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 contribute to ABA synthesis, and whether a previous cycle of soil dehydration-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 primary root tips, from both well watered plants and previously drought-rewatered plants, were subjected to different degrees of dehydration and their ABA concentration changes were assayed. All categories of roots from always well watered plants, including mature tissues containing no apex, could synthesize ABA when dehydrated. Mature primary root sections and their previously associated secondary roots accumulated less ABA in response to dehydration than the young primary roots and primary root tips did, and their ABA accumulation was not substantial until dehydration was below 65% of relative water content (RWC). Previous soil dehydration-rehydration cycles substantially reduced ABA accumulation in these roots in response to dehydration again. Young primary root sections and primary root tips accumulated ABA much more sensitively in response to dehydration than mature root sections, although considerable variations existed among different batches of young primary root sections. Results are discussed in the context of the relative contribution 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.