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Plants, as sessile organisms, have evolved many strategies to survive in a changing environment. Three key phenomena are considered in the present study: (1) morphological changes associated with C allocation to roots and shoots in favour of the most limiting resource; (2) differential C allocation to root pieces growing in relatively high nutrient patches and (3) physiological adjustment to the nutrient environment by up- or down regulating specific nutrient uptake rates. Plants exist as individuals within populations or communities. The aim of this study was to explore some of the dynamics, with respect to the above phenomena, that even-aged monocultures display. To this end, an individual-based modelling approach was developed (Chapter 2). A Moore neighbourhood cellular automation was designed so that each plant had nine roots (eight in its eight neighbouring cells and one in the central cell below the shoot). Roots could deplete N and P from neighbouring cells. A diffusion term was introduced to allow the movement of N and P between cells following a concentration gradient. To avoid edge effects, a 30 x 30 cell array was implemented with a toroidal configuration. As well as homogenous nutrient distribution, heterogeneity was introduced into the array at fine, medium or coarse grain. The total N available within the array (either high or low) remained constant but was distributed differently according to the grain of heterogeneity. The individual unit of the cellular automation was a single plant growth model (SPM) that incorporates the three phenomena listed above (Chapter 3). Plants (SPM) were randomly allocated to cells within the array from a Gaussian distribution of initial plant weights. Each plant could grow and interact with its eight immediate neighbours. The SPM growth followed an expo-logistic curve.

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Nitrate reductase is known to be the enzyme regulating the reduction of nitrate to nitrite in plants. This reduction is the first and rate-limiting step in the transformation of inorganic nitrate to a cellular nitrogen form. Since this reduction process is essential to phytoplankton growth when nitrate is the only available nitrogen source, the species with the highest capability to reduce nitrate, or the highest nitrate reductase activity, should be the species most favored to dominate a phytoplankton population under nitrate-limiting conditions. To study this hypothesis, the nitrate reductase activities of two species were studied; these species are co-occurring yet dominate under different conditions. Thalassiosira nordenskioldii is the dominant species in Auke Bay, Alaska in early spring when nitrate levels are decreasing from 15 μM to 1 μM, and the temperature is about 5 C. Skeletonema costatum is dominant in mid-summer when nitrate levels are below 1 μM and the temperature is above 12 C. The results show that at 15 C, Skeletonema has a higher enzyme activity at all nitrate levels than does Thalassiosira, which is consistent with the hypothesis. In addition, Thalassiosira shows a higher enzyme activity at 10 C, nearer its optimal temperature for growth, than at 15 C, suggesting that temperature affects species succession through its influence on enzyme activity. The applicability of Michaelis-Menten kinetics to this reduction reaction, based on the enzyme activities measured for these two species, is doubtful but inconclusive. The results also have implications for such concepts as nitrate-limited growth and the Redfield model relating nutrient, O₂ and CO₂ changes in the ocean. / Graduation date: 1971

Thesis (M.S.)--University of North Carolina at Wilmington, 2004. / Includes bibliographical references (leaves : [49]-52).

No description available.

No description available.

Nitric oxide (NO) alters chondrocyte metabolism, and is thought to be a key catabolic mediator in osteoarthritis. NO is also an important modifier of bone metabolism, and may partially mediate the bone-sparing effects of oestrogen. Oestrogen has also been linked to the modulation of osteoarthritis, though its role is not clear. The aim of this study was to examine the structural and metabolic effects of ovariectomy and the NO donor glyceryl trinitrate (GTN) on (1) normal ovine femoro-tibial joint tissues, and (2) the progression of joint lesions in the established ovine meniscectomy model of osteoarthritis. Preliminary investigations tested a novel computer-assisted histomorphometric method of assessing osteochondral changes post-meniscectomy, in a trial of a putative disease-modifying osteoarthritis compound. Quantitative assessment revealed a subtle protective effect not evident by qualitative methods. These techniques were then used to test the experimental hypotheses in a combined trial involving 48 aged ewes, variously subjected to ovariectomy, bilateral lateral meniscectomy, and/or topical GTN therapy. At six months, joint tissues were analysed using histology, histomorphometry, dynamic biomechanical testing, serum markers, bone densitometry, and tissue culture of synovial fibroblasts and explants of cartilage and bone. Ovariectomy modified cartilage structure and chondrocyte metabolism, and induced subchondral bone remodelling. Prior ovariectomy altered the development of OA lesions post-meniscectomy, producing thicker but biomechanically inferior cartilage and elevated metabolic activity in subchondral bone. GTN treatment of normal sheep induced thinner, structurally-altered cartilage in normal sheep, and accentuated cartilage and subchondral bone lesions post-meniscectomy. These results support an important homeostatic role for oestrogen in joint tissues, and show that GTN, a commonly used angina therapy, can induce structural alterations in joint tissues and potentially accelerate the progression of concurrent OA. Results also advance understanding of the role of synovial and subchondral bone changes in the pathogenesis of this OA model.

Thesis (Ph. D.)--University of Adelaide, Dept. of Plant Science, (1995). / Includes bibliographical references (leaves 153-179).

Thesis (M.S. - Soil and Water Science)--University of Arizona. / Includes bibliographical references (leaves 86-91).