Nitrogen is one of the major requirements for plant growth, plentiful in some habitats, though scarce in others. A selection of plants from mires, with an emphasis on bryophytes, was examined for possible adaptations to an ammonium-based nutrition at the level of the assimilating enzymes. Measurements of nitrogen-sources in a valley mire demonstrated the presence of both nitrate and ammonium, and either could be used by the component bryophytes. Levels of about O.05mr.contrasted sharply with those of eutrophic habitats , ammonium approaching 1 m in a sewage works discharge. The ammonium-assimilating enzymes glutamate dehydrogenase , glutamine synthetase, and glutamate synthase were found in all genera except Sphagnum, where glutamate dehydrogenase could not be detected. The availability of nitrogen in Sphagnum-dominated habitats was not demonstrably less than environments occupied by genera possessing this enzyme. Levels of some ammonium-assimilating enzymes responded to the amount of substrate supplied within the overall constraints of the culture systems, although behaviour was not characteristic for plants of particular habitats. The affinity of glutamate dehydrogenase and glutamine synthesese for ammonium was similar in species from contrasting habitats. Glutamate dehydrogenase showed a negative cooperative effect with ammonium and some Km values were not incompatible with a role in ammonium assimilation.

Many popular UK garden plants have their origins in the Mediterranean Basin. Adapted for hot summers with prolonged periods of drought they exhibit features that conserve water. Climate change is predicted to bring warmer drier summers to the UK over the next century but also wetter winters: winter rainfall is predicted to increase by up to 30 % by the 2080s. It is uncertain how these species will tolerate the increased flooding associated with greater winter rainfall, or possible summer flooding resulting from high rainfall intensity storms. This research sought to assess the impact of waterlogging on Mediterranean species through a series of glasshouse and field experiments investigating plant growth and physiological changes, in response to increasing soil water content and decreasing oxygen status. In initial potplant experiments using four species, all plants survived winter flooding, whereas a 17-day summer flood (22 -39°C) resulted in the death of about one third Salvia officinalis and Cistus hybridus. Root growth and stomatal conductance were found to be key factors in survival. Previous workers have shown that some species survive waterlogging through the formation of aerenchyma and the development of shoot borne roots: responses found to be initiated by hypoxia. In hydroponic experiments considered here, Salvia officinalis also showed an ability to acclimate to anoxia by a preceding period of hypoxia, achieved largely through the rapid production of lateral roots close to the stem base: 60 % of the lateral roots were located within 40mm compared to 10% in the control. These results were supported by experiments in soil columns where the rate of root production doubled in the unsaturated zone above a rising water table. Such adaptation responses show an ability to withstand short flooding events in the predicted warmer and wetter winters of the future.

No description available.

Reactive oxygen species (ROS), such as superoxide (O2 •‾), hydrogen peroxide (H2O2), singlet oxygen (1O2) and the hydroxyl radical (•OH) can damage essential biomolecules including nucleic acids, proteins and lipids, causing damage to various cellular components. However, ROS also participate in signalling networks that are essential in plant stress responses, and also in the regulation of growth and development. Given the apparent importance of ROS, it is surprising that very little of their beneficial aspects have been researched in seeds and seedlings. Using desiccation sensitive ("recalcitrant") seeds of sweet chestnut (Castanea sativa Mill.) and desiccation tolerant ("orthodox") seeds of garden pea (Pisum sativum L.), the production of ROS was investigated during germination and seedling development, and in response to abiotic stress. Putative extracellular ROS-producing enzymes in both species were characterised to elucidate mechanisms of ROS production. Desiccating C. sativa seeds led to viability loss while the intracellular antioxidant glutathione became increasingly oxidised. Wounding and desiccation induced extracellular ROS production in C. sativa embryonic axes and P. sativum seedling axes. A pivotal role for extracellular peroxidases in producing O2 •‾ as a stress response became evident in both species as well as for the development of P. sativum seedlings. Wounding also induced amine oxidases in P. sativum embryonic axes to produce a burst of H2O2 that was essential for O2 •‾ production. Lipoxygenases were identified as putative O2 •‾- producing enzymes that may contribute to stress signalling in response to wounding. Treating desiccation-stressed material with H2O2 improved seed germination, seedling vigour and the establishment of secondary root growth. In conclusion, cell wall peroxidases, amine oxidases and lipoxygenases may work in synergy to produce O2 •‾required for stress signalling. Such extracellular ROS produced by seeds appear to be important signalling components involved in wound and desiccation response, regeneration and growth.

