This study was conducted to determine whether xylem abscisic acid (ABA)
concentration is altered by mycorrhizal
symbiosis of cowpea plants grown in drying soil, and to determine whether
stomatal sensitivity to xylem ABA is
altered by the symbiosis. We allowed the entire root zone to dry and found that
at high soil water contents (θ),
mycorrhizal plants had higher stomatal conductance (gs) and
lower xylem [ABA] than did non-mycorrhizal plants,
but the difference disappeared at low θ, probably because of stomatal closure.
The altered gs and xylem [ABA] were
apparently not related to plant water status since shoot water potential,
xylem sap osmotic potential and shoot
water content were similar for mycorrhizal and non-mycorrhizal plants across
the range of soil moisture. These
differences were also not related to P nutrition or plant size. The relationship
of gs to xylem [ABA] was not affected
by the symbiosis, indicating that either stomatal sensitivity to xylem ABA was
not affected by other xylem
constituents, or that more than one xylem constituent was altered by the
symbiosis but was offsetting in its effect
on gs. We conclude that the symbiosis altered
gs non-hydraulically, and that the factor might
be xylem ABA.