Root-supplied ABA affects water use efficiency of ABA-deficient tomato scions

To evaluate whether root-supplied ABA affects tomato leaf and whole-plant water use efficiency (WUE) and tissue ABA status, wild-type (WT) and ABA-deficient flacca (flc) genotypes were self- and reciprocally-grafted. Root xylem ABA concentration was rootstock-dependent irrespective of sap flow rate, thus ABA delivery to the scion of reciprocal grafts was substantially altered compared to self-grafted plants. However, a flc rootstock had minimal effects on WT leaf and whole plant photosynthesis or stomatal and canopy conductance, as it didn't affect leaf ABA concentration (ABAleaf) and water potential (Ψleaf) of WT scions. A WT rootstock had variable effects on a flc scion. At ambient vapour pressure deficits (VPD) exceeding 1.5 kPa (but not at lower VPDs) or with deficit irrigation (supplying 30% less water), it increased WUE of a flc scion by diminishing stomatal and canopy conductance while increasing ABAleaf and Ψleaf. Across all graft combinations, WUE and ABAleaf were positively correlated independent of variation in Ψleaf or soil moisture. While WT scions regulated their WUE and ABA status independently of the root system, acropetal root-to shoot ABA signalling regulated flc scion ABA status (and thus WUE) under high evaporative demand or soil drying, independently of changing Ψleaf.