Xylem sap abscisic acid concentration and stomatal conductance of mycorrhizal Vigna unguiculata in drying soil

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 (theta), mycorrhizal plants had higher stomatal conductance (g(s)) and lower xylem [ABA]than did non-mycorrhizal plants, but the difference disappeared at low theta, probably because of stomatal closure. The altered g(s) 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 g(s) 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 g(s). We conclude that the symbiosis altered g(s) non-hydraulically, and that the factor might be xylem ABA.