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.

A new split-root system was established through grafting to study cotton response to non-uniform salinity. Each root half was treated with either uniform (100/100 mM) or non-uniform NaCl concentrations (0/200 and 50/150 mM). In contrast to uniform control, non-uniform salinity treatment improved plant growth and water use, with more water absorbed from the non- and low salinity side. Non-uniform treatments decreased Na⁺ concentrations in leaves. The [Na⁺] in the ‘0’ side roots of the 0/200 treatment was significantly higher than that in either side of the 0/0 control, but greatly decreased when the ‘0’ side phloem was girdled, suggesting that the increased [Na⁺] in the ‘0’ side roots was possibly due to transportation of foliar Na⁺ to roots through phloem. Plants under non-uniform salinity extruded more Na⁺ from the root than those under uniform salinity. Root Na⁺ efflux in the low salinity side was greatly enhanced by the higher salinity side. NaCl-induced Na⁺ efflux and H⁺ influx were inhibited by amiloride and sodium orthovanadate, suggesting that root Na⁺ extrusion was probably due to active Na⁺/H⁺ antiport across the plasma membrane. Improved plant growth under non-uniform salinity was thus attributed to increased water use, reduced leaf Na⁺ concentration, transport of excessive foliar Na⁺ to the low salinity side, and enhanced Na⁺ efflux from the low salinity root.