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"Major Subject: Horticultural Sciences" Title from author supplied metadata (automated record created 2010-03-12 12:08:51). Includes bibliographical references.

Numerous studies in high-Si accumulators found physiological and biochemical adaptation processes were enhanced by Si application in crops stressed by cold conditions. Meanwhile, there is relatively little information on the impact of Si fertilization on low-Si accumulator crops (dicots) in respect of physiological and biochemical adaptation to stress, including citrus, which is classified as a chilling-sensitive species. The objectives of this study were: 1) To validate the method for Si analysis in citrus plants; 2) To determine factors that influenced Si uptake in citrus and its uptake mechanism; 3) To establish the impact of Si fertilization and cold stress on the photosynthetic efficiency of citrus plants; 4) To determine the impact of Si fertilization and cold stress on fruit quality and yield of citrus plants; 5) To determine the impact of Si fertilization and cold stress on the levels of key sugars, and proline in citrus trees. The method validation showed that the ICP-OES analytic method was fast and sensitive, with a detection limit five times lower than a colorimetric analysis. The uptake study showed that Si uptake in citrus increased significantly (P< 0.05) with the duration of application, leaf age and concentration applied, with 1000 mg L-1 being the optimum concentration tested. Si fertilization onto the roots of citrus trees was shown to be the best method of application compared to foliar application. Dry matter production was significantly increased by Si application. Electron microscopy studies suggested that Si accumulates in the adaxial epidermal cell regions of leaves and constitutes a double Si layer. The levels of Si in citrus leaves increased from young leaves<mature leaves<roots. Winter uptake of Si was close to zero, whereas summer uptake was substantially higher, which suggested that the uptake and transfer of Si into tissues was an active process depending on the prevailing temperature conditions and the physiological activity of the roots. The investigation of the impact of Si fertilization on the physiological adaptations of citrus to cold stress suggested that an improvement in photochemical efficiency occurred in both citrus cultivars (Delta and Nules). Si fertilization of citrus trees made them more cold stress tolerant via physiological factors as well as improvements in the osmotic balance of their sugar (sucrose and fructose) and proline content, resulting in improved membrane rigidity during cold stress. Both factors impacted on water use efficiency via osmoregulation processes. Overall, the current study provided substantial information on the response of citrus to Si fertilization under both normal and lower temperature conditions. The present study also identified crucial parameters in citrus adaptation to cold stress that may be useful in improving the tolerance of citrus crops to abiotic stress and particularly in frost-prone production areas in South Africa, or similar environments. / Thesis (PhD (Horticultural Science))--University of Pretoria, 2020. / -NRF-THRIP -Citrus Research International (CRI) / Plant Production and Soil Science / PhD (Horticultural Science) / Unrestricted

Increased demand for sustainably produced, healthy, and nutritious food has seen certain segments of the world agricultural sector flourish in the past few decades. The macadamia nut industry in particular has expanded at a tremendous rate, with more than 10 000 hectares of trees being planted annually across a range of environments. The greatest portion of these expansions occur in semi-arid areas, which are characterized by highly variable rainfall patterns, and are as a result irrigated to minimize the risk of yield, quality and income losses, brought about by water stress. The recently commercialized nature of the crop, in combination with lack of water use research specific to macadamia, has created great uncertainty amongst producers. This study has therefore firstly aimed at gaining a fundamental understanding of leaf gas exchange and macadamia transpiration (Ec) in response to a range of environmental and physiological variables, in an attempt to identify the driving variables of transpiration. Secondly, the study aimed to identify crop water use models that best incorporate the driving variables of Ec, in order to transfer results obtained from this study, to a range of growing environments. Measurements of leaf gas exchange, hydraulic conductance, canopy dimensions, weather, and Ec were made over an approximate three year period, in a fully irrigated commercial mature bearing (MB) and immature bearing (IB) macadamia orchard in the Mpumalanga province of South Africa. Leaf gas exchange measurements, included, but were not limited to net CO2 assimilation rate (A) and stomatal conductance (gs). Transpiration measurements were obtained using sap