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The effects of the natural phenylpropanoid α-asarone on growth pattern, photosynthesis, respiration and cell structure of two microalgae have been investigated. In cultures ofAnkistrodesmus braunii α-asarone decreases in the medium and induces a lag in growth. Both phenomena were dependent on the number of cells inoculated. By contrast, in cultures ofSelenastrum capricornutum a constant decrease of the growth rate at all inocula was observed and only a slight decrease of α-asarone in the medium occurred. In both algae α-asarone caused an initial inhibition of photosynthesis, followed by a resumption of control values. The respiratory rate ofA. braunii was not significantly affected by α-asarone, whereas inS. capricornutum respiration lowered to 60% of the control in the first 48 h and subsequently rose to values exceeding the controls by 20%. Ultrastructural observations carried out 24 and 72 h after the addition of α-asarone showed modifications of cell wall inA. braunii, an increase in the number of mithocondrial profiles per cell section inS. capricornutum, and an accumulation of electron-dense deposits in the vacuoles of both algae.

Continuous white light induced a secondary dormancy in 40% of radish seeds (Raphanus sativus L. “Saxa”); this value was increased to 80% by coumarin 2 x 10 4 M. Either darkness or removal of the coat were found to be dormancy breaking agents for seeds not germinated in the above conditions. © Taylor & Francis Group, LLC.

Thirteen natural and synthetic phenylpropanoids as well as coumarin (2×104M) were tested for their biological activity on radish germination and subsequent root growth in light and darkness. Coumarin was the most potent inhibitor. With some exceptions, phenylpropanoids with a carboxylic group in the side chain inhibited root growth. Coumarin was formed spontaneously by photooxidation of 2-hydroxycinnamic acid. Microscopic observations of root treated with coumarin suggest that this substance inhibits the elongation of cells of the differentiating zone of the root. © 1993 Plenum Publishing Corporation.

Two strains of Cyanidium caldarium which possess different biochemical and nutritional characteristics were examined with respect to their ability to utilize amino acids or 2-ketoglutarate as substrates. One strain utilizes alanine, glutamate or aspartate as nitrogen sources, and glutamate, alanine or 2-ketoglutarate as carbon and energy sources for growth in the dark. The growth rate in the dark on 2-ketoglutarate is almost twice as high or higher than that on glutamate or alanine. During growth or incubation of this alga on amino acids, large amounts of ammonia are formed; however, ammonia formation is strongly inhibited by 2-ketoglutarate. The capacity of the alga to form ammonia from amino acids is inducible and develops fully only when the cells are grown or incubated in the presence of glutamate. By contrast, the other strain of Cyanidium caldarium cannot utilize alanine or aspartate as nitrogen sources. It utilizes glutamate only very poorly and does not excrete ammonia into the external medium. This strain is unable to utilize amino acids or 2-ketoglutarate as carbon and energy sources for heterotrophic growth. Cell-free extracts were tested for the occurrence of enzymes which could account for amino acid metabolism and ammonia formation. © 1976 Springer-Verlag.

When intact cells of the acidophilic red alga Cyanidium caldarium, grown at pH 1.9 with nitrate as sole nitrogen source, are transferred to pH 7.5, they actively reduce nitrate to nitrite which accumulates in the external medium. At pH 7.5 nitrite is not further reduced to ammonia. Formation of nitrite from nitrate occurs in the light or in the dark, either in the presence or in the absence of oxygen. In cell suspensions supplied with ammonia in addition to nitrate, the formation of nitrite at pH 7.5 is strongly prevented. However ammonia is not able to prevent nitrite formation at pH 9.4. Nitrate reductase is 40% latent in cells resuspended at pH 7.5 in the presence of nitrate and up to 90% latent in cells resuspended in the presence of both nitrate and ammonia. It is proposed that the inhibition by ammonia of nitrate reduction to nitrite is based on the latent state that nitrate reductase exhibits in the presence of ammonia. In fact, at pH 9.4, when the production of nitrite is not inhibited by ammonia, nitrate reductase is only 14–25% latent even in cells resuspended in the presence of nitrate and ammonia.

A strain of Cyanidium caldarium has been studied which is able to grow in darkness using amino acids as sole energy sources. During growth ammonia was released into the external medium as a catabolic end product. With either threonine or glutamate similar rates of ammonia formation and similar kinetics of growth were observed. These observations suggest that the amounts of energy made available for cell growth from the two amino acids are equivalent. Deamination of threonine and glutamate by whole cells exhibited similar temperature-dependence profiles and similar Arrhenius energies of activation. Thus it is suggested that a partially common pathway is involved in the catabolism of these amino acids. Threonine dehydrase may play a role in this pathway. The threonine dehydrase of C. caldarium was inhibited by isoleucine and activated by valine. In the absence of isoleucine no cooperative effect of threonine was observed. Succinate or 2-ketoglutarate supported a faster growth than did amino acids. Growth tests in the presence of both a krebs cycle intermediate and an amino acid have shown that the oxidative metabolism of amino acids is in some way controlled by the more suitable energy sources, presumably through catabolite inhibition and catabolite repression. © 1977 Springer-Verlag.