Differential xylem phytohormone export from dry and wet roots during partial rootzone drying is independent of shoot‐to‐root transport in soybean

Different phytohormones can act as root-to-shoot signalling molecules in response to soil drying. Recent findings suggest that root ABA levels are predominantly leaf-sourced and not locally synthesized, thus, ABA exported from the roots in the xylem is mostly recycled from the shoot. To explain the differential root hormone accumulation observed under partial rootzone drying (PRD) that imposes distinct dry and wet parts of the root zone, we grafted "two-root, one-shoot" soybean plants to independently assess xylem export of different phytohormones from either part of the root zone. Grafts were subjected to a combination of girdling (either part, all, or none of the rootzone) and irrigation (homogenously well-watered (WW) and PRD). PRD did not increase foliar ABA but decreased stomatal conductance, attributed to decreased leaf water potential and/or increased xylem sap ABA, JA, or ACC concentrations. In contrast, the foliar ABA increments that accompanied girdling-induced stomatal closure were proportional to the root fraction to which phloem transport was interrupted. Irrespective of girdling, root ABA accumulation (and xylem ABA export from) was highest in the dry PRD rootzone, xylem jasmonic acid (JA) in the wet PRD rootzone, and xylem ACC in both rootzones of PRD plants. Thus, soil drying of the dry root zone and transient overwatering of the wet root zone enhanced ACC export in PRD plants. We conclude that root water status during PRD enhances root ABA, JA and ACC synthesis and xylem export, independent of shoot-to-root transport.