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Mn, Cu and Zn requirements for wheat root growth

Webb, Amanda Jane (1992) Mn, Cu and Zn requirements for wheat root growth. PhD thesis, Murdoch University.

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The effects of Mn, Cu and Zn deficiencies on the growth of the root system of wheat (Triticum aestivum cv. Gamenya) seedlings in solution culture were examined in three series of glasshouse experiments. Firstly, simple deletion experiments were undertaken in order to identify early root deficiency symptoms. Secondly, split root experiments were used to determine whether all of the root system requires an external supply of Mn, Cu and Zn; and thirdly, radioisotopes were used to investigate the extent of redistribution of Mn and Zn to roots without an external supply of Mn and Zn.

Since roots prepared using conventional procedures for microscopical examination were unsuitable for monitoring effects of nutrients on root anatomy, preliminary studies were undertaken to improve root preservation. Minimum shrinkage was achieved by reducing the osmolality of the primary fixative and increasing the period of resin infiltration using graded concentrations of resin.

In the deletion experiments, germinants were transferred (DO) into complete basal nutrient solution without either Mn, Cu or Zn for up to 25 days. Shoot and root Mn content and concentration were depressed in Mn-deficient plants as early as Day 5 (DS). Lignification of the cell walls of the root protoxylem was reduced at DlO, 5 days prior to a depression in root dry matter (DM) and expression of foliar deficiency symptoms. By D15, Mn deficiency depressed the length and number of seminal lateral roots, and by D20 had delayed nodal root formation, increased the number of lateral initials on the seminal root axes, and increased the distance from the root tip to the youngest emergent lateral root. The meristematic zone in roots of Mn-deficient plants was disorganized and highly vacuolated.

Withholding Cu led to reduced root protoxylem cell wall lignification, and decreased root Cu content and shoot Cu concentration (DIO), 9 days prior to foliar symptoms. By D25, Cu deficiency decreased root and shoot DM yield. the length of the seminal lateral roots and the nodal root axes, the number of lateral root primordia on the root axes. and increased the distance from the root tip to the youngest emergent lateral root.

Foliar symptoms of Zn deficiency were observed in Zndeficient plants on D9, one day prior to a depression in the Zn concentration of roots and shoots, and the Zn content of shoots. On D15, the root Zn content, root and shoot DM yield, root length, and root number were reduced in Zn-plants. Zn deficiency increased the thickness of the inner radial and tangential cell walls of the endodermis at D15. Furthermore, the size of the meristematic zone of Zn-deficient root tips was reduced and cells were highly vacuolated. By D20, the mean root length of lateral roots was depressed, and the distance from the root tip to the youngest emergent lateral root was reduced in Zn deficient plants.

In the second series of experiments, the effect of supplying one half of the root system with either Mn, Cu or Zn on root growth was examined using a split root system. The half of the root system without external Mn exhibited early symptoms of Mn deficiency; increased number of lateral initials/plant, increased distance from the root tip to the youngest emergent first order lateral root on the seminal axes, and reduced lignification of protoxylem in the young regions of the root. Supplying one half of a Cu-deficient root system with Cu caused renewed lignification of protoxylem cell walls in the younger regions of the roots supplied with external Cu, but did not ameliorate the decline in cell wall lignification in the part of the root system without an external supply of Cu. Withholding Zn from one half of the root system depressed the root Zn concentration and content, depressed the mean root length of nodal axes and lateral roots. These results suggest that Mn, Cu and Zn may be required in the rhizosphere for maximum root growth as retranslocation was insufficient to maintain normal root growth and development.

In the third series of experiments, the half of the root system grown continuously with complete nutrient solution was labelled with either roots without an external supply of either Mn or Zn was very low, and was estimated to be less than 1.0% of the total activity of the shoots. After the 24 h. uptake period 54Mn was present in the younger segments of the seminal root from the scutellar node, and after the 48 h. uptake period, 54Mn was present along the length of the seminal root from the scutellar node, and in the root tip of the seminal root. Autoradiography revealed a homogeneous distribution of 54Mn along the length of both seminal roots. By contrast the low levels of 65Zn were concentrated in the tips of Zn-deficient roots.

That Mn, Cu and Zn are not readily redistributed to roots of glasshouse grown wheat suggests that for plants in the field in which the shoot is adequately supplied with Mn, Cu and Zn, root growth may be limited into subsoils which are deficient, or have an uneven distribution of nutrients with very low phloem mobility.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental 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): Dell, Bernard
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