Does engineering abscisic acid biosynthesis in Nicotiana plumbaginifolia modify stomatal response to drought?

The consequences of manipulating abscisic acid (ABA) biosynthesis rates on stomatal response to drought were analysed in wild-type, a full-deficient m utant and four under-producing transgenic lines of N. plumbaginifolia. The roles of ABA, xylem sap pH and leaf water potential were investigated under four experimental conditions: feeding detached leaves with varying ABA con centration; injecting exogenous ABA into well-watered plants; and withholdi ng irrigation on pot-grown plants, either intact or grafted onto tobacco. C hanges in ABA synthesis abilities. among lines did not affect stomatal sens itivity to ABA concentration in the leaf xylem sap ([ABA](xyl)), as evidenc ed with exogenous ABA supplies and natural increases of [ABA](xyl) in graft ed plants subjected to drought. The ABA-deficient mutant, which is uncultiv able under normal evaporative demand, was grafted onto tobacco stock and th en presented the same stomatal response to [ABA](xyl) as wild-type and othe r lines. This reinforces the dominant role of ABA in controlling stomatal r esponse to drought in N. plumbaginifolia whereas roles of leaf water potent ial and xylem sap pH were excluded under all studied conditions. However, w hen plants were submitted to soil drying onto their own roots, stomatal res ponse to [ABA](xyl) slightly differed among lines. It is suggested, consist ently with all the results, that an additional root signal of soil drying m odulates stomatal response to [ABA](xyl).