The generation of nonphotochemical quenching of chlorophyll fluorescence (qN) in the antenna of photosystem II (PSII) is accompanied by the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin. The function of zeaxanthin in two mechanisms of qN, energy-dependent quenching (qE) and photoinhibitory quenching (qI), was investigated by measuring the de-epoxidation state in the antenna subcomplexes of PSII during the generation and relaxation of qN under varying conditions. Three different antenna subcomplexes were separated by isoelectric focusing: Lhcb1/2/3, Lhcb5/6, and the Lhcb4/PSII core. Under all conditions, the highest de-epoxidation state was detected in Lhcb1/2/3 and Lhcb5/6. The kinetics of de-epoxidation in these complexes were found to be similar to the formation of qE. The Lhcb4/PSII core showed the most pronounced differences in the de-epoxidation state when illumination with low and high light intensities was compared, correlating roughly with the differences in qI. Furthermore, the epoxidation kinetics in the Lhcb4/PSII core showed the most pronounced differences of all subcomplexes when comparing the epoxidation after either moderate or very strong photoinhibitory preillumination. Our data support the suggestion that zeaxanthin formation/epoxidation in Lhcb1-3 and Lhcb5/6 may be related to qE, and in Lhcb4 (and/or PSII core) to qI.

The development of the photosynthetic apparatus of intermittent light grown pea plants under continuous illumination has been investigated. We determined the formation of antenna proteins and the synthesis of pigments at different stages of greening and compared the data with the changes in the xanthophyll cycle reactions. The limited convertibility of violaxanthin in the de-epoxidation reactions of the cycle was found to be closely related to the presence of antenna proteins and could be attributed to direct (pigment binding) and indirect (grana formation) functions of antenna proteins. The reduced epoxidation rate in intermittent light plants was found to be accelerated with increasing amounts of antenna proteins. However, the changes in the epoxidation rates were not consistent with the assignment of the epoxidase activity to LHC II, the major light harvesting complex protein of photosystem II. This interpretation was further supported by an unchanged epoxidase activity in - also LHC II depleted - bundle sheath cells of the C4 plant Sorghum bicolor and stroma fractions of isolated spinach thylakoids. We assume that the basic function of antenna proteins in the xanthophyll cycle of higher plants is mainly related to the binding of the substrate and/or to interactions with the de-epoxidase/epoxidase. By that antenna proteins seem to be responsible for the limited violaxanthin convertibility as well as they are required for highest epoxidation rates. Copyright 1998 Elsevier Science B.V.

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Dass die Prinzipien wissenschaftlichen Erkennens a priori sein, sie also aus reiner Vernunft entspringen müssen, diese Überzeugung ist für Kant ebenso selbstverständlich wie für die Rationalisten. Wie aber diese Prinzipien selbst begründet werden können, darüber bestehen höchst unterschiedliche Auffassungen. Descartes und Spinoza beabsichtigen, das mathematische Erkenntnisideal der Evidenz zu universalisieren und in einer Substanzlehre metaphysisch zu fundieren. Kant dagegen liefert nicht nur eine scharfe Kritik der dogmatischen Substanzmetaphysik, sondern setzt sich auch eingehend mit dem rationalistischen Programm einer Mathesis universalis auseinander: Ihm zufolge muss der intuitive (mathematisch-anschauend verfahrende) Vernunftgebrauch aus der Philosophie gänzlich ausgeschlossen werden, so dass in der philosophischen Prinzipienlehre allein diskursiv verfahren werden kann. Dann aber verbleibt einzig das transzendentalphilosophische Verfahren, um dennoch zu einer apriorischen Grundlegung der (Natur-)Wissenschaften zu gelangen. Die Arbeit bietet eine detaillierte Analyse der beiden Begründungsversuche. Besonderes Gewicht liegt dabei auf dem Zusammenhang der methodischen Grundlegung mit der jeweiligen Metaphysik: Ohne Rekurs auf die letztere muss das Verständnis der ersteren mangelhaft bleiben.

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