Graduate Thesis Or Dissertation
 

The effects of cadmium on the nitrogen fixation system in Alnus rubra

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  • Alnus rubra (Bong. ) seedlings were grown in sand culture and irrigated with nutrient solution containing CdCl₂ ranging from 5 μg to 100 mg per liter. Treatment of A. rubra seedlings for 4 weeks with 50 and 100 mg CdCl₂ per liter of nitrogen-free nutrient solution decreased in situ nitrogenase activity 93 and 99%, respectively, when compared to controls. Nitrogen fixation was decreased 32 and 65% at CdCl₂ concentrations of 50 and 100 mg per liter, respectively. Growth was decreased to about the same extent as nitrogen fixation. Cadmium concentrations in the organs of A. rubra increased with increasing CdCl₂ concentrations in the nutrient solution and increasing duration of treatment. Treatment of A. rubra seedlings for 11 weeks with CdCl₂ - concentrations ranging from 0.1 to 25 mg per liter of nitrogen-free nutrient solution decreased in situ nitrogenase activity 25 to 89% when compared to controls. These treatments resulted in nitrogen fixation decreases of 23 to 98% and the number of nodules per plant decreased 29 to 74%. Similar reductions were observed in plant growth. Treatment with lower cadmium concentrations (0.01 to 0.1 mg CdCl₂ per liter) decreased nitrogenase activity 6 to 31%. Nitrogen fixation was not significantly reduced in the 0.01 to 0.1 mg CdCl₂ per liter treatment range and treatments in this range resulted in an increased number of nodules formed. Cadmium concentrations in the plant organs decreased with increasing CdCl₂ concentration in the nutrient solution. Either the Mo-Fe or the Fe protein component of nitrogenase was pre-incubated with CdCl₂ and then combined with the other nonincubated component to reconstitute the enzyme. Pre-incubation of the Mo-Fe protein with 136 μM CdCl₂ decreased in vitro nitrogenase activity to a minimum of 45% of the control assay without cadmium. Incubation of the Fe protein with cadmium non-specifically increased activity up to 175% (at 27 μM CdCl₂), Activity then decreased to 70% of control at 136 μM CdCl₂. When the two proteins were combined and then pre-incubated with CdCl₂ the results were similar to those obtained by pre-incubating the Fe protein. A. rubra seedlings without nodules were inoculated at the start of the experiment. The growth period prior to apparent nodulation increased from 5 to 8 weeks as the CdCl₂ concentration increased from 10 to 100 μg per liter of nitrogen-free nutrient solution. Nitrogen fixation decreased 52 and 89%, when compared to control plants, at 10 and 20 μg CdCl₂ per liter, respectively. No detectable nitrogen fixation was observed at higher cadmium concentrations. Decreases in plant growth from CdCl₂ treatment were roughly parallel to de-creases in nitrogen fixation. When seedlings without nodules were given 12 mM NH₄NO₃ nitrate reductase activity in the roots decreased 22 to 25% as the CdCl₂ concentration increased from 10 to 100 μg per liter. Nitrogen gain and growth were not decreased from this range of CdCl₂ treatments. When the seedlings were given 6mM concentrations of either NH₄NO₃ or Ca(NO₃)₂ nitrate reductase activity decreased 22 and 24%, respectively, at 100 μg CdCl₂ per liter. Nitrogen gain and growth were decreased in plants supplied with Ca(NO₃)₂ and 50 and 100 μg CdCl₂ per liter. Cadmium concentrations in the plant organs increased as CdCl₂ in the nutrient solution increased. The ultrastructure of root and nodule cells was investigated by electron microscopy. Spaces lacking cristae were observed in mitochondria and endophyte resorption advanced as the cadmium concentration increased. The number of starch grains in root xylem parenchyma cells increased as the CdCl₂ concentration increased from 20 to 100 μg per liter. Nucleoli increased in prominence and mitochondrial cristae became less well defined over the same range of CdCl₂ concentrations. These results indicate that cadmium in nutrient media inhibits nitrogenase activity, and therefore nitrogen fixation in Alnus rubra. Growth, modulation, and nitrate reductase activity were inhibited by the element. Observations of root and nodule cell ultrastructure suggest that cadmium exerts a portion of its effect by influencing the structure of organelles.
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