No description available.

No description available.

Carbon dioxide fixation in excised barley roots has been investigated, particularly in relation to mineral ion absorption, using carbon-14 dioxide and radiochromatography. As unsatisfactory results were obtained in the determination of radioactivity were obtained in the determination of radioactivity in the ethanol-insoluble root residues, the ethanol-soluble activity was taken as the basis for carbon-14 assimilation. A linear zonation of fixation was demonstrated within the roots, the greatest uptake occurring in the region of the spices. Maintaining the root material in distilled water, rather than a nutrient solution, prior to exposure to carbon-14 dioxide, led to a marked increase in the carbon-14 incorporated in the eathol-soluble fraction. This increase was principally in malic acid. Mineral deficiency experiments, involving the omission of potassium, nitrogen, and phosphorus in turn from the pretreatment medium, resulted in differences in the level of carbon dioxide fixation compared with that for the complete mineral solution. Results for the radioactivity of the 80% ethanol extracts expressed as percentages of that found in the complete mineral solution were 70.6, 137.33, and 96.11 respectively. Addition of glucose to the mineral solution resulted in a small increase in the level of carbon dioxide assimilation. The increase was in glutamic acid. Experiments using single salt solutions provide support for the hypothesis that carbon dioxide fixation is largely determined by organic acid formation in response to excess cation absorption. It is suggested that the relative activity of the labelled products may be related to the metabolic status of the roots. Malonate reduced carbon dioxide fixation, but the same range of labelled ethanol-soluble compounds was formed as in uninhibited roots, and there was no accumulation of labelled succinate. An experiment with cyanide was inconclusive, due to unsatisfactory experimental conditions.

A broad range of aquatic plants has been examined by means of three experimental techniques for the purpose of describing their abilities to use the different forms and concentrations of inorganic carbon which may be presented to them. Experiments were carried out on macrophyte shoots or algal suspensions in a specially designed apparatus under uniform conditions of light (560 ?Einsteins m-2 s-1) and temperature (20°C) while measuring the rate of oxygen production, and the rate of carbon uptake 3S determined indirectly by pH measurements. Kinetic experiments showed that macrophytes had more diffusive resistance to CO2 uptake than microalgae; that microalgae had a high affinity for CO2 and HCO3 whereas the macrophytes, including previously established bicarbonate users, had high affinities for CO2 only. The ratio of [HCO3] to [CO2] needed to produce the same rate of photosynthesis ranged from less than 1 in two of the microalgae to more than 100 in two macrophytes, implying a gradation in capability for bicarbonate use. Results of experiments at constant with varied pH showed that, over the range 7.4 to 9.0, net photosynthesis of the macrophytes was primarily dependent upon [CO2] in a given CT concentration but that of Anabaena cylinrica and Chlorella emersonii was not; their rates only became pH-dependent above pH 9.5. Bicarbonate use by macrophytes above pH 9 became apparent if the [CT] was high enough. The question of bicarbonate use by any specie depends upon the concentration of bicarbonate in the bathing solution, that species' affinity for bicarbonate and its bicarbonate compensation point. Using the pH-drift technique, a method is described for distinguishing CO2 and HCO3 compensation points. With this method, CO2 compensation points were determined as 0.9 muM for Cosmarium botrytis, 4.5 muM for Elodea Canadensis, 5.5 muM for Eurhynchium rusciforme, and 8 muM for Fontinalis antipyretica; HCO3 compensation points ranged from 0.05 mM in Cosmariuni to, respectively, 0.9, 0.95, and 3.3 mM in the macrophytes. From evidence from these three types of experiment, bicarbonate users and non-users do not exist as such but there is a graded capacity to use bicarbonate in which strong and weak users can be distinguished. Their poor affinity for bicarbonate means that most macrophytes can barely photosynthesise with this source at natural alkalinities when pH rises above 9. Macrophytes are largely CO2 dependent and, since CO2 is rarely saturating, they become pH- dependent. The photosynthetic activities of the plants in a lake, particularly those with a high affinity for bicarbonate can reduce the CT concentration below the air-equilibrium level. The resulting rise in pH, and at times precipitation of marl, causes a sever reduction in the free CO2 concentration which the weak users of bicarbonate rely on. It is suggested that competition for carbon arising from variations in alkalinity and from free CO2 depletion constitute an important factor in determining the composition of the plant community in lakes, particularly the macrophytes, and these plants can serve as indicators of the long-term carbon status of a lake.

