MYCORRHIZAL INFLUENCE ON HYDRAULIC AND HORMONAL FACTORS IMPLICATED INTHE CONTROL OF STOMATAL CONDUCTANCE DURING DROUGHT

During drying, mycorrhizal plants often maintain higher stomatal condu ctance (g(s)) than similarly-sized and -nourished non-mycorrhizal plan ts, but the mechanism of mycorrhizal influence remains unclear. Severa l 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 Sm ith isolate UT143. At low soil water contents (theta), mycorrhizal pla nts maintained higher g(s), transpiration and shoot water potential (p si) than non-mycorrhizal plants. These higher foliar water status char acters were associated with lower xylem-sap abscisic acid concentratio ns ([ABA]) and lower ABA fluxes to leaves in mycorrhizal plants at low soil theta. Stomatal conductance was most closely correlated with xyl em-sap [ABA], ABA flux to leaves and shoot psi. Stomatal conductance w as not correlated with xylem-sap concentrations of calcium or zeatin r iboside equivalents, or with xylem-sap pH, nor were these xylem-sap co nstituents affected by mycorrhizal symbiosis. Stomata of mycorrhizal a nd non-mycorrhizal leaves showed similar sensitivities to ABA, whether leaves were intact or detached. It is concluded that mycorrhizal fung i 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.