The fate of photosynthetically-fixed carbon in Lolium perenne grassland as modified by elevated CO2 and sward management.

Prediction of the impact of climate change requires the response of carbon (C) flow in plant-soil systems to increased CO(2) to be understood. A mechanism by which grassland C sequestration might be altered was investigated by pulse-labelling Lolium perenne swards, which had been subject to CO(2) enrichment and two levels of nitrogen (N) fertilization for 10 yr, with (14)CO(2). Over a 6-d period 40-80% of the (14)C pulse was exported from mature leaves, 1-2% remained in roots, 2-7% was lost as below-ground respiration, 0.1% was recovered in soil solution, and 0.2-1.5% in soil. Swards under elevated CO(2) with the lower N supply fixed more (14)C than swards grown in ambient CO(2), exported more fixed (14)C below ground and respired less than their high-N counterparts. Sward cutting reduced root (14)C, but plants in elevated CO(2) still retained 80% more (14)C below ground than those in ambient CO(2). The potential for below-ground C sequestration in grasslands is enhanced under elevated CO(2), but any increase is likely to be small and dependent upon grassland management.

Source-sink coupling in young barley plants and control of phloem loading.

Phloem loading of carbohydrate within a mature exporting leaf of a barley seedling is shown to respond quickly to a change in the temperature of the root and the shoot meristem. This is interpreted as a close coupling between source supply and sink demand for carbohydrate, through the hydrostatic pressure gradient linking source and sink generated by the solute concentration within the sieve tubes. This interpretation was tested by using anoxia to alter solute concentration within the sieve tubes of one region of a leaf while observing phloem loading in an adjacent region. Responses to anoxia could not be explained by the above model, suggesting that either this model is incorrect or other signalling pathways are involved. There is evidence in the literature for coarse control of phloem loading but no evidence was found of fine control by solute content of the loaded sieve elements.

Spatial and temporal distribution of solutes in the developing carrot taproot measured at single-cell resolution.

The time-course and spatial distribution of sugars and ions in carrot (Daucus carota L.) was studied at fine resolution using single cell (SiCSA) and tissue analysis. Four phases of osmolyte accumulation in the taproot were identified: an amino acid (germination) phase, when internal sources of amino acids provide seedlings with osmotica; an ion phase, when inorganic and organic ions were the main solutes; a hexose phase, when concentrations of glucose and fructose sharply increased and reached their maximum; and a sucrose phase, when sucrose became the major solute. Spatial distribution of sugar in taproot cells showed a general trend of highest concentration on both sides of the vascular cambium (some 200 mM sucrose, 150 mM glucose) and a minimum in the pith (some 100 mM sucrose, 60 mM glucose) and in periderm. Electrolytes (e.g. potassium) followed a distribution generally reciprocal to that of sugars; minimum in the tissue adjacent to the cambium (some 10 mM) and maximum in the pith and periderm (some 60-100 mM). The cambial cells contained unexpectedly low concentrations of sugars and potassium. These spatial and temporal patterns indicate that amino acids, other electrolytes and sugars are interchangeable in the tissue osmotic balance. The nature of the solute is developmentally determined both temporally and spatially. During the accumulation of electrolytes following the initial amino acid phase, osmotic pressure to 420 mosmol kg-1 rises and then remains constant despite large changes in the concentration of individual solutes. This indicates that osmotic pressure is regulated independently of the individual concentrations of solutes.

Age-related changes in microvascular permeability: a significant factor in the susceptibility of children to shock?

