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New material collected from the Kliphoek Member of the Nama Group (Kuibis Subgroup, Dabis Formation) on Farm Aar, southern Namibia, offers insights concerning the morphology of the Ediacaran organism Pteridinium. Pteridinium fossils previously described as being preserved in situ have been discovered in association with scour−and−fill structures indicative of transport. Additionally, two Pteridinium fossils have been found within sedimentary dish structures in the Kliphoek Member. A form of organic surface with a discrete membrane−like habit has also been recovered from Farm Aar, and specimens exist with both Pteridinium and membrane−like structures superimposed. The association between Pteridinium fossils and membrane−like structures suggests several possibilities. Pteridinium individuals may have been transported before burial along with fragments of microbial mat; alternately they may have been enclosed by an external membranous structure during life.
The levels of three endogenous cytokinin equivalents: zeatin (Z), iso-pentenyladenine (iP) and dihydrozeatin (dZ) in two Arabidopsis thaliana (L.) Heynh genotypes - wild type (wt) and ethylene-insensitive mutant (eti5), were compared using enzyme immunoassay (ELISA). Cytokinin content was measured after exposure to low (4 °C for 24 h in darkness) or high temperature (38 °C for 24 h in darkness). Measurements were performed immediately and 24, 48 and 120 h after treatments. It was found that at normal growth conditions eti5 plants contained more endogenous cytokinins compared to the wild type. At both temperature treatments mutant plants had decreased total cytokinin levels. Wild-type plants treated with high temperature (HT) exhibited reduced total cytokinins (with the exception of rates at 48 h), while low temperature (LT) treatment resulted in elevated total amount of the studied equivalents (except at 24 h). The obtained results suggested that HT had greater effect on cytokinin levels than LT since it caused more profound changes in the total content. We assume that this was due to the natural chilling tolerance of Arabidopsis plants.
The specific activity of cytokinin oxidase/dehydrogenase (EC 1.5.99.12) (CKX) was determined in leaves of wild type (wt) and ethylene-insensitive mutant (eti5) of Arabidopsis thaliana (L.) Heynh plants. Comparative studies showed that this mutation has lower basal CKX activity than wt. Application of 4PU-30 (N1-(2-chloro-4-pyridyl)-N2-phenylurea) resulted in decreased CKX activity in both wt and mutant plants. The treatment increased leaf blade thickness and the volume of chlorophyll-containing cells per unit leaf area in wt but these changes were not observed in the eti5 mutant. The reduction in chlorophyll “a” and “b”, as well as in carotenoids content in the treated wt tissues resulting from altered leaf morphology was not detected in eti5 plants.
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