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

During drying, mycorrhizal plants often maintain higher stomatalconductance (g_s) than similarly-sized and -nourished non-mycorrhizalplants, but the mechanism of mycorrhizal influence remains unclear.Several hydraulic and non-hydraulic factors previously implicatedin 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 Glomusintraradices Schenck and Smith isolate UT143. At low soil watercontents (), mycorrhizal plants maintained higher g_s, transpirationand shoot water potential () than non-mycorrhizal plants. Thesehigher foliar water status characters were associated with lowerxylemsap abscisic acid concentrations ([ABA]) and lower ABAfluxes to leaves in mycorrhizal plants at low soil . Stomatalconductance was most closely correlated with xylem-sap [ABA],ABA flux to leaves and shoot . Stomatal conductance was notcorrelated with xylemsap concentrations of calcium or zeatinriboside equivalents, or with xylem-sap pH, nor were these xylem-sapconstituents affected by mycorrhizal symbiosis. Stomata of mycorrhizaland non-mycorhizal leaves showed similar sensitivities to ABA,whether leaves were intact or detached. It is concluded thatmycorrhizal fungi probably increased the capability of rootsystems to scavenge water in drier soil, resulting in less strainto foliage and hence higher g_s, and shoot at particular soil. Key words: Abscisic acid, cytokinins, Glomus intraradices