners simultaneously. These networks are capable of transporting water from established nurse trees to establishing receiver seedlings, thus contributing to the functioning and recovery of fine roots, extramatrical hyphae, and the surrounding plant/rhizosphere community under drought conditions. It is currently unknown if water transported through a CMN is capable of directly reducing drought stress in plants connected to the network. Consequently, I attempted to examine the role of a CMN in ameliorating plant drought stress by testing the hypotheses that plants connected via a CMN will experience reduced effects of drought on transpiration rate, stomatal conductance, and net photosynthetic rate compared to plants that are not connected to the network. I attempted to test these hypotheses by comparing the transpiration rate, stomatal conductance, and net photosynthetic rate of Douglas-fir (Pseudotsuga menziesii) seedlings tenuously connected via a CMN to a nurse tree during an experimental dry down. Six experimental mesocosms were constructed, each of which contained four outer chambers to house receiver seedlings, and a central chamber to house a nurse tree. Layers of stainless-steel mesh were installed between each outer chamber and the central chamber to apply one of four water pathway treatments: 1) Mycorrhizal+Soil; 2) Mycorrhizal only; 3) Soil only; and 4) No pathway. A WALZ GFS-3000 gas exchange system was used to quantify all physiological measures from receiver Douglas-fir seedlings. The experiment was conducted in a greenhouse on the California State University, Sacramento campus. No significant difference in transpiration rate, stomatal conductance, and net photosynthetic rate of Douglas-fir receiver seedlings during dry down was observed within or among the four water treatment pathways. A significant decline in stomatal conductance and net photosynthetic rate was detected, but no significant difference was found in transpiration rate. Notably, there is strong circumstantial evidence that a CMN did not establish between trees. Consequently, the results of this experiment are inconclusive. Although these results may indicate that CMNs do not function to reduce drought-stress in plants, a small sample size, unforeseen effects of Douglas-fir anatomy, a relatively small age difference between nurse and receiver trees, and a malfunctioning greenhouse climate control system may have increased variability and masked detection of any treatment effects. Most notably, the inconclusive formation of a CMN precluded standard regard for results gathered during the experiment. These issues could be addressed with methods to confirm the establishment of a CMN, the use of mycorrhizal spores more likely to form a CMN, an increased sample size, the use of a different tree species, and modifications to experimental design.