flow measurements using the heat ratio method of the heat pulse velocity technique. Macadamia A was found to be slightly lower than that of other evergreen subtropical crops, which is largely attributed to substantial stomatal and non-stomatal limitations to A. Non-stomatal limitations to A were linked to an internal light limitation resulting from the sclerophyllous nature of leaves. Stomatal limitations stem from the predominantly isohydric nature of macadamias, where gs is carefully controlled in order to maintain midday leaf water potential within certain safety margins. Isohydric behaviour suggested an underlying hydraulic limitation, which was found to exist within the stem to leaf interface of macadamias. Responses of gs to leaf vapour pressure deficit (VPDleaf) showed that gs declined as VPDleaf exceeded 2.5 kPa. The response of gs to VPDleaf, however, varied substantially throughout the season, being significantly higher during fruiting periods compared to non-fruiting periods, implying isohydrodynamic behaviour and emphasizing the influence of phenology on leaf gas exchange. Similar results were found on both fruiting and non-fruiting branches implying that an upregulation of gs at leaf level would most likely lead to an upregulation at the canopy level, which would lead to increased Ec. During fruiting periods, macadamia Ec was ~20% higher compared to non-fruiting periods, with no significant difference in weather variables or canopy size, which could act as confounding factors. Increased Ec during fruiting periods was associated with a greater response of Ec to air vapour pressure deficit (VPDair) in the 0.0 – 3.0 kPa range, which was similar to the observed increases of gs in response to VPDleaf > 1.5 kPa. An examination of transpiration crop coefficients (Kt), confirmed that increased Ec during fruiting periods stem from a physiological upregulation of gs and subsequently canopy conductance (ga). Besides physiological and phenological variables influencing macadamia Ec, physical attributes (i.e. canopy size) and weather variables remained the key driving variables of Ec. Macadamia Ec increased in a linear fashion when VPDair < 0.8 kPa, solar radiation (Rs) <0.3 MJ m-2 h-1 and reference evapotranspiration (ETo) <0.13 mm day-1, but failed to increase at the same rate when these limits were exceeded. The reduction in the rate of Ec in response to increases in environmental evaporative demand under non-limiting soil water conditions, indicates that Ec in macadamias is a supply-controlled system. Supply controlled Ec was confirmed upon examination of maximum daily recorded Ec (Ec max) in response to increases in the aforementioned weather variables, with daily Ec max failing to increase at VPDair >1.5 kPa, Rs > 15 MJ m-2 day-1 and ETo > 3.5 mm day-1. The response of Ec and Ec max to these weather variables did not vary between the two orchards, the magnitude of both Ec and Ec max, however, differed between orchards, being highest in the MB orchard. Higher Ec in the MB orchard was largely attributed to a ~60% larger canopy, with Ec in the MB orchard being ~60% more than Ec in the IB orchard. Transpiration measured in this study, however, remains site specific, and identification and validation of crop water use models were therefore needed to extrapolate data to a broader range of growing environments. The study therefore evaluated three models including the widely used FAO-56 dual crop coefficient, a canopy conductance (gc) model in conjunction with the Penman-Monteith equation, and a canopy transpiration model. The study showed, that a poor estimation of daily Kt and subsequently Ec was obtained using the FAO-56 dual crop coefficient model, which was largely attributed to overestimation of Kt and therefore Ec when daily reference evapotranspiration (ETo) rates exceeded 4.0 mm day-1, and an underestimation of Kt and Ec when ETo < 2.0 mm day-1. The model, however, provided reasonable estimates of Kt and Ec on a monthly or seasonal basis, with only slight discrepancies observed between measured and simulated Kt and Ec from January to April in each season, which was attributed to physiological upregulation of Ec in the presence of fruit. The gc estimations in conjunction with the Penman-Monteith equation, provided more accurate estimates of daily Ec in both the MB and IB orchards, compared to the empirical FAO-56 dual crop coefficient model, but was particularly sensitive to seasonal changes in leaf area index (LAI), with adjustments of maximum canopy conductance (gc max) being required to achieve accurate estimates of Ec. An adjustment for variations in LAI, however, failed to provide increased estimates of Ec during the January to April period reaffirming the phenological and physiological influence of fruit on gc and Ec during this period. Measurements of macadamia gc in this study was rather low (0.3 – 0.7 mm s-1) in relation to ga (37 - 75 mm s-1), confirming that macadamias are well coupled to the atmosphere. The high degree of coupling in macadamias implies that changes