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The effect of excess manganese on nitrate assimilation in lettuce

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Jones, Richard Harold (1974) The effect of excess manganese on nitrate assimilation in lettuce. PhD thesis, University of Tasmania.

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Abstract

Growth of lettuce may be limited in acid soils by manganese
toxicity which may be alleviated by applications of lime, phosphate
fertilisers, iron and molybdenum. However few investigations have
been undertaken to determine the biochemical effect of excess manganese
in plants. The aim of this project was to demonstrate that manganese
toxicity could reduce lettuce yield by inhibiting nitrate reductase
activity.
Manganese toxicity inhibited nitrate reductase activity in
lettuce plants grown in an acid krasnozem soil. The yield of affected
plants, which was significantly lower than the yield of control plants,
was significantly correlated with the decrease in nitrate reductase
activity. Associated with the decrease in nitrate reductase activity
was an accumulation of nitrate in leaves and a decrease in the total
nitrogen content (mgN/plant). Following applications of molybdenum
foliar sprays there were significant increases in yield and nitrate
reductase activity. The molybdenum content of plants increased in
plants which had high tissue levels of manganese. The increase was
probably due to the lower yield in these plants. It is not known
how molybdenum reverses the manganese inhibition of nitrate reductase.
Similar responses were observed in lettuce grown in nutrient
solution. The Critical solution concentration of manganese,at which
there was a decrease in yield and in nitrate reductase activity, was
found to be 36uM. Nitrate reductase activity of young and mature leaves
was significantly correlated with leaf manganese levels. Plants grown
in nitrate-nitrogen nutrient solutions were more susceptible to
manganese toxicity than plants grown in ammonium nitrogen nutrient solutions.
A study of the time-course of manganese inhibition of nitrate
reductase in vivo (indicated that after a short lag period, manganese significantly
decreased nitrate reductase activity. The half life for this decrease
in enzyme activity was found to be 2.5 hours. These results indicated
that manganese was not inducing molybdenum deficiency and that manganese
may have inhibited nitrate reductase by binding to the enzyme.
Enzyme kinetic analysis of the manganese inhibition of nitrate
reductase (in vitro) showed that manganese was a competitive inhibitor
of nitrate reductase with respect to NADH and a non-competitive
inhibitor of nitrate reductase with respect to nitrate. These results
indicated that manganese was bound to the enzyme at or near the binding
site for NADH. When manganese was added to the enzyme before NADH
the degree of inhibition was greater than when the NADH was added to
the enzyme before manganese. These results also indicated that
manganese was interfering with the binding of NADH to the enzyme.
EPR studies supported the results obtained with the kinetic
analysis of the inhibition of nitrate reductase. Results of the EPR
study showed that manganese was not bound by NADH but could be bound
by the partially purified enzyme extract.

Item Type: Thesis (PhD)
Keywords: Plants
Copyright Holders: The Author
Copyright Information:

Copyright 1973 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.) - Tasmania, 1974. Bibliography: l. 207-220

Date Deposited: 19 Dec 2014 02:23
Last Modified: 01 Dec 2016 03:39
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