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Zn function in P accumulation by plants

Webb, Michael John (1987) Zn function in P accumulation by plants. PhD thesis, Murdoch University.

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Wheat plants (Triticum aestivum L. cv. Gamenya) were grown for up to 40 days in a complete culture solution containing 1000 uM phosphate (Pi) with zinc (Zn) added (Zn+) or no Zn added (Zno). Plants were grown under various light regimes with either natural or artificial light. In some experiments, the root system was split into equal halves for either a long term growth period or a short term experimental period with each half root system was bathed in a separate solution.

The effects of Zn treatment on dry matter, P content, P concentration, and Pi absorption rates were studied in an attempt to elucidate the mechanism by which Zn functions in relation to P accumulation in wheat plants. The effects of other factors, such as light, localised Zn supply, and pH of solution, were studied to determine how these may affect the response of P accumulation to Zn treatment.

Decreasing total light irradiation, either by decreasing intensity or daylength, generally delayed growth but did not directly affect the response to Zn treatment. Thus, omitting Zn generally depressed growth when Zn concentration in the youngest emerged blade (YEB) had fallen to 10 to 12 µg/g dry matter irrespective of the light treatment imposed. Several days after Zn deficiency had depressed growth, Zn deficient plants developed necrotic symptoms in the mid to basal region of the young leaf blades.

In addition to inducing these symptoms, Zn deficiency enhanced P concentrations in older leaves to 2% dry matter or greater. Under conditions of high light intensity, it also enhanced the necrotic symptoms of the leaf tips and margins attributed to P toxicity. Although it enhanced P concentration in shoots, Zn deficiency depressed P concentration in roots and generally depressed or had no effect on the rate of Pi absorption (Pi absorbed per unit root weight per unit time) or total P content of wheat plants. On the rare occasions that Z n deficiency enhanced Pi absorption rate or P content, its effect was only transient.

When Zn was omitted from only one half of a paired root system, plants appeared identical to those with Z n supplied to both root systems even though omitting Zn from both root systems had severely depressed growth. Although not affecting dry matter of roots or shoots, omitting Zn from only one half root system slightly depressed the rate of Pi absorption by that root system. That omitting Zn from the root environment depressed Pi absorption rate by that root and did not enhance P concentration in shoots suggests that the effect of Zn in roots may be independent of its effect in shoots in relation to P accumulation.

In separate experiments, radioactive phosphate (32Pi) was either painted on the oldest leaf blade or supplied to one half of a paired root system. These short term studies suggested that developing Zn deficiency enhanced accumulation of P in old leaves as a result of enhanced gross transport of P to shoots and depressed export of P out of these leaves. This effect of Zn on P movement is discussed in relation to the reported effects of Zn deficiency on auxin concentration.

When Zn adequate plants were grown in culture solution with ammonium (NH4+), they lowered their culture solution pH to approximately 3.2. However, Zn deficient plants failed to lower the pH of their culture solution to t he same extent as Zn adequate plants. Indeed, Z n deficient plants raised the pH of their culture solution in spite of the presence of NH4+. This suggests that Zn may have a function in integrity of root membranes.

When it was strictly controlled at differing levels, increasing the pH of the culture solution markedly enhanced subsequent short term absorption rate of 3 2 Pi . The effect of pH was much greater than the effect of either the Zn treatment or the nitrogen source supplied to wheat plants. This effect of long term pH treatment was consistent with other long term studies on the effects of pH on P content of plants but was inconsistent with the short term effects of pH on Pi absorption rate which have been previously reported. These results indicate that the pH at which wheat plants were grown was of far greater importance in determining Pi absorption rate than the pH of the solution from which Pi was absorbed. Failure to recognise the long term effect of pH on subsequent short term Pi absorption rate explains why previously published results for short term 32Pi absorption rates conflict with those for long term P accumulation in plants.

Zn deficiency affected rhizosphere pH and this may, in turn, affect Pi absorption rate. However, in experiments in which culture solution pH was maintained at near constant levels, this effect of Zn on rhizosphere pH would probably have been of little importance in determining Pi absorption rate. Thus, the effect of Zn deficiency in enhancing P concentration to toxic levels in wheat leaves in the experiments presented in this thesis, can not be attributed to an enhanced Pi absorption rate as has been shown for other species. However, its effect in enhancing P concentration in emerged leaves can be explained by its effects on the distribution and translocation of P within wheat plants. As Zn deficiency became more severe, its effects on dry matter distribution accentuated its effects on P concentration.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Environmental and Life Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Loneragan, Jack
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