The measurement of photosynthesis of submerged aquatic macrophytes was investigated with particular reference to water movement over a leaf surface. Light and dark 14C incorporation rates of two broadleaved pondweeks, Potamogeton perfoliatus and P. praelongus, were measured in the laboratory using procedures developed for use during in situ field productivity estimates. These measurements are used to evaluate the errors involved in the estimation of photosynthesis rates by this method and to provide recommendations for reducing these errors. Large variations in the 14C uptake, of replicate leaves or cut discs, were correlated with the position of the leaf on the stem and with the position of the disc on the leaf. Variation due to the size of the disc and to the effect of cutting were much less pronounced. Measurement of the rate of release of previously incorporated 14C from leaves showed that the release of carbon dioxide is greater in the dark than in the light, suggesting that extensive refixation is occurring and that this method will overestimate net photosynthesis and underestimate gross photosynthesis in light/dark bottle experiments. The movement of 14CO2 from the roots to the leaves of a plant was shown to be small and it is concluded that this may be disregarded as a significant source of carbon dioxide for photosynthesis. The ratio of leaf area to fluid volume in experimental enclosures was shown to correlate with the size of the pH changes caused by photosynthesis. Changes in pH occurring during typical in situ experiments were shown to be significantly larger than those occurring naturally and it is recommended that large enclosures with small quantities of leaf tissue are used. Reynolds number calculations shown that laminar boundary layers might be expected to predominate for broad leaves in both the aquatic and terrestrial situation. Theoretical boundary layer thicknesses, for leaves of similar sizes at similar bulk fluid velocities, show that the laminar boundary layer in water will be approximately four times less than that in air. Turbulent flow produced increases of more than 40% in measured 14C incorporation over unstirred enclosures. Different laminar flow rates over the surface of leaf discs produced measurable changes in the rate of 14C incorporation, showing a correlation between laminar boundary layer thickness and the rate of 14CO2 uptake. These measurements show that the diffusion of free carbon dioxide across the average laminar boundary layer would not be fast enough to support the flux of 14C, which must be assisted by the diffusion of the bicarbonate ion.

No description available.

Most plants complete their life cycle in a single location and therefore are affected by the changing environment. As a result, plants have evolved physiological and developmental adaptations to overcome stress. The work presented in this thesis has examined the regulation of photosynthetic electron transport in barley, rice and Thellungiella salsuginea. Barley is considered as a crop which is comparatively tolerant to soil salinity. The focus of this study was to evaluate the physiological responses of photosynthesis in barley under salinity and to characterize traits responsible for the regulation of photosynthesis. At low salt concentrations, barley plants protect PSII centres from excitation pressure by down-regulating the electron transport chain and maintaining ΔpH, by cyclic electron transport associated with PSI, to support nonphotochemical quenching (NPQ). However, at the highest concentration of salt examined, this regulation starts to fail. The failure might result from a specific loss of PSI, resulting in reduced cyclic electron flow, or an increase in the leakiness of the thylakoid membranes, resulting in loss of ΔpH. The effects of salinity on the regulation of electron transport through Photosystem I and Photosystem II have been studied in two rice varieties from Sri Lanka. The regulation of photosynthesis in the salt-tolerant At-354 is more prominent than in the salt-sensitive Bg-352 when plants are exposed to salt. Exposure of Bg-352 to salt resulted in a substantial decrease in gas exchange, PSII photochemistry, leaf area and loss of chlorophylls. The decrease in the photosynthesis in AT-354 is caused by stomatal limitations, which restrict the CO2 entry into the plants, whereas the decrease of photosynthesis in Bg-352 is caused by non-stomatal limitations. Results suggest that At-354 protects PSII centres from excitation pressure by down-regulating the electron transport chain and maintaining ΔpH by cyclic electron transport associated with PSI to support NPQ. At high salt concentration, this regulation starts to fail in Bg-352.Tolerance to abiotic and biotic stress has evolved in many wild plant species, termed extremophiles. These plants contain essential genes which may used to improve crop production in changing environments. Thellungiella salsuginea is an extremophile, able to grow and reproduce in extreme environments. Stepien and Johnson (2009) identified a protein, known as the plastid terminal oxidase (PTOX) which acts as an alternative electron sink in T. salsuginea under salt stress. The current study showed that, in addition to salt, T. salsuginea showed increases in PTOX protein content and activity when exposed to drought, different growth irradiances and cold with high light. Semi-natural conditions also triggered the activity of PTOX. This study also showed that salt caused an up-regulation of PTOX gene transcripts in the leaves of salt treated T. salsuginea plants compared to control plants. Direct electron transport from PSII to PTOX and then to oxygen via the PQ pool accounted for up to 30% of total PSII electron flow in T. salsuginea (Stepien and Johnson, 2009). Efficient electron flow from PSII to PTOX would however, probably require co-location of these complexes in the same thylakoid fraction. To examine the location of PTOX in the thylakoid membrane, immunoblot analyses were performed, to test for changes in other protein complexes which may be associated with PTOX. In addition blue-native polyacrylamide gel electrophoresis and immunoblots were performed to isolate and detect the PTOX protein with any associated complexes. Although immunoblot analysis showed a prominent signal, mass spectrometry data did not allow identification of PTOX. This results suggests that further studies are needed to identify the precise localisation of the PTOX protein in the thylakoid membranes in T. salsuginea.

