Effects of cotton rootstock on endogenous cytokinins and abscisic acid in xylem sap and leaves in relation to leaf senescence

Leaf senescence varies greatly among cotton cultivars, possibly due to their root characteristics, particularly the root-sourced cytokinins and abscisic acid (ABA). Early-senescence (K1) and late-senescence (K2) lines, were reciprocally or self-grafted to examine the effects of rootstock on leaf senescence and endogenous hormones in both leaves and xylem sap. The results indicate that the graft of K1 scion onto K2 rootstock (K1/K2) alleviated leaf senescence with enhanced photosynthetic (Pn) rate, increased levels of chlorophyll (Chl) and total soluble protein (TSP), concurrently with reduced malondialdehyde (MDA) contents in the fourth leaf on the main-stem. The graft of K2 scion onto K1 rootstock enhanced leaf senescence with reduced Pn, Chl, and TSP, and increased MDA, compared with their respective self-grafted control plants (K1/K1 and K2/K2). Reciprocally grafted plants differed significantly from their self-grafted control plants in levels of zeatin and its riboside (Z+ZR), isopentenyl and its adenine (iP+iPA), and ABA, but not in those of dihydrozeatin and its riboside (DHZ+DHZR) in leaves in late season, which was consistent with variations in leaf senescence between reciprocally and self-grafted plants. The results suggest that leaf senescence is closely associated with reduced accumulation of Z+ZR, and iP+iPA rather than DHZ+DHZR, or enhanced ABA in leaves of cotton. Genotypic variation in leaf senescence may result from the difference in root characteristics, particularly in Z+ZR, iP+iPA, and ABA which are regulated by the root system directly or indirectly.