http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Studies on the cytokinin physiology of senescent and nonsenescent grain sorghum cultivars
Nonsenescent sorghums have characteristics that resemble cytokinin responses; therefore, their cytokinin physiology was compared with that of senescent sorghums. First, effects of kinetin on chlorophyll retention of disks from mature leaves placed in the dark for 5 days were studied. Two nonsenescent sorghums, 'Tx2817' (tropically-adapted) and 'SC599-6' (late-season drought resistant), had maximal retention at lower concentrations than did senescent 'RTx7000' and 'BTx378' and in some cases had greater responses to 10^-8 M kinetin and to 10^-5 M gibberellic acid. SC599-6 had high initial chlorophyll content and low loss of chlorophyll without kinetin; RTx7000 and Tx2817 showed characteristics opposite of this, and BTx378 was intermediate. Second, xylem sap from Tx2817 and RTx7000 was analyzed for cytokinins using high-performance liquid chromatography (HPLC). Bioassay studies on HPLC-fractionated sap from vegetative plants indicated that zeatin (Z), zeatin riboside (ZR), and a substance eluting similar to ZR 5'-monophosphate were probably responsible for most of the eell-division type of cytokinin activity. HPLC analyses showed that zeatin riboside, zeatin, and isopentenyl adenosine were present in sap from mature plants. The amount of Z + ZR was 51% higher for Tx2817 than that of RTx7000 on a shoot dry weight basis but only 17% higher on green leaf dry weight basis. For vegetative plants grown with solution culture, Z + ZR amounts in sap per g shoot fresh weight were consistent between duplicates. When RTx7000 and Tx2817 were grown with high N and high light, they had nearly the same Z + ZR amounts. However, they differed greatly with moderately suboptimal treatments of 10% N and 50% light for 7 or 8 days. Suboptimal N caused Z + ZR to be 30% and 59% of the control for RTx7000 and Tx28I7, respectively. Suboptimal light decreased Z + ZR for RTx7000 to 55% of the control but increased that of Tx2817 to 136%. The results with suboptimal light may indicate that Tx2817 has high cytokinin production under carbohydrate shortage, a phenomenon which could delay senescence and enhance resistance to adverse conditions. The results are consistent with the hypothesis that nonsenescence in sorghum is associated with enhanced cytokinin levels.
Includes bibliographical references (leaves 159-186).