ACCUMULATION OF ABA IN MAIZE ROOTS IN RESPONSE TO ROOT SEVERING

When isolated maize (Zea mays L.) roots were incubated for 3 h in a fully hydrated state in air, ABA content was found to increase by around 100% in all the different root tissues examined - i.e. mature primary root sections, young and growing primary root sections, secondary roots and primary root tips - from both well watered plants and previously droughted plants. This ABA increase was not turgor-related, as usually is the case in droughted plants, and was not quantitatively related to the injury on the cutting surface. A time sequence of ABA accumulation showed a peak accumulation at about 2-3 h after initial incubation. Oxygen was essential for this increase in ABA accumulation because when roots were incubated in pure nitrogen, no such increase was found. Evidence suggests that this increase was derived from xanthophylls, the common precursors for stress-induced ABA. Maize leaves did not show such an accumulation when incubated under the same conditions. When half the root system was severed, ABA content in the remaining half root system increased by 100-200% in 3 h. We suggest that there is a systemic effect on the ABA biosynthesis in the root system following the shock of root severing, and that roots may supply substantial amounts of ABA to the shoots even at time when there is no apparent dehydration.