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.

Electron flow through Photosystem II (PSII) is essential to all life on Earth. The efficiency of this process in freshwater phytoplankton can be depressed by nutrient limitation, ultraviolet radiation (UVR), and excessive photosynthetically active radiation (PAR). The effects of nutrients and radiation on PSII function of natural communities were assessed using changes in the variable fluorescence of PSII (PSII VF), as determined by a pulse amplitude modulation (PAM) fluorometer. The net susceptibility to photoinhibition of PSII depends upon the sensitivity of the assemblage to photodamage and the efficiency of recovery. Damage and recovery rates were quantified by fitting changes in FV:FM during experimental spectral exposures to the model of Kok (1956). Phytoplankton from deep chlorophyll maxima were found to exhibit much higher photosynthetic impairment under UVR exposure than epilimnetic communities in two lakes, due to differences in both damage and recovery rates. In six temperate lakes of the Dorset-Haliburton region, the susceptibility to UVR-induced photoinhibition of PSII was found to be a function of the water transparency (dissolved organic carbon content) of the systems from which the plankton were isolated, with no obvious taxonomic pattern to the responses. Nutrient (nitrogen & phosphorus) supplementation of communities from the Dorset Lakes and from Lake Ontario did not have strong effects on PSII VF, and did not alter the response of Lake Ontario phytoplankton to spectral irradiance. Diurnal changes in FV:FM of Lake Ontario phytoplankton were modeled and average values for the upper half of the water column showed reasonable agreement with observed data; however it is suggested that the addition of a model to simulate vertical mixing could improve the depth-specific accuracy of the predictions. It is concluded that the light history (photoacclimation status) of phytoplankton is the major determinant of the sensitivity of PSII to UVR, and that the nutrient status and taxonomic composition of phytoplankton communities have a comparatively minor influence.

No description available.

Electron flow through Photosystem II (PSII) is essential to all life on Earth. The efficiency of this process in freshwater phytoplankton can be depressed by nutrient limitation, ultraviolet radiation (UVR), and excessive photosynthetically active radiation (PAR). The effects of nutrients and radiation on PSII function of natural communities were assessed using changes in the variable fluorescence of PSII (PSII VF), as determined by a pulse amplitude modulation (PAM) fluorometer. The net susceptibility to photoinhibition of PSII depends upon the sensitivity of the assemblage to photodamage and the efficiency of recovery. Damage and recovery rates were quantified by fitting changes in FV:FM during experimental spectral exposures to the model of Kok (1956). Phytoplankton from deep chlorophyll maxima were found to exhibit much higher photosynthetic impairment under UVR exposure than epilimnetic communities in two lakes, due to differences in both damage and recovery rates. In six temperate lakes of the Dorset-Haliburton region, the susceptibility to UVR-induced photoinhibition of PSII was found to be a function of the water transparency (dissolved organic carbon content) of the systems from which the plankton were isolated, with no obvious taxonomic pattern to the responses. Nutrient (nitrogen & phosphorus) supplementation of communities from the Dorset Lakes and from Lake Ontario did not have strong effects on PSII VF, and did not alter the response of Lake Ontario phytoplankton to spectral irradiance. Diurnal changes in FV:FM of Lake Ontario phytoplankton were modeled and average values for the upper half of the water column showed reasonable agreement with observed data; however it is suggested that the addition of a model to simulate vertical mixing could improve the depth-specific accuracy of the predictions. It is concluded that the light history (photoacclimation status) of phytoplankton is the major determinant of the sensitivity of PSII to UVR, and that the nutrient status and taxonomic composition of phytoplankton communities have a comparatively minor influence.

No description available.

No description available.