in gc would lead to direct changes in Ec, which contributed to the success of the use of a simplified Ec model. This model provided reasonable estimates of daily Ec without multiple adjustments for canopy size being needed within each of the orchards. The Ec model, similar to the other models tested, however, failed to provide reasonable estimates of Ec during the January to April period. The results from this study have shown that macadamias are predominantly isohydric in nature, a trait which ultimately dictates leaf gas exchange and Ec in this recently domesticated subtropical crop. Strict stomatal control in response to increased atmospheric evaporative demand, is also evident in the supply controlled nature of macadamia Ec, which has added to the success of mechanistic models in accurately estimating macadamia Ec. Although the study has reaffirmed that Ec is largely driven by environmental demand and canopy size, it demonstrated that physiological and phenological factors can have a significant effect on leaf level gas exchange and subsequently Ec of macadamias. / Thesis (PhD)--University of Pretoria, 2020. / Water Research Commission and Macadamias South Africa NPC / Plant Production and Soil Science / PhD / Unrestricted

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There has been interest in developing a cashew industry in Maputaland, the far north-eastern corner of Natal/KwaZulu. Flowering and fruit development coincide with a rainy period, with accompanying serious flower diseases (Oidium anacardii and Colletotrichum gloeosporioides). Glasshouse studies were carried out at Pietermaritzburg, concurrently with field trials in Maputaland, in an attempt to manipulate flowering and growth of cashew trees. Two glasshouse trial were carried out. A factorial design with treatments 0, 3, 6 and 9 weeks of low temperatures (24°C day/9°C nights)(factor A) and 0, 3, 6 and 9 weeks of water deficit (Factor B) was used, with both factors in all combinations. During the second season the durations were increased to 0, 4, 8 and 12 weeks for both factors. No flowering occurred in this trial. Tree growth was not affected significantly by drought and/or cold duration. Temperature appeared to be the dominant factor at low temperatures, stomatal conductance and transpiration being suppressed by cold regardless of soil water potential. At more optimum temperatures for growth, stomatal conductance was dependent on soil water potential (r² = 0.756). Starch levels in the roots, dry matter production in the leaves, roots and stems, as well as leaf area were decreased significantly (P≤0.01) with increasing low temperature duration. Another glasshouse trial to test the effects of foliar urea at concentrations of 0, 1, 2, 4 and 8 g urea 100 l⁻¹, applied once, twice or thrice at fortnightly intervals was undertaken. The treatments were applied in late autumn/early winter of 1990 and 1991. Tree growth and flowering were monitored, and starch and leaf NH₃/NH₄⁺ analyses carried out. The highest urea concentration (8%) resulted in leaf scorch and abscission, extremely low stem diameter growth rates, and was too high for glasshouse trees. The starch contents of the 8% urea treatment were depleted significantly (P≤0.01) more than the other concentrations. The other urea treatments resulted in vigorous growth and high dry matter production. There were no significant effects of the number of sprays on cashew growth. Only seven trees flowered, and therefore no definite conclusions could be drawn regarding urea effects on flowering. Most hermaphrodite flowers (max. 76.8% hermaphrodite) opened soon after first anthesis of a panicle, and all terminal flowers of panicle branches were hermaphrodite. Flowers generally opened basipetally in a panicle, starting with hermaphrodite flowers and with progressively more male flowers. Urea sprays resulted in NH₃/NH₄⁺ build-up in the leaves, concentrations in flowering trees ranging from 100 to 700 μg g⁻¹ DM for approximately a month. A field trial at was carried out at Makatini Research Station to determine the effects of timing of a two month winter drought period on flowering and growth. An observational trial to determine the effects of girdling on growth and flowering was incorporated in the border rows of the irrigation trial. The trial tested five treatments (no irrigation during May and June, June and July, July and August, August and September, and a control treatment which received irrigation throughout Winter). Mean monthly temperatures were below 20°C, and mean minimum temperatures below 15°C for the 5 winter months during treatment application. There were no significant differences in tree growth, flowering, flushing, or yields between drought stressed treatments and control, indicating that, under the conditions at Makatini, autumn and winter temperature was the overriding factor controlling initial flower induction. Flowering occurred from early October (when mean temperatures exceeded 23 to 24°C) to late April (7 months - a prolonged flowering period), when