AMOUNTS OF ZEATIN AND ZEATIN RIBOSIDE IN XYLEM SAP OF SENESCENT AND NONSENESCENT SORGHUM

Xylem sap from decapitated vegetative and mature plants of nonsenescent 'Tx2817' and senescent 'Tx7000' sorghum [Sorghum bicolor (L.) Moench] cultivars was analyzed for cytokinins to determine whether the delayed leaf senescence of nonsenescent sorghums may be associated with transport of greater quantities of cytokinins from roots. Cytokinins were assayed by high-performance liquid chromatography (ultraviolet absorption) with C-14-labeled zeatin (Z) as an internal standard. For field-grown plants, the amount of Z + zeatin riboside (ZR) in xylem sap per gram shoot dry weight was 1.51 times higher for the nonsenescent sorghum. Vegetative plants were grown in solution culture for almost-equal-to 1 mo and then irradiance and N supply were altered for 7 or 8 d prior to collection of sap for 24 h. With high N and high light, Tx7000 and Tx2817 had nearly identical Z + ZR in the sap per gram shoot fresh weight. Treatments of 10% N supply or 50% light reduced growth only slightly. With 10% N, however, Z + ZR averaged 38% of the control for Tx7000 and 59% for Tx2817; with 50% light, Z + ZR averaged 55% of the control for Tx7000 and 136% for Tx2817. Thus, Z and ZR transport from roots to shoots of nonsenescent Tx2817 was less susceptible to reduction by low N and was actually increased by low light. High xylem sap cytokinin contents for Tx2817 with 50% light suggests that its delayed senescence and enhanced resistance to adverse conditions may be due to high cytokinin production by roots when the carbohydrate supply to the roots is reduced.