XVI International Botanical Congess
When ABA was fed into leaves of sunflower through xylem flux, the dynamic ABA accumulation in the leaves was computed taking into consideration the variation of ABA flux, water flux and ABA metabolism. Results show a poor link between the kinetics of ABA-induced stomatal closure and the xylem-derived ABA accumulation. When tetcyclacis was added into the feeding solution, less than 30% of ABA was metabolised after 24 h of incubation, in comparison of 70% degradation within 2 h in control leaves. However, the inhibition of ABA metabolism by tetcyclacis did not interfere with ABA-induced stomatal closure, which was still a concentration-dependent function. Results strongly indicate that stomata mainly respond to the prevailing ABA concentration in the xylem stream, rather than to the accumulated amount of xylem-derived ABA in the leaves.