mean monthly temperatures dropped below 23 to 24°C. Girdling of cashew trees in March and May, using girdle widths of 1, 5 and 10 mm was not successful in improving flowering and yields under the conditions of the trial. A field trial was carried out at Mosi Estate in Maputaland to test the following chemicals as tree and/or flower manipulators: foliar applied ethephon (50,100,200,500,2000 mg l⁻¹), KNO₃ (1 %, 2%, 4%), urea (1 %, 2%, 4%) and paciobutrazol (500, 1000, 2000 mg l⁻¹). A phenological model for cashew in Maputaland showed a dormant period during winter, followed by a generative flush, from which panicles and flowers were produced (peak November-January). The harvest period peaked in February and March. A strong post-harvest flush preceded the winter dormant period. Trunk starch levels were at their highest after the dormant winter period, and at their lowest following the harvest. Ethephon at high concentrations (500 and 2000 mg l⁻¹) resulted in excessive leaf drop, disturbed the root shoot balance and normal phenological patterns, and gave poor yields. The best ethephon concentrations were 100 to 200 mg l⁻¹. KNO₃had no significant effect on tree growth, flushing, flowering or yields when compared to control trees. Urea at 2% concentration gave a significant increase (P≤0.05) in flushing and simultaneous decrease in flowering. Paclobutrazol at 500 to 2000 mg l⁻¹ resulted in significantly lower growth rates, and early panicle production. The mean yields of all paclobutrazol treated treatments were higher than controls, despite a hail storm which damaged the flowers. From results of this trial, the use of these chemicals to improve yields and manipulate flowering may not be economically justified. The most promising chemical for further research was paclobutrazol. / Thesis (M.Sc.Agric.)-University of Natal, Pietermaritzburg, 1994.

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Thesis (M.S.)--University of Delaware, 2008. / Principal faculty advisor: Robert E. Lyons, Dept. of Plant & Soil Sciences. Includes bibliographical references.

Thesis (M.S.)--Ohio State University, 1987. / Includes bibliographical references (leaves 98-99). Available online via OhioLINK's ETD Center

Master of Science / Department of Horticulture, Forestry, and Recreation Resources / Candice A. Shoemaker / Educational opportunities in horticultural therapy at colleges and universities are currently experiencing a minimal level as compared to the time of their development in the early 1970’s. While there is an education decline in horticultural therapy, there is a rebirth of interest in this therapeutic modality taking place within the medical profession. Due to the lack of availability of university introductory-level horticultural therapy courses throughout the United States, it is important to ascertain what topics are needed for inclusion in an introductory course. A survey was sent to 13 horticultural therapy practitioners, 11 university instructors, and 8 students. Overall, responses of practitioners and professors were varied in their opinions. The one topic of importance in which they agreed was horticultural therapy definitions, as definitions for horticultural therapy, horticultural therapist, and horticultural therapy programs. A majority of horticultural therapy practitioners indicated that horticultural therapy history and theories are important topics for an introductory horticultural therapy course. A majority of the professors reported that horticultural therapy settings are a somewhat important topic. Topics of interest to students who had taken an introduction to horticultural therapy course included interaction with special populations (for example, elders, children, patients in physical rehabilitation), the use of adaptive gardening techniques and tools, and measuring the benefits of horticultural therapy. Topics of interest about which students wanted to learn more were ways in which to help each population through planned activities and how to reach horticultural therapy goals planned for patients and clients. Results of a literature search was conducted in which sources of information indicated that there have been declines in educational opportunities over time and a lack of growth in these opportunities. Based on the survey results and literature review, an introductory horticultural therapy course was developed. An introduction-type course is one that briefly introduces most, if not all, topics that will be studied in depth in more advanced courses. It should be designed in such a way that interests students and encourages them to continue their studies in the program. At the same time, the concerns and interests of the practitioners and professors need to be considered in the design of the course. As a result, the course was planned and includes the topics that are of interest to the participants in the surveys. The topics include horticultural therapy history, theories, definitions, program types, client/patient groups, settings, and the people-plant relationship.