Cytokinin dynamics in xylem sap and leaves of fruiting and de-fruited sunflower hybrids of contrasting post-anthesis canopy senescence patterns during flowering and grain-filling

To determine whether xylem sap and leaf cytokinin levels were associated with live root length density (LRLD) and leaf senescence, these variables were measured in intact (control) plants and those from which florets were removed, in hybrids of contrasting post-anthesis senescence patterns. Field experiments were carried out during two consecutive seasons using sunflower hybrids (Paraiso75, stay-green [SG]; and Paraiso65, fast dry down [FDD]) of contrasting canopy senescence patterns. Crop carbon partitioning was altered by removing all florets (-F treatment) before anthesis. At crop level, the dynamics of LRLD, xylem sap flow rate, cytokinins in the xylem sap, and green leaf area index (GLAI) were measured, while at leaf level, dynamics of total chlorophyll content, trans-Zeatin content, and net photosynthesis were recorded for leaf positions 17, 20, 22, 24 and 27. In both hybrids, the –F treatment had higher LRLD, cytokinins in the xylem sap, and leaf trans-Zeatin concentration at anthesis and during the grain filling period than control plants. All measured variables decreased earlier in the control treatment than the -F treatment (in both hybrids), and in the FDD hybrid than the SG hybrid. The dynamics of xylem sap cytokinins were related to the dynamics of LRLD and that of leaf trans-Zeatin levels, contributing to contrasting canopy senescence patterns and underlying the responses to defruiting in both hybrids.