The absorption of chloride by detached leaves of Elodea densa and the absorption of chloride and calcium by isolated pea chloroplasts was investigated using radioactive isotopes. Chloride absorption by Elodea is stimulated by light and dependent upon photophosphorylation as the energy source for accumulation. Both photosystems could support uptake although photosystem II was more efficient than photosystem I. DCMU severely inhibited uptake. Accumulation in the light could proceed in the absence of oxygen and was stimulated under C02-free conditions, whereas uptake in the dark was dependent upon oxygen and C02. Bicarbonate inhibited both uptake in the light and dark. When detached pea leaves were fed with radioactive chloride the isolated chloroplasts contained high levels of chloride. Preliminary experiments with isolated chloroplasts indicated that ATP was involved in chloride uptake. Using a faster method of separation of chloroplasts from experimental solutions, very rapid and large chloride movements occurred in the light with slower movements taking place in the dark. These results indicated that the chloroplast membrane in vitro was permeable to chloride. The accumulation of calcium was stimulated by very low light intensities, The mechanism was very labile and showed a high degree of specificity. Uptake was not associated with phosphorylative processes but with electron flow. Inhibitor experiments indicated that the accumulatory mechanism probably lies close to the electron flow chain but not directly upon it. Approximately 60% of the accumulated calcium was present in a water-soluble fraction, though not associated with protein. A further 30% was present associated with a component that could exchange with nonradioactive calcium. A small fraction of the accumulated calcium was present in a lipid-chlorophyll fraction. The calcium accumulation reported here shows several new features. The results are discussed in terms of a mechanism of ion exchange induced by light in chloroplasts.

The earliest stages of leaf development in rice occur without direct exposure to the prevailing light environment, yet the mature leaf shows a number of morphological and physiological adaptations to light. Systemic signals generated in mature leaves are thought to play an important role in controlling the response of the young leaf primordia, but the precise developmental window during which this response can occur requires further characterisation. Stomatal size, density and distribution are likely to be key elements of this response since they play an important role in controlling gas exchange in the mature leaf, yet the cell division processes that control stomata formation are limited to relatively early stages of leaf development. The work reported here describes the process of stomata formation during rice leaf development and, in particular, identifies the stages up to and including the P3 stage as the phase during which the stomatal differentiation and patterning system is responsive to a shift of the plant from high to low irradiance. Careful staging of the transfer revealed that various aspects of stomatal size could be altered during this early phase of rice leaf development. Variable guard cell width (linked to variation in epidermal cell file width in the rice leaf) was identified as a key variable in this morphological response. Physiological analysis of mature leaves with altered stomatal properties revealed the extent to which biochemical/physiology adaptations could compensate for altered potential parameters of gas exchange. Finally, a whole-mount in situ hybridisation (WISH) procedure was optimized and proven to work in a reliable fashion using probes for various genes, thus providing a new tool for the visualisation of gene expression in rice leaves. This paves the way for the analysis of gene expression during the very early stages of leaf development when stomatal differentiation occurs. The work advances our understanding of the control of stomatal formation in rice and its potential influence on leaf photosynthetic performance. The method developed for the analysis of gene expression will enable future work to characterise the genetic mechanism underlying the environmental control of stomatal properties.

No description available.

