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The continuum theory provides a framework in which the growth of a plant root as a dynamic process involving interactions among transport of water and solute, cell division, and the subsequent cell elongation can be described. A plant root is modeled as a one-dimensional, multi-phase, mathematical continuum. The network of cell walls constitute the solid phase of the system. The symplast and the apoplast pathways reside in this network of cell walls. Water and carbohydrates move in opposite directions through the apoplast and symplast pathways within the deforming network of cell walls. The division and elongation of cells depends on the mechanical stress imposed on the cell walls, the rate of metabolic stress relaxation process, and the physical properties of the cell walls. The model consists of five systems of differential equations. The kinematic equations are derived which allow, specifically, the different roles of cell division and elongation in root growth to be considered. These provide the reference system of the model. Equations of water transport in the coupled system of apoplast and symplast pathways are derived from considerations of theories of transport in the porous media and the cellular and membrane properties of the plant root. Equations of solute transport are derived by considering, specifically, the mechanisms involved in solute transport both at the membranes separating individual cells and within the cytoplasm. The rate of cell elongation is described as a function of the mechanical stress in the cell walls, the viscoelastic properties of the cell walls, and a metabolically controlled strain energy relaxation process. Growth in the meristem is modeled as the result of continuous cell elongation and division. The equations of water and solute transport, cell elongation, and meristem growth are solved simultaneously under the reference system provided by the kinematic theory. The model is used to examine the effects of soil water stress, soil resistance to root penetration, and temperature, as well as the carbohydrate supply from the upper part of the plant on the dynamic process of root elongation. The close correspondence between the material coordinate system and the underlying cellular structure of the root allows the comparison between the continuum theory and the results of cell growth studies. Agreement of the model predictions of the pattern of growth along the root axis, as well as the effects of temperature and soil water stress on root growth, with the experimental measurements reported in the literature provides the justification for the theories. / Graduation date: 1991

In the N$ sb2$ fixing legume symbiosis, suboptimal root zone temperatures (RZTs) not only decrease N$ sb2$ fixation. but reduce the formation and development of nodules. The purpose of this thesis was to elucidate the mechanism by which suboptimal RZTs affect nodulation and nodule development in legumes, such as soybean (Glycine max (L.) Merr.) and to attempt to find ways to overcome this inhibition. Initial studies characterized the RZT response in soybean plants inoculated with Bradyrhizobium japonicum. In plants grown at RZTs from 25 to 17$ sp circ$C, the time between soybean inoculation with B. japonicum and the beginning of N$ sb2$ fixation increased by 2.5 days for every $ sp circ$C decrease, whereas below 17$ sp circ$C RZT each $ sp circ$C appeared to delay the onset of N$ sb2$ fixation by 7 days. RZTs less than 17$ sp circ$C strongly inhibited the nodulation process and, as a result also sharply decreased N$ sb2$ fixation per plant. The greater sensitivity below 17$ sp circ$C is due to events related to, or occurring before infection initiation. Coinoculation of soybean with B. japonicum and other microorganisms beneficial to legumes, either vesicular-arbuscular (VA) mycorrhizae or plant growth promoting rhizobacteria (PGPR), increased soybean nodulation and N$ sb2$ fixation, but these increases were temperature dependent. Vesicular-arbuscular mycorrhizal colonization had a negative effect on nodule establishment below 18.5$ sp circ$C RZT, but a positive one above this RZT. At each temperature tested some PGPR increased the amount of fixed N and number of nodules formed, whereas some decreased the level of these variables. The most stimulatory strain at each temperature was: 15$ sp circ$C-Serratia proteamaculans 1-102, 17.5$ sp circ$C - S. proteamaculans 1-102 and Aeromonas hydrophila P73, and 25$ sp circ$C - S. liquefaciens 2-68. / Because our research indicated that an event before infection thread initiation was most sensitive, and because the first known step in establishment of the symbiosis is production of a plant-to-bacterial signal molecules. I tested whether the poor nodulation at suboptimal RZTs was related to disruption of plant-to-bacterium signalling. Inocula bacteria were preincubated with genistein, a major isoflavonoid signal molecule in soybean. This shortened the period between inoculation and root hair curling, and hastened the onset of N$ sb2$ fixation under both controlled environment and field conditions. At 15 and 17.5$ sp circ$C RZTs, 20 and 15 $ mu$M genistein was found to reduce the inhibition of suboptimal RZTs, increase nodulation, and accelerate the onset of ${ rm N} sb2$ fixation. When applied to the plant rhizosphere in the field, genistein also reduced the inhibitory effects of cold spring soils on nodulation and N$ sb2$ fixation. Direct measurements of genistein accumulation in soybean roots indicated that, with decreasing RZTs, genistein accumulation decreased. B. japonicum USDA110 containing plasmid ZB977 with nodY-lacZ fusion genes incubated with genistein under different temperatures indicated that higher genistein concentrations and longer incubation times were required to activate the lacZ gene to a maximum level under low incubation temperature. Overall, these findings suggested that plant-to-bacteria signal molecules such as genistein may be an important limiting factor in the nodulation of legume plants at low RZT.

Three sugarbushes were selected for the study of root distribution. Saint-Hippolyte was the best site for the distribution and production of fine roots. A high water-table and an abrupt change in texture limited the extension of the root system to the top 40 cm at the Vaudreuil site. In Tingwick, an acid mineral B horizon with a low Ca saturation restricted the production of fine roots in that horizon. / A sugarbush in Norbertville was used as a replacement for the Vaudreuil site for the study on root chemistry. The Saint-Hippolyte site had the best overall nutrient status. The molar ratio Ca/inorganic Al and Mg/inorganic Al in solution never reached values low enough to be considered toxic for the growth of fine roots. However, Al seems to increase the imbalance between Ca and K and may be responsible for the low foliar P found at Norbertville. Aluminium may also inhibit Ca uptake by roots due to very low Ca content in Tingwick.

Thesis (Ph. D.)--Oregon State University, 1996. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Thesis (MScAgric)--Stellenbosch University, 1969. / ENGLISH ABSTRACT: no abstract available / AFRIKAANSE OPSOMMING: geen opsomming

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