Mycorrhizal influence on hydraulic and hormonal factors implicated in the control of stomatal conductance during drought

During drying, mycorrhizal plants often maintain higher stomatal conductance (g(s)) than similarly-sized and -nourished non-mycorrhizal plants, but the mechanism of mycorrhizal influence remains unclear. Several hydraulic and non-hydraulic factors previously implicated in control of stomatal behavior during drought were measured, to learn which are affected when roots of cowpea (Vigna unguiculata [L.] Walp. cv. White Acre) are extensively colonized by Glomus intraradices Schenck and Smith isolate UT143. At low soil water contents (theta), mycorrhizal plants maintained higher g(s), transpiration and shoot water potential (psi) than non-mycorrhizal plants. These higher foliar water status characters were associated with lower xylem-sap abscisic acid concentrations ([ABA]) and lower ABA fluxes to leaves in mycorrhizal plants at low soil theta. Stomatal conductance was most closely correlated with xylem-sap [ABA], ABA flux to leaves and shoot psi. Stomatal conductance was not correlated with xylem-sap concentrations of calcium or zeatin riboside equivalents, or with xylem-sap pH, nor were these xylem-sap constituents affected by mycorrhizal symbiosis. Stomata of mycorrhizal and non-mycorrhizal leaves showed similar sensitivities to ABA, whether leaves were intact or detached. It is concluded that mycorrhizal fungi probably increased the capability of root systems to scavenge water in drier soil, resulting in less strain to foliage and hence higher g(s) and shoot psi at particular soil theta.