No description available.

No description available.

The New Zealand avocado industry is worth $39.7 million in exports of 'Hass' avocados. Crop yields grew steadily from 1996 to 2001 to reach an average of 8.86 tonnes/ha. Since then however, crop yields have remained steady. To increase returns to growers, crop yields must increase. Avocado leaves in New Zealand become yellow in winter and it is hypothesised that chilling, followed by photoinhibition, is leading to photooxidation. Leaf yellowing leads to reduced photosynthetic capacity and early leaf abscission, at a time when carbon fixation and carbohydrate reserves are needed to support developing flowers, subsequent fruit set and vegetative flush, in addition to the existing mature crop. The focus of this research was to determine the underlying causes of yellowing in 'Hass' avocado leaves during winter. It is suspected that it is a result of the creation of free-radical oxygen that causes photooxidation of leaf components under excess light during low temperature conditions, such as experienced on clear winter mornings in the Bay of Plenty. An orchard in Katikati, in the Bay of Plenty, New Zealand was selected has it had a history of leaf yellowing. Two open flow, differential gas exchange measurement systems, The CIRAS-1 and the CMS-400 were used to monitor leaf photosynthetic performance over the course of the 2006 winter, with particular focus on the month of August. Chlorophyll a fluorescence was measured with a Walz Mini-PAM, leaf colour with a Minolta Chroma meter CR-200b and chlorophyll content with Minolta SPAD chlorophyll meter (in addition to traditional extraction techniques). There was conclusive evidence that the cold nights resulted in decreased net photosynthesis over the winter, with the depression starting in May and ending around the middle of August, dates that coincide closely with the period when days with mean temperatures less than 10 C occurred. The decrease in photosynthesis appears to be due to a direct effect on the carbon reduction pathway and in unusual in that full recovery seems to occur at the same time during the day. No photodamage of significance was found and the avocado seems to be highly protected against high light when photosynthesis is inhibited. This investigation found that leaf yellowing is not caused by photodamage following depressed photosynthesis. A new hypothesis is proposed which suggests that leaf yellowing is produced by the re-allocation of nitrogen from leaves during cold weather during flowering. It is suggested that the chilled leaves are seen as unproductive, old or shaded leaves by the plant and nutrient resources are re-allocated away from these leaves. A foliar application of 1% low biuret urea and 0.5% magnesium sulphate is currently used by avocado growers to restore leaf colour in leaves that have become yellow over winter. An experiment was carried out on yellowed leaves on 23rd August 2006 to determine the effectiveness of the treatment. This study concluded that the treatment was able to restore some leaf colour, but had no effect on leaf photosynthetic function.

Regensburg, Univ., Diss., 2007

University, Diss., 2001--Heidelberg.

Universiẗat, Diss., 2004--Bremen. / Erscheinungsjahr an der Haupttitelstelle: 2003.

Regensburg, Univ., Diss.,

Techn. Universiẗat, Diss., 2002--Braunschweig.

Arsenic is a notorious poison due to its high toxicity, worldwide distribution, and lack of any taste and colour once dissolved. The abundance of arsenic in Earth’s crust makes that it can naturally find its way into food and drinking water. Rapid and reliable detection of arsenic, directly in the field, is critical to support evidence-based decision-making in choosing irrigation or drinking water sources. Current cost-effective colourimetric techniques are associated with poor accuracy, health risks, and unacceptable levels of false negatives. Arsenic-specific cellular sensors, or biosensors, may present an inexpensive, safe, and renewable alternative, yet they have long been criticized for unsatisfactory sensing performance, and inconsistency of the outcome. This, in addition to the lack of suitable instruments capable of measuring the signals produced by these biosensors, has led to very few solutions reaching market. The goal of my thesis research was to test hypotheses that improve our fundamental understanding of As species biogeochemistry in simple and complex environmental matrices to then develop a new arsenic monitoring interface, one that would be both simple and accessible to the general public. Using a combination of wild-type and mutant strains, I managed to detail both the internal regulation of arsenic, and the external drivers of arsenic bioavailability. I started by designing a defined exposure protocol that achieved, for the first time, equimolar uptake of over 94% of the added As(III) and As(V) into the cells. By developing this control early into my thesis, I then worked to reintroduce commonly found constituents of environmental waters that are thought to impact arsenic uptake. This direct testing approach uncovered fundamentals of environmental arsenic redox chemistry such as As(III) photooxidation in solution, environmental ligand exchanges, and biological transport pathways. Simplifying a complex exposure protocol for use by the general public required automation of the data analysis steps. This consists of several hundred lines of code, capable of analyzing, normalizing and stabilizing biosensor output to improve the consistency and robustness of this system. These algorithms were then integrated into a new arsenic monitoring interface, one that was built and designed specifically for dehydrated biosensors. This portable, low-cost spectrometer achieved a fluorescent detection range that rivals expensive and sophisticated laboratory equipment at a fraction of the price, and without the need for a computer to compile the measurements. In contrast to highly criticized colorimetric techniques, the biosensor exposure protocol exceeds in operational use, reliability and detection limit. At its core, my thesis research provides a new and complete arsenic testing solution, one capable of measuring both As(III) and As(V) at levels relevant to the World Health Organization and Canadian guidelines for arsenic content in water (10 µg/L). It also provides a new method capable of selectively discriminating between arsenic species, thereby providing an inexpensive and high-throughput arsenic speciation method. I hope this work will help kickstart development of a marketable solution that empowers individuals to test and to monitor the quality of their water sources.