This thesis considers the elongation of root and shoots in relation to matric potential and soil strength in contrasting species. The role of root-particle contact for root and shoot elongation in relation to particle/aggregate size and bulk density at various matric potentials is discussed. Root and shoot elongation of maize and lupin in soil and vermiculite at matric potentials ranging from -0.03 MPa to -1.6 MPa were investigated. Both root and shoot elongation rate of maize and lupin were significantly slower in vermiculite than in soil (p < 0.001). As vermiculite has very different particle size distribution and hydraulic properties from soil, the degree of contact between root and vermiculite was thought to provide a possible explanation for the slower elongation rates. A new method was developed to quantify root-particle contact using X-ray microtomography and verified using ‘phantoms’ (model systems of known dimensions). Root-particle contact was approximately 25 % greater in soil than in vermiculite. The greater root-particle contact in soil was thought to provide better growth conditions than in vermiculite. Root and shoot elongation were examined when plants were place in humid air above an osmotic solution (KC1) to evaluate the degree to which root elongation could occur in the absence of solution contact. No significant shoot elongation occurred and root elongation was more reduced than in vermiculite. Hairless maize and barley mutants and their wildtypes were used to investigate further the effects of root-particle contact when water availability is limiting in both soil and vermiculite systems. Root elongation rates of the hairless mutants were slower than those of the wildtypes, when the growth medium was wetter than -1.6 MPa. However the reduction in root elongation of hairless maize may have been due to pleiotropic effects slowing the elongation (the elongation rate relative to the maximum elongation rate was not significantly different). The combined effects of mechanical impedance and decreasing matric potentials on root and shoot elongation were tested. Maize and lupin were grown in soil packed to five bulk densities (bulk densities 1.1, 1.2, 1.3, 1.4 and 1.5 g cm'3) and wetted to three matric potentials ranging from -0.01 MPa to -1.2 MPa (Chapter 6). Root elongation rate decreased with increasing penetrometer resistance and maize was considerably more sensitive than lupin towards increase in these soil physical stresses. The effects of soil mechanical impedance dominated any improvement in root-soil contact. The averaged length of the root elongation zone (estimated from the distance between root hair zone and root tip) was linearly related to elongation rates. This is a possible method for estimating root elongation rates in situ. To manipulate root-soil contact in loosely packed aggregates, seedlings were grown at various matric potentials in soils of different aggregate sizes (4—2 mm, 2-1 mm, 1- 0.5 mm and <0.5 mm). The finer the aggregates were the faster the roots elongated, they also had better root-soil contact (72-79 % at <0.5 mm and 23-25 % at 4 -2 mm). A method was developed to investigate the role of liquid and solid contact with roots. Roots were exposed in different portions to the mist produced in an aeroponic system. In another experiment roots were placed above a water surface and supplied at different parts with water through cotton wool. Different portions of the total surface of the root were in contact with liquid or solid phase. Root and shoot elongation were not significantly affected by the portion of root surface in contact with water or solid phase. A good root-particle contact can can improve plant growth when water availability is limiting growth but the effects of greater contact area are dependant on matric potential, plant species, and soil strength.

Roots are essential for nutrient uptake and anchorage for the plant, however there is published evidence to suggest that the physical structure of soil has a strong influence on their abilities to grow and develop healthily. Observing roots in 3-dimensions, in situ and non-destructively is important for understanding the complex nature of the physical root/soil relationship, however roots are notoriously difficult to observe due to the opaque nature of soil. This problem can be partially negated by using techniques such as X-ray micro-computed tomography, but is an expensive and time-consuming technique. Furthermore, soil is a growth medium prone to spatial and temporal variation in terms of water, nutrient availability, and microbial populations, making it difficult to observe the effects of soil physical structure alone. The development of transparent soil (TS) by Downie et al. (2012) has brought about a new era in the study of root/soil interactions. TS is a growth medium with the transparency of agar and some of the physical heterogeneity of soil. TS has particles and pores, so roots can explore it in much the same way as they would soil, however the water and nutrient levels can be better controlled and microbial influences are less of an issue, due to the semi-sterile conditions that transparent soil cores are kept under. Downie et al (2012) used TS to study root growth of small Arabidopsis thaliana roots and also imaged Psuedomonas fluorescens colonising lettuce seedling roots. This project scaled the TS system up in order to image larger root systems of Hordeum vulgare (barley) seedlings under different physical conditions. Comparisons of barley roots growing in soil and TS were made, and it was found that roots grew longer in natural soil than in TS. The TS was then sieved into different particle size ranges and it was found that barley roots grew more successfully in the smaller particles (850-1250 μm) than the larger particles (>1676 μm). Vertically stratified split pots, containing large particles down one side and small particles down the other were also used and non-destructively imaged at 24-hour intervals. It was found that the presence of the large particles had an inhibitory effect on root growth across the entire root system, including the roots that were growing in the smaller particles. Finally a device was designed which allowed the application of compression to the TS system. It was found that root growth decreased proportionally with the level of pressure that was applied to the TS cores. Manipulation of TS structure and the development of techniques to quantitatively record root growth and physical soil conditions from 3-D images has enabled us to measure root growth in barley roots under different physical conditions. The results showed that root growth is heavily influenced by particle size, pore structure and soil strength. Root/soil contact was consistently observed as an important soil property for root growth across experiments.

No description available.