The purposes of this study were to determine if Rhododendron macrophyllum has the potential to inhibit the growth of other species through the production of water-soluble toxins which are leached out of its litter by rainfall, and to determine if this potential is realized in the field. The study was therefore composed of two part: 1) a series of bioassays to determine the presence an activity of water-soluble phytotoxins in R. macrophyllum leaf litter, and 2) a field study to describe vegetational patterns associated with R. macrophyllum in a western Cascades clearcut.

Thesis (Ph. D.)--University of Hong Kong, 2000. / Includes bibliographical references.

Thesis (Ph.D.)--University of Hong Kong, 2000. / Includes bibliographical references. Also available in print.

No description available.

Although leaf movements have been documented for over a century, there are few studies focused on the adaptive significance of leaf movements, especially with experiments under controlled conditions. The major objective in this study is to determine the ecological significance of leaf movements in Rhododendron maximum, which is a subcanopy, evergreen species distributed in seasonally cold environments. Leaf movements could be necessary for maintaining a favorable energy balance and/or avoiding photoinhibition and photooxidation. A series of leaf manipulation treatments were established to verify these potential explanations. These leaf treatments were designed to separate the influence of leaf curling and leaf angle on leaf energy budget, gas-exchange characteristics, chlorophyll contents and leaf longevity. Leaf movements were found to have a significant influence on leaf physiology and longevity of R. maximum. Without changes in leaf angle, chlorophyll contents decreased, and permanent photoinhibition occurred due to excess irradiance absorption in the winter. Leaf angle also influenced leaf temperature although the changes in leaf temperature were within the physiological tolerances of R. maximum leaves. Leaf curing had little or no effect on the parameters measured in the study. / M.S.

Phytophthora pini Leonian, recently re-established from P. citricola I, is a pathogen with a wide range of forest and nursery hosts. It causes foliar infections in horticultural nurseries in Oregon, where recirculating irrigation systems are common. Increased use of recirculating irrigation systems may contribute to disease caused by waterborne plant pathogens. Simulated nursery chamber experiments were utilized to investigate the relationship between Phytophthora pini zoospore inoculum dose and disease on Rhododendron. Disease incidence in this system was unexpectedly low despite high inoculum levels tested, so further experiments under lab conditions were conducted to explore possible causes. Detached leaf assays were conducted to determine how inoculum dose, leaf wounding, and agitation of zoospore inoculum affected foliar infection of Rhododendron. Wounded and nonwounded leaves were dipped into suspensions of zoospores that were either untreated, mechanically agitated by vortexing to cause encystment, or pumped through an irrigation sprayer system. Disease severity (lesion area) and incidence (number of lesions per leaf area) were measured over seven days. At inoculum levels of ���10,000 propagules/mL, motile zoospores infected both wounded and nonwounded leaves. Vortexing or pumping resulted in zoospore encystment, and inoculation with these treatments caused disease almost exclusively on wounded leaves. No disease symptoms were observed following inoculation with any inocula at ��� 2,000 propagules/mL. Scanning electron microscopy of leaves inoculated with encysted propagules showed germinated cysts with hyphae growing over and around stomata without entering leaf tissue until reaching a wound site. Nonwounded leaves inoculated with motile spores showed stomata penetrated by hyphae. These findings indicate the importance of zoospore motility in reaching suitable infection sites, and demonstrate the impact of zoospore encystment on disease development. This has implications for disease management in nurseries where pruning wounds are common and the pumping of infested irrigation water may influence zoospore motility and infectivity. / Graduation date: 2013

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1991. / Vita. Includes bibliographical references.

Microbial respiration on sticks and leaves, measured as oxygen uptake rate, was compared among four (2nd-4th order) sites along an elevational gradient at Coweeta Hydrologic Laboratory, North Carolina. Rhododendron and birch leaves were placed in mesh bags in the stream at each site on 21 Oct 90. Oxygen uptake rates were measured for both leaf species beginning 15 Dec 90 and continuing monthly until leaves were no longer intact (birch-2 months, rhododendron-7 months). Microbial oxygen uptake rates were also measured monthly for one year on qualitative collections of sticks (1-3 cm diameter) found in the stream at each site. Oxygen uptake rates (mg O₂/hr) were calculated on both a surface area and AFDM basis. Overall mean respiration rate per unit surface area was highest for sticks, followed by rhododendron, and birch. When expressed on an AFDM basis, respiration rates were highest for birch, followed by rhododendron, and sticks. Based on continuous measurements, mean monthly stream temperatures were significantly different among sites, but mean incubation temperatures were not significantly different among sites, Respiration rates were significantly correlated with temperature for both rhododendron leaves and sticks on both an AFDM and surface area basis. Respiration rates on rhododendron also increased with exposure time. Results indicate that differences in respiration rates among sites, for both rhododendron leaves and sticks, can be explained primarily by changes in temperature. Additionally, the high respiration rates per unit surface area on sticks indicate that small woody debris may play a significant role as a substrate for microbial metabolism. / Master of Science

Propagation by tissue culture is effective for many woody ornamental plants, but propagules often become desiccated and die during the acclimatization period. This loss is due in part to stomata that fail to close in response to the reduced humidity outside of the culture environment. KCl was used in in vitro treatments to determine if an additional K supply would improve microshoot stomatal function and water status during acclimatization. The effects of the KCl treatments upon subsequent microshoot rooting and percent fresh weight gain were also evaluated. In preliminary experiments, microshoots of the flame azalea (Rhododendron calendulaceum) were subcultured onto modified Woody Plant Medium amended with a wide range of KCl concentrations for various time periods. It was determined that microshoots did not grow well when cultured at in vitro KCl levels above 50 mM, so treatments were adjusted to 0, 30, and 60 mM KC1, with 9 days of in vitro exposure. After treatment, percent tissue K was determined by atomic absorption spectrophotometry and microshoot water potentials were measured by thermocouple psychrometry. The capacity for the microshoots to resist desiccation after in vitro KCl treatment was determined by percent rooting and fresh weight gain after exposure to dehydration stress, and by gravimetric weight loss of microshoots placed in isopiestic tubes. In addition, microshoots from in vitro KCl treatments were evaluated for percent stomatal closure and water potential during a 38-day acclimatization period. In vitro KCl treatments induced elevated tissue K levels in microshoots and reduced microshoot water potentials, but it could not be shown that these effects specifically enabled the microshoots to resist desiccation. Rooting and percent fresh weight gain were not affected by in vitro treatments, nor was gravimetric weight loss. Microshoot maturation, as a function of days out of culture, had the greatest effect upon increased stomatal function, which, coupled with the onset of rooting, improved microshoot water status. / Master of Science

published_or_final_version / Ecology and Biodiversity / Doctoral / Doctor of Philosophy