Obligate shade plants possess adaptations that enable them to photosynthesise in the low light environment of the forest floor. Adaptations that facilitate light scavenging may compromise capacity for high rates of photosynthesis. This study compares the responses of obligate shade and facultative shade plant species upon exposure to elevated light. The obligate shade plants were two commercially grown medicinal herb species of ginseng, Panax ginseng C.A. Meyer and Panax quinquefolius L.; and goldenseal - Hydrastis canadensis L. Comparison was made to Arabidopsis thaliana and Pisum sativum L. as facultative shade species. Panax ginseng (Korean ginseng) and Panax quinquefolius (American ginseng) are obligate shade plants found in broadleaf forests of Eastern Asia and North America, respectively. Studies on these plants have shown optimal growth at light intensities between 200-300 [mu]mol photons. m⁻�. s⁻�, or 10-15% of full sunlight, and at intensities greater than 500 [mu]mol photons. m⁻�. s⁻� characteristic photoinbibitory symptoms develop. An atypical response to methyl viologen in photosynthetic electron transport assays was observed in ginseng in both isolated thylakoid membranes and whole leaves. No correlation was found between detectable superoxide dismutase activity and altered methyl viologen reactions. In a mutagenesis study using the model cyanobacterium Synechocystis sp. PCC 6803, a unique amino acid residue in the terminal electron acceptor PsaC, found only in ginseng, was changed and found to have no effect on methyl viologen reactions. Electron transfer to methyl viologen was examined in both isolated thylakoid membranes and whole leaves using chlorophyll a fluorescence and the apparent ability for methyl viologen to act as an electron acceptor was observed to differ between ginseng species. Obligate shade species were observed to possess alternate pools of photosystem II centres that potentially provide a mechanism to maximise photosynthetic gain under low light and during short periods of increased illumination. In experiments designed to identify physiological processes that contribute to increased susceptibility to photoinhibition in obligate shade plants, responses were observed and characterised following a moderate increase in illumination (140 to 400 [mu]mol photons. m⁻� . s⁻�) using chlorophyll a fluorescence induction curve analysis. The obligate shade species exhibited varied responses to elevated light and showed increased susceptibility, to photoinhibition. Photoprotective non-photochemical dissipative capacity was quantified and found to be comparable between all species studied.

Obligate shade plants possess adaptations that enable them to photosynthesise in the low light environment of the forest floor. Adaptations that facilitate light scavenging may compromise capacity for high rates of photosynthesis. This study compares the responses of obligate shade and facultative shade plant species upon exposure to elevated light. The obligate shade plants were two commercially grown medicinal herb species of ginseng, Panax ginseng C.A. Meyer and Panax quinquefolius L.; and goldenseal - Hydrastis canadensis L. Comparison was made to Arabidopsis thaliana and Pisum sativum L. as facultative shade species. Panax ginseng (Korean ginseng) and Panax quinquefolius (American ginseng) are obligate shade plants found in broadleaf forests of Eastern Asia and North America, respectively. Studies on these plants have shown optimal growth at light intensities between 200-300 [mu]mol photons. m⁻�. s⁻�, or 10-15% of full sunlight, and at intensities greater than 500 [mu]mol photons. m⁻�. s⁻� characteristic photoinbibitory symptoms develop. An atypical response to methyl viologen in photosynthetic electron transport assays was observed in ginseng in both isolated thylakoid membranes and whole leaves. No correlation was found between detectable superoxide dismutase activity and altered methyl viologen reactions. In a mutagenesis study using the model cyanobacterium Synechocystis sp. PCC 6803, a unique amino acid residue in the terminal electron acceptor PsaC, found only in ginseng, was changed and found to have no effect on methyl viologen reactions. Electron transfer to methyl viologen was examined in both isolated thylakoid membranes and whole leaves using chlorophyll a fluorescence and the apparent ability for methyl viologen to act as an electron acceptor was observed to differ between ginseng species. Obligate shade species were observed to possess alternate pools of photosystem II centres that potentially provide a mechanism to maximise photosynthetic gain under low light and during short periods of increased illumination. In experiments designed to identify physiological processes that contribute to increased susceptibility to photoinhibition in obligate shade plants, responses were observed and characterised following a moderate increase in illumination (140 to 400 [mu]mol photons. m⁻� . s⁻�) using chlorophyll a fluorescence induction curve analysis. The obligate shade species exhibited varied responses to elevated light and showed increased susceptibility, to photoinhibition. Photoprotective non-photochemical dissipative capacity was quantified and found to be comparable between all species studied.