Dentre os diversos fatores responsáveis pela instabilidade química das vitaminas nos alimentos, a exposição à radiação luminosa é determinante, principalmente em alimentos contendo vitamina B2. O presente trabalho investigou a degradação fotossensibilizada das vitaminas do complexo B (ácido fólico, piridoxal, biotina e niacina) por flavinas. O piridoxal-5\'-fosfato (PLP) mostrou-se reativo frente aos estados singleto e tripleto excitado das flavinas com constante de desativação de 1,03 1011 L mol-1 s-1 para o estado singleto, valor este superior ao valor esperado para reações bimoleculares controladas por difusão em meio aquoso. Foi observada uma dependência significativa da constante de velocidade de desativação do estado singleto excitado com a temperatura, onde o aumento da temperatura proporciona um decréscimo da constante de velocidade sugerindo a existência de um complexo [FMN...PLP] no estado fundamental o qual foi confirmado por espectroscopia de fluorescência resolvida no tempo. O PLP mostrou-se reativo frente ao estado tripleto excitado da FMN com constante de velocidade de 3kq = 3,0 108 L mol-1 s-1 em meio de tampão fosfato pH 6,4 ou em meio de óxido de deutério a 25 °C. Não foi observada diferença significativa entre as constantes de desativação do estado tripleto excitado em meio aquoso e de óxido de deutério o que corrobora com um processo direto de transferência de elétrons do PLP para a 3FMN* ao invés de um processo de transferência de átomo de hidrogênio. As vitaminas (biotina e niacina) mostraram-se não reativas frente aos estados singleto e tripleto excitados da vitamina B2 o que pode ser atribuído aos altos potenciais de oxidação, Eo > 2 V vs. NHE, observados para estas vitaminas em meio aquoso. A voltametria cíclica do PLP apresentou um processo anódico e irreversível (E= 1,07 V vs. NHE), controlada cineticamente por transferência de elétrons heterogênea do PLP para o eletrodo. O rendimento quântico de fotodegradação do PLP em meio aquoso e aerado é 2,5 vezes superior ao encontrado para a reação em meio anaeróbico, o que sugere a participação do íon superóxido no processo global de degradação do PLP. O ácido fólico demonstrou-se igualmente reativo frente ao estado tripleto excitado das flavinas (3kq= 4,8 108 L·mol-1 s-1 e Φ = 0,26 (meio aerado) e Φ = 0,32 (meio anaeróbico)) e a sua complexação pela β-LG (Φ = 0,032 (meio aerado) e Φ = 0,055 (meio anaeróbico)) uma eficiente abordagem na proteção desta vitamina frente a fotodegradação sensibilizada por flavinas. / Among several factors responsible for the chemical instability of vitamins in food, exposure to light radiation is decisive, especially in supplemented or fortified food with vitamin B2. This study investigated the photosensitized degradation of B-vitamins (folic acid, pyridoxal, biotin and niacin) by flavins. The pyridoxal-5\'-phosphate (PLP) reacted with singlet and triplet excited states of flavins with rate constant for quenching of 1kq = 1,03 1011 L mol-1 s-1 for the singlet state, this value is higher than expected value of bimolecular reactions controlled by diffusion in an aqueous solvent. A significant dependence was observed for the rate constant for deactivation of singlet excited state with temperature, the increase of the temperature leads to a decrease of the rate constant suggesting the existence of a complex [FMN...PLP] in ground state confirmed by time resolved fluorescence spectroscopy. PLP reacted with FMN triplet excited state with rate constant 3kq = 2,96 108 L mol-1 s-1 in phosphate buffer pH 6,4 or deuterium oxide at 25 °C. There was no significant difference between the rate constant of deactivation of the triplet-excited of FMN in aqueous solution or deuterium oxide which confirms a direct process of electron transfer to PLP for 3FMN* rather than a process of transfer of hydrogen atom. Biotin and niacin unreacted with singlet and triplet excited states of vitamin B2 which can be attributed to high oxidation potentials, Eo > 2 V vs. NHE, observed for this vitamins in aqueous solution. The cyclic voltammetry of PLP had an irreversible anodic oxidation process (E = 1.07 V vs. NHE) kinetically controlled by heterogeneous electron transfer from PLP to the electrode. The quantum yield of photodegradation of PLP in aerobic condition is 2.5 times higher than that found for the reaction in anaerobic condition, which suggests the of participation of superoxide ion in the PLP global degradation process. Folic acid demonstrated reactive with triplet excited state of flavins (3kq= 4,8 108 L·mol-1 s-1 and Φ = 0,26 (aerobic condition) and Φ = 0,32 (anaerobic condition)) and the complexation with β-LG (Φ = 0,032 (aerobic condition) and Φ = 0,055 anaerobic condition)) an efficient approach in protecting the vitamin against photodegradation sensitized by flavins.