ABA synthesized in the roots can also be transported to the shoot via the xylem. The concentration of ABA in the xylem sap of well-watered sunflower plants is between 1.0 and 15.0 nM, the ABA concentration in water stressed sunflower plants increases to as much as 3000 Nm (3.0 μM)
#Unit 6. System Physiology – Plant#Abscisic Acid - A Seed Maturation and Antistress Signal#Part B Pointers