Les hydrogénases FeFe sont des métalloenzymes qui catalysent l'oxydation et la production de H$_2$. Le cycle catalytique et de nombreux aspects de la réactivité de ces enzymes, y compris l'inactivation aérobie et anaérobie, ne sont toujours pas complètement compris. J'ai développé une nouvelle technique appelée photo-électrochimie directe et je l'ai utilisée pour étudier l'effet de l'irradiation sur la cinétique d'inhibition par le CO de trois hydrogénases FeFe distinctes. J'ai déterminé le spectre d'action de la photo-dissociation de l'inhibiteur CO et j'ai décrit le processus au niveau QM pour la première fois, en obtenant un bon accord entre les expériences et la théorie.J'ai également étudié la photoinhibition de l'enzyme. J'ai effectué des expériences de photoélectrochimiques en irradiant la protéine avec des diodes laser monochromatiques dans le domaine de la lumière visible, une lampe halogène ou une lampe au xénon et j'ai observé que les hydrogénases FeFe de C. reinhardtii et C. acetobutylicum sont irréversiblement inactivées par la lumière UVB. En utilisant la DFT et la TDDFT, j'ai conclu que les étapes initiales de la photoinhibition consistent en la photodissociation d'un ligand carbonyle intrinsèque du site actif, suivie de la formation d'une espèce inactive stable.J'ai aussi effectué des expériences préliminaires pour examiner l'effet de la lumière sur l'activité de deux autres métalloenzymes: la CO déshydrogénase et la hydrogénase NiFe.Mes résultats illustrent la force de l'approche méthodologique qui combine électrochimie directe et TDDFT, et apportent de nouvelles connaissances sur les propriétés chimiques et photochimiques de plusieurs métalloenzymes. / FeFe hydrogenases are metalloenzymes that catalyze the oxidation and production of H$_2$. The catalytic cycle and many aspects of the reactivity of these enzymes, including their aerobic and anaerobic inactivation, are still the subjects of intense investigations. I developed a new technique called direct photo-electrochemistry and I used it to study the effect of monochromatic irradiation in the visible range on the kinetics of inhibition by CO of three distinct FeFe hydrogenases. I determined the action spectrum of the photo-dissociation of the inhibitor CO and I described the process at the QM level for the first time, obtaining good agreement between experiments and theory.I also studied the photoinhibition of the enzyme. I carried out photoelectrochemistry experiments irradiating the protein with monochromatic visible light laser diodes, a halogen lamp or a xenon lamp, and I observed that the FeFe hydrogenases from C. reinhardtii and C. acetobutylicum are irreversibly inactivated by UVB light. Using DFT and TDDFT, I concluded that the initial steps of photoinhibition consist in the photodissociation of one carbonyl intrinsic ligand of the active site, followed by the formation of a stable inactive species.I also performed preliminary experiments to examine the effect of light on the activity of two other metalloenzymes: Carbon monoxide dehydrogenase (CODH) and NiFe-hydrogenase.My results illustrate the strength of the methodological approach that combines direct electrochemistry and TDDFT, and reveal new insights in the chemical and photochemical properties of several metalloenzymes.

It is well known that photosynthetic health impacts the overall fitness of the mature plant. This study aims to determine photosynthetic vigour of spring wheat cultivars during field development as well as their biomass composition at maturity to determine which cultivars/varieties would be optimum for cellulosic ethanol production. Additionally, specimens were grown at non-acclimating (20˚C), cold acclimating (5˚C), non-acclimating high CO2 (20˚C/750 µmol mol-1 CO2) and cold-acclimating high CO2 (5˚C/750 µmol mol-1 CO2) to resolve photosynthetic responses to different environments. Plants were photoinhibited under high irradiance (5 fold growth irradiance) and low temperature (5˚C) while photochemical efficiency of PSII was monitored throughout using chlorophyll fluorescence imaging. Vegetative production was monitored using normalised difference vegetation index. De-epoxidation of xanthophyll photoprotective pigments were also recorded using HPLC and photochemical reflectance index. Additionally, carbon assimilation rate was recorded with infra-red gas analysis methods. It was discovered that no one wheat cultivar demonstrated any photosynthetic advantage in the field or under photoinhibitory conditions. However, photosynthetic differences were observed between wheat grown in different environments. Plants that were cold-acclimated or grown under high CO2 were more resilient to photoinhibitory stress. This was also reflected by most cold-acclimated cultivars having increased triose phosphate utilization, electron transport and zeaxanthin induction. Plants acclimated to high CO2 at room temperature also displayed increased electron transport and triose phosphate utilization but had decreased zeaxanthin induction. It is hypothesized increased excitation pressure in cold acclimated and high CO2 cultivars allowed for their increase in the development of photoinhibitory tolerance.