Photosynthesis, photorespiration and differences in their interactions among species were studied. In the first of two parts of the investigation, the CO₂ compensation concentration of members of the Gramineae and other plants was determined with an infrared CO₂ analyzer. In some cases the Initial products of ¹⁴C0₂ fixation and leaf anatomy were also examined. In plants with low compensation values (lacking photo-respiration) the initial products of photosynthesis were formed by the C₄-dlcarboxylic acid pathway. High compensation plants (with photorespiration) produced compounds typical of the Calvin cycle. The leaf veins of low compensation species were surrounded by a specialized parenchyma bundle sheath containing a high concentration of chloroplasts with large quantities of starch. Low compensation members of the Gramineae belonged to the arlstidoid, chloridoid-eragrostoid and panicoid lines of evolution. Of the genera confined to these phylogenetic lines, Panlcum was found to differ in photosynthetlc physiology and bundle sheath characteristics at the subgeneric level. This type of photosynthetlc diversity also occurred within Cyperus (Cyperaceae), Atriplex and Bassia (Chenopodiaceae). Despite these differences, the correlation between type of photosynthetlc pathway, compensation value and leaf anatomy was consistent. The literature indicates that low compensation grasses have photosynthetic rates that are about double those of plants with photorespiration correlated with a temperature optimum for photosynthesis of about 35°. Species with photorespiration have optima within the range 10-25°. Some simple assay procedures proposed on the basis of these correlations allow rapid determination of the physiological and biochemical status of plants with respect to photosynthesis. In the second part of the investigation, some published studies of photorespiration and glycolate oxidation were reexamined and correlated by infrared CO₂ analysis. Photosynthetic rate data at different O₂ tensions for wheat, oat and corn seedlings fed 3-(3,4-dichlorophenyl)-l, 1 dimethyl urea (DCMU) indicated that dark respiration continued in the light when photosynthesis was completely inhibited. Photorespiration was also inhibited. The O₂ sensitivity of glycolate-stimulated CO₂ production was found to be compatible with the proposal that glycolate is a substrate of photorespiration. Both 'in vivo' and 'in vitro' studies of the alga Nitella flexilis revealed a pathway of glycolate oxidation resembling that of higher plants. DCMU inhibition of photosynthesis by Nitella gave results similar to those for the monocotyledons tested. Under very low light intensity, CO₂ compensation in corn was measurable but was not sensitive to high O₂ concentration. It appears that the lack of photorespiration in this plant is not the end result of efficient internal recycling of CO₂ to photosynthesis. / Science, Faculty of / Botany, Department of / Graduate

No description available.

No description available.