Mycorrhizal symbiosis and nonhydraulic root signals of soil drying in cowpea plants

The objective of this work was to test the hypothesis that VA mycorrhizal fungi modify nonhydraulic root to shoot communication of soil drying, thereby altering stomatal response to declining soil water status.

Cowpea plants (Vigna unguiculata [L.] Walp. 'California Blackeye') were grown with roots divided between two pots. One pot of each plant was inoculated with a vesicular-arbuscular (VA) mycorrhizal fungus (Glomus intraradix Schenck & Smith), the other pot remained nonmycorrhizal. Plants were grown under natural light in a greenhouse and under well-watered conditions.

Two experiments were performed. In each, split-root cowpea plants were divided into the following three treatments: (1) control plants with both mycorrhizal and nonmycorrhizal roots well-watered (MwNw), (2) mycorrhizal roots well-watered and nonmycorrhizal roots allowed to dry (MwNd), (3) nonmycorrhizal roots well-watered and mycorrhizal roots allowed to dry (MdNw). In experiment two, half root systems of four control plants were severed to determine if the remaining half root systems were able to maintain similar stomatal conductance (Cs) as MwNw plants.

Two to three days after withholding water from one pot, Cs of half-dried plants was significantly lower than that of control plants. In experiment one, Cs of half-dried plants eventually dropped to 30% or below that of control plants, after 4 days of soil drying. In experiment two, Cs of half-dried plants dropped to about 40% of control plants, after one week of soil drying. Differences in leaf water potential or leaf relative water content were not observed among treatments in experiment two. Severing half of the root system from the control plants showed that within five days Cs was similar in plants with severed roots and non-severed roots.

These results indicate that declines in Cs were probably caused by a nonhydraulic rather than a hydraulic root-sourced signal, at least during the first five days of soil drying. Cs in plants whose mycorrhizal roots were dried tended to be lower than that of plants whose nonmycorrhizal roots were dried. This suggests that mycorrhizal fungi altered the nonhydraulic root signaling process.