Root produced DHZR-, ZR- and IPA-like cytokinins in xylem sap in relation to coppice shoot initiation and growth in cloned trees of Betula pubescens

Six-year-old cloned Betula pubescens Ehrh. trees, grown outdoors at 65 degrees 01' N, were cut on six dates during the growing season to study coppice shoot development in relation to root-produced cytokinin-like compounds. Bleeding sap was collected over timed intervals for two days after cutting, and endogenous cytokinin-like compounds were measured by ELISA assay in HPLC-purified fractions of xylem sap. Initiation and development of coppice shoots on the clonally propagated plants were comparable to those in seedlings. Coppice shoot initiation was affected by the time of cutting, diminishing significantly after June. Of the cytokinin-like compounds detected in the xylem sap, zeatin riboside-like (ZR) compounds were present in the highest concentrations, and the concentrations of dihydrozeatin riboside-like (DHZR) and isopentenyladenoside-like (IPA) compounds were approximately one third and one eighth of the ZR concentrations, respectively. The concentration of cytokinin-like compounds was positively correlated with xylem sap flow rate. The export of cytokinin-like compounds, especially DHZR- and ZR-types, was positively correlated with the initiation and elongation rate of coppice shoots, the number of lateral branches, and the radial growth of the more slowly growing coppice shoots. The export of cytokinin-like compounds collected immediately after cutting may represent the basal value for each tree. This value is probably affected by the size and activity of the root system and may be a relevant estimate for predicting the success of coppicing.     

Topographic forcing of the atmosphere and a rapid change in the length of day

During June to September 1992, a special campaign was held to measure rapid changes in Earth's rotation rate and to relate these measurements to variations in the atmosphere's angular momentum, due principally to changes in zonal winds. A strong rise in both length of day and atmospheric momentum during a particular 6-day subperiod is documented, and this example of a short-period perturbation is identified with a specific regional coupling mechanism. Mountain torques within the southern tropics appear to account for most of the rapid momentum transfer between the solid Earth and atmosphere, with those across South America especially important.