Effects of root restriction on growth and associated cytokinin levels in cotton (Gossypium hirsutum)

Root restriction experiments on cotton plants were conducted under conditions where water and nitrogen supplies were not limiting. Xylem sap and leaf tissues were obtained before significant biomass reduction as a result of root restriction. Xylem-derived cytokinins (CTKs) entering a leaf and the CTK levels in the same excised leaf were measured, thus, enabling xylem sap and foliar CTKs to be compared concurrently under root-restrictive and control conditions. Abscisic acid (ABA) was also measured simultaneously in the same xylem sap samples. Root restriction reduced photosynthesis and ¹⁸O of leaf matter showed that leaves of root-restricted plants generally had lower stomatal conductances. Root-restriction significantly decreased (37%) the delivery rate per unit leaf area of CTK bases (dihydrozeatin (DZ) and N ⁶-(2-isopentenyl)adenine (iP)) to the leaf but did not alter the export of CTK O-glucosides and ABA from the roots to leaf 4. Root-restriction also did not alter significantly the levels of total CTK bases in the leaves but increased (54-57% higher) the CTK O-glucosides (especially zeatin O-glucosides). Root restriction also slowed down the apparent CTK turnover rates in the leaves. The slow down in CTK turnover rate can be interpreted as a reduction in leaf CTK metabolism during root restriction.