RELATIVE CONTRIBUTION OF APICES AND MATURE TISSUES TO ABA SYNTHESIS IN DROUGHTED MAIZE ROOT SYSTEMS

We investigated whether different parts of maize root systems could co ntribute to ABA synthesis, and whether a previous cycle of soil dehydr ation-rehydration would modify the ability of roots to synthesize ABA. Maize (Zea mays L.) root tissues, i.e. mature primary root sections, young and unbranched primary root sections, secondary roots and primar y root tips, from both well watered plants and previously drought-rewa tered plants, were subjected to different degrees of dehydration and t heir ABA concentration changes were assayed. All categories of roots f rom always well watered plants, including mature tissues containing no apex, could synthesize ABA when dehydrated. Mature primary root secti ons and their previously associated secondary roots accumulated less A BA in response to dehydration than the young primary roots and primary root tips did, and their ABA accumulation was not substantial until d ehydration was below 65% of relative water content (RWC). Previous soi l dehydration-rehydration cycles substantially reduced ABA accumulatio n in these roots in response to dehydration again. Young primary root sections and primary root tips accumulated ABA much more sensitively i n response to dehydration than mature root sections, although consider able variations existed among different batches of young primary root sections. Results are discussed in the context of the relative contrib ution of different categories of roots to ABA synthesis when the root system is in drying soil. We concluded that primary root apices should not contribute by more than 2% to the total ABA synthesis by the root system.