Abstract. Chloroplasts redistribute and/or reorientate in the cell as a response to the light direction, resulting in patterns typical for light of low or high fluence rate, respectively. Usually, the main photoreceptor pigment is a blue‐UV‐absorbing pigment (‘cryptochrome’), but in a few exceptional cases, the reversible red/far‐red system phytochrome is involved. Detection of light direction is based on light refraction and/or on dichroic orientation of photoreceptor molecules. Membrane effects, intracellular calcium redistribution and calcium‐calmodulin interaction are discussed as likely steps in signal transduction. In the response mechanism the actin‐myosin system is involved. However, several details of perception, transduction and response are still unsolved and open for discussion. Particularly interesting are the cases of multiple photoreceptor systems, i.e. those where separate transduction chains are started which coact or interact with each other. This raises the question as to the evolution of multiple photoreceptor systems under the assumption that light‐oriented chloroplast movements serve to optimize photosynthesis.

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SummaryWe have analysed the centromere 1 (CEN1) of Arabidopsis thaliana by integration of genetic, sequence and¯uorescence in situ hybridisation (FISH) data. CEN1 is considered to include the centromeric core and the¯anking left and right pericentromeric regions, which are distinct parts by structural and/or functional properties. CEN1 pericentromeres are composed of different dispersed repetitive elements, sometimes interrupted by functional genes. In contrast the CEN1 core is more uniformly structured harbouring only two different repeats.The presented analysis reveals aspects concerning distribution and effects of the uniformly shaped heterochromatin, which covers all CEN1 regions. A lethal mutation tightly linked to CEN1 enabled us to measure recombination frequencies within the heterochromatin in detail. In the left pericentromere, the change from eu-to heterochromatin is accompanied by a gradual change in sequence composition but by an extreme change in recombination frequency (from normal to 53-fold decrease) which takes place within a small region spanning 15 kb. Generally, heterochromatin is known to suppress recombination. However, the same analysis reveals that left and right pericentromere, though similar in sequence composition, differ markedly in suppression (53-fold versus 10-fold). The centromeric core exhibits at least 200-fold if not complete suppression. We discuss whether differences in (®ne) composition re¯ect quantitative and qualitative differences in binding sites for heterochromatin proteins and in turn render different functional properties. Based on the presented data we estimate the sizes of Arabidopsis centromeres. These are typical for regional centromeres of higher eukaryotes and range from 4.4 Mb (CEN1) to 3.55 Mb (CEN4).

Climate mitigation and climate adaptation are crucial tasks for urban areas and can involve synergies as well as trade-offs. However, few studies have examined how mitigation and adaptation efforts relate to each other in a large number of differently sized cities, and therefore we know little about whether forerunners in mitigation are also leading in adaptation or if cities tend to focus on just one policy field. This article develops an internationally applicable approach to rank cities on climate policy that incorporates multiple indicators related to (1) local commitments on mitigation and adaptation, (2) urban mitigation and adaptation plans and (3) climate adaptation and mitigation ambitions. We apply this method to rank 104 differently sized German cities and identify six clusters: climate policy leaders, climate adaptation leaders, climate mitigation leaders, climate policy followers, climate policy latecomers and climate policy laggards. The article seeks explanations for particular cities’ positions and shows that coping with climate change in a balanced way on a high level depends on structural factors, in particular city size, the pathways of local climate policies since the 1990s and funding programmes for both climate mitigation and adaptation.