Date of Award:

5-1982

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

M. A. Walsh

Committee

M. A. Walsh

Committee

R. Lanner

Committee

R. Shaw

Committee

F. B. Salisbury

Committee

W. F. Campbell

Abstract

Root phloem structure was researched in taxonomically related C3 and C4 grass species. The functional aspect of phloem unloading in the root was then discussed in terms of this observed structure. In Triticum aestivum L. (C3) each phloem bundle consists of one protophloem sieve-tube element, two companion cells, and one metaphloem sieve-tube element. Protophloem sieve-tube elements and companion cells are contiguous with the pericycle. Centripetally the companion cells are in contact with the metaphloem sieve-tube element. All are separated from tracheary elements by stelar parenchyma. Eighty-four percent of the lateral connections observed in protophoem sieve-tube elements are associated with the companion cells and 15% with the pericycle cells. Plasmodesmata occur in decreasing number between companion cells and stelar parenchyma (78%), companion cells and companion cells (16%), and companion cells and pericycle (6%). Lateral connections in metaphloem sieve-tube elements are distributed evenly between companion cells and stelar parenchyma. The phloem bundles of Zea mays L. (C4) have similar numbers, types, and organization of cells as in T. aestivum. The one difference is there are two to six metaphloem sieve-tube elements in each bundle. In both species, the major cellular elements responsible for solute movement towards the cortex appears to be the stelar parenchyma cell located between the companion cell and protoxylem element. Similar phloem anatomy and stelar tissue organization was observed in the roots from the following C3, C4, and C3-C4 intermediate monocotyledonous species; T. aestivum (C3), Panicum bisulcatum Thumb. (C3), P. Clandestinum L. (C3), Z. Mays (C4), P. maximum Jacq. (C4), P. milloides Nees. (C3-C4 intermediate), and P. laxum Swartz. (C3-C4 intermediate). It is suggested here that the similar number and types of cells in each phloem buncle as well as the species specificity in percent phloem area to stele area is not linked to high photoassimilate transport rates oberved from the leaves of the C4 species Z. mays and P. Maximum. The supposition that increased rates of photoassimilate translocation in C4 species are dependant, in part, on phloem and chlorenchyma anatomy is not supported by a unique phloem and stelar parenchyma anatomy in the sink.

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