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Notes: Abstract Isoprene emission from plants represents one of the principal biospheric controls over the oxidative capacity of the continental troposphere. In the study reported here, the seasonal pattern of isoprene emission, and its underlying determinants, were studied for aspen trees growing in the Rocky Mountains of Colorado. The springtime onset of isoprene emission was delayed for up to 4 weeks following leaf emergence, despite the presence of positive net photosynthesis rates. Maximum isoprene emission rates were reached approximately 6 weeks following leaf emergence. During this initial developmental phase, isoprene emission rates were negatively correlated with leaf nitrogen concentrations. During the autumnal decline in isoprene emission, rates were positively correlated with leaf nitrogen concentration. Given past studies that demonstrate a correlation between leaf nitrogen concentration and isoprene emission rate, we conclude that factors other than the amount of leaf nitrogen determine the early-season initiation of isoprene emission. The late-season decline in isoprene emission rate is interpreted as due to the autumnal breakdown of metabolic machinery and loss of leaf nitrogen. In potted aspen trees, leaves that emerged in February and developed under cool, springtime temperatures did not emit isoprene until 23 days after leaf emergence. Leaves that emrged in July and developed in hot, midsummer temperatures emitted isoprene within 6 days. Leaves that had emerged during the cool spring, and had grown for several weeks without emitting isoprene, could be induced to emit isoprene within 2 h of exposure to 32°C. Continued exposure to warm temperatures resulted in a progressive increase in the isoprene emission rate. Thus, temperature appears to be an important determinant of the early season induction of isoprene emission. The seasonal pattern of isoprene emission was examined in trees growing along an elevational gradient in the Colorado Front Range (1829–2896 m). Trees at different elevations exhibited staggered patterns of bud-break and initiation of photosynthesis and isoprene emission in concert with the staggered onset of warm, springtime temperatures. The springtime induction of isoprene emission could be predicted at each of the three sites as the time after bud break required for cumulative temperatures above 0°C to reach approximately 400 degree days. Seasonal temperature acclimation of isoprene emission rate and photosynthesis rate was not observed. The temperature dependence of isoprene emission rate between 20 and 35°C could be accurately predicted during spring and summer using a single algorithm that describes the Arrhenius relationship of enzyme activity. From these results, it is concluded that the early season pattern of isoprene emission is controlled by prevailing temperature and its interaction with developmental processes. The late-season pattern is determined by controls over leaf nitrogen concentration, especially the depletion of leaf nitrogen during senescence. Following early-season induction, isoprene emission rates correlate with photosynthesis rates. During the season there is little acclimation to temperature, so that seasonal modeling simplifies to a single temperature-response algorithm.

Notes: 1. Um ein etwa vorhandenes stabiles Poloniumisotop aufzufinden, wurden größere Mengen der Tellurminerale: Hessit, Calaverit, Nagyagit und der Wismutminerale: Wismuttellurid, Wismutglanz und metallisches Wismut aufgearbeitet unter Verwendung von Polonium als Indikator. Das Polonium wurde der Lösung der Minerale zugesetzt und am Ende der Aufarbeitung wieder abgeschieden. Ein etwa vorhandenes, stabiles Poloniumisotop hätte sich gemeinsam mit dem Polonium ausgeschieden. Es zeigte sich jedoch, daß der das gesamte Polonium enthaltende Niederschlag nur etwa 1/10 mg Gewicht hatte und wie eine röntgenspektroskopische Untersuchung ergab, auch dieser Niederschlag weniger als 1% des gesuchten Elementes enthielt. Daraus folgt, daß die untersuchten Tellur- und Wismutminerale, in denen ein etwaiges stabiles Poloniumisotop hätte vorhanden sein müssen, kein solches Element enthalten, d. h. pro Gramm Mineral ist weniger als 10-11 g des gesuchten Elementes vorhanden.2. Es ist eine Methode ausgearbeitet worden, um Polonium chemisch und elektrolytisch von großen Mengen von Fremdsubstanzen, insbesondere Tellur und Wismut zu trennen.3. Eine systematische Untersuchung der elektrolytischen Abscheidung an verschiedenen Metallen ergab die günstigsten Ergebnisse bei Verwendung von Molybdän als Elektrodenmaterial.

Notes: Es wurden homologe Linienpaare für die Systeme: Li, Cu, Zn: Au, Tl und Bi in Pb sowie Ag in Sn mitgeteilt, die sich our quantitativen optischen Bestimmung im Bereich von etwa 10 - 0,0001% als günstig bewährt haben.