The detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history. Objective evidence for the existence of such structures can only be provided by geophysics and boreholes. The combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures. Their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density, and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character. Seismic measurements provide the most detailed information about the internal structure of the maar sediments. Zones of low seismic reflectivity and very low density represent sediments of the late maar-lake period. The early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors. The seismic sections clearly reveal the bowl-like structure of the maar. Outside this bowl-like structure, there are only a few reflections, which represent the basement. Taking into account the shape of the gravity anomaly, seismic information allows geometrical modelling of the maar structure. Optimal drilling sites can be selected based on the results of geophysical surveying. Comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern.

No abstract

The unicellular green alga Scenedesmus obliquus adapts to different irradiances and wavelengths of light by altering the molecular organization of the photosynthetic apparatus. Regulation occurs on the level of pigment-accumulation, assembly of pigmentprotein complexes and gene-expression. Action spectroscopy revealed the involvement of at least two photoreceptor-systems regulating adaptation antagonistically with markedly different thresholds (Thielmann et ai., 1991;. One of them was a BL-(flavin-type-) receptor, which mediates weak-light adaptation, resulting in increased Chi, LHC II (Humbeck et ai., 1988;Senger and Bauer, 1987) and accumulation of cab-mRNA (Hermsmeier et ai., 1991); the other one was a VL/RL-receptor with action peaks at 404 and 650 nm, inducing high-irradiance responses indicated by decreased amounts of Chi, LHC II, LHCP and cab-mRNA (Hermsmeier et ai., 1991).

Recently we reported on the stimulating effect of a cold dark pretreatment (cold-dp) on the chlorophyll accumulation (Schonbohm et al., 1988) and on the CO 2-and 02-gas exchange (Schonbohm and CzeskaWerner, 1990) during de-etiolation of wheat leaves under white light.The goal of the present paper was to study the effect of a cold-dp on the accumulation of chloroplast pigments. on the kinetics of fluorescence quenching. on the synthesis of Cab-proteins and on the amounts of cabtranscripts during de-etiolation under white light.The following results were obtained: 1. Not only the chlorophyll synthesis but also the synthesis of xanthophylls and of fJ-carotene is highly increased by a cold-dp. 2. The kinetics of the PAM-fluorescence curves demonstrate that the activity and co-operation of the two photosystems start earlier and with higher efficiency during de-etiolation in cold-dp than in warm-dp wheat leaves. 3. The amount of cab-transcripts can be elevated by a cold-dp. 4. The amount of Cab-proteins during de-etiolation is much higher in cold-dp than in warm-dp wheat leaves.The presented data are in accordance with our earlier published results. Consequences of these results on seedlings growing under natural conditions with fluctuating temperatures during day and night are pointed out.

Time courses of the accumulation of chlorophyll (Chl) a, Chl b, of mRNA transcribed from the genes encoding the Chl a/b-binding proteins of the light-harvesting complexes (LHCs) (Lhc mRNAs) and LHC apoproteins have been investigated in Scenedesmus wild type (WT) and the mutant C-2A' upon transfer from darkness to light. The WT develops the complete photosynthetic apparatus in darkness, whereas C-2A' exhibits a light-dependent formation of Chls and functional chloroplasts. Accumulation kinetics of Chls and LHC apoproteins were paralleled in green (WT) and greening (C-2A') cells. Illumination induced a steady increase in these compounds which was only moderate in the WT but pronounced in C-2A'. Accumulation of Lhc mRNAs, however, was strongly enhanced by light in both WT and C-2A' cells and showed a transient time course with a maximum after 4 h illumination. Amounts of Lhc mRNAs and LHC approproteins were correlated only during the initial 4 h of greening of C-2A'. In green cells (WT and later stages of greening of C-2A'), amounts of Lhc mRNAs and LHC apoproteins diverged markedly. These findings and results provided by in-vitro translation of polysomal RNA lead to the following conclusion: the light-regulation of Lhc-gene expression in Scenedesmus takes place mainly at the transcriptional level in greening cells and at the post-translational level in green cells.