During studies of the pathogenesis of dengue shock syndrome, a condition largely confined to childhood and characterized by a systemic increase in vascular permeability, we observed that healthy controls, age-matched to children with dengue shock syndrome, gave high values of filtration capacity (K(f)), a factor describing vascular permeability. We hypothesized that K(f) might be age dependent. Calf K(f) was studied in 89 healthy Vietnamese subjects aged 5 to 77 years. The K(f) was highest in the youngest children [7. 53 (1.96-15.46) K(f)U; median (range); where the units of K(f), K(f)U=ml.min(-1).100 ml(-1).mmHg(-1)]. Values were 3- to 4-fold lower towards the end of the second decade [4.69 (1.91-7.06) K(f)U]. Young mammals are known to have a larger microvascular surface area per unit volume of skeletal muscle than adults. During development the proportion of developing vessels is greater. Moreover, the novel microvessels are known to be more permeable to water and plasma proteins than when mature. These factors may explain why children more readily develop hypovolaemic shock than adults in dengue haemorrhagic fever and other conditions characterized by increased microvascular permeability.

Sinks--integral parts of a whole plant.

It is argued that it is not possible to predict the flux of assimilate into a sink by making a measurement on that sink alone. Rather carbon flux is a whole-plant property and can only be predicted from measurements made simultaneously in source leaves and sinks, and the transport system itself.

Visualizing the distribution of elements within barley leaves by energy dispersive X-ray image maps (EDX maps).

X-ray image maps of transverse sections of barley leaf blades were compared with individual X-ray emission spectra from vacuoles of barley leaf mesophyll, epidermis and parenchymatous bundle sheath cells. Both forms of X-ray analysis revealed distinct patterns in element distribution within the leaf blade. Calcium and Cl predominated within the epidermal cell vacuoles whilst P appeared mainly in the mesophyll cell layer. Apparent accumulation of S and Mg within the bundle sheath cell vacuoles is discussed in relation to xylem unloading and sulphur assimilation.

Respiration and photosynthesis in oats exhibiting different levels of partial resistance to Erysiphe graminis D.c. ex Merat f. sp. avenae Marchal.

Rates of photosynthesis and respiration were measured by two methods, oxygen electrode and infrared gas analysis (IRGA), in infected and control, seedling and adult leaves of oat genotypes exhibiting different levels of partial resistance. Measurements were carried out up to 9 d after inoculation, sporulation commencing on day 5. There was no decrease in the rate of photosynthesis, except in the second leaves of one genotype, and no decrease in any of the fifth leaves relative to controls. The two methods of measuring respiration gave different results, with no consistent differences being found. Measurements by oxygen electrode, up to 6 d after inoculation, revealed that disease treatment had a significant effect on respiration in only two cases; the susceptible first leaf of one genotype and the resistant fifth leaf of another. Measurements by IRGA, up to 9 d after inoculation, did not confirm these differences, but did reveal a significant increase in respiration in the diseased fifth leaf of another resistant genotype. SHAM inhibited respiration, indicating the presence of the alternative oxidase, but there was no significant difference in its activity between diseased and control plants. Thus changes in photosynthesis and respiration previously associated with powdery mildew infection are either delayed or compensated for in oats. Photosynthetic and respiratory responses do not appear to be involved in partial resistance of oats to powdery mildew.

Temperature and the partitioning and translocation of carbon.

The partitioning of carbon within sources and sinks, and its transport between them, is considered in relation to temperature. The characteristic accumulation of non-structural carbohydrates in both sources and sinks at low temperature is due partly to growth being more sensitive than photosynthesis to reductions in temperature, and partly to the differential sensitivity that enzymes of carbohydrate metabolism show to temperature. Translocation in the phloem is reduced by low temperature, due partly to viscosity and partly, possibly, to displacement of the contents of sieve elements; cooling slowly has much less effect than cooling rapidly. Partitioning in the whole plant has two partial processes: acquisition, the rate of import into a sink region, and allocation, the proportional distribution of assimilate between two or more competing sinks. Each of these can be affected by temperature treatment of the sink, of the source, or of the transport path. Allocation between the two halves of a barley root system held at different temperatures could not be explained by effects of temperature on metabolism, sucrose uptake or viscosity of transport in phloem.

INCREASED RATES OF PHOTOSYNTHESIS IN LOCALIZED REGIONS OF A BARLEY LEAF INFECTED WITH BROWN RUST.

The rate of photosynthesis was examined in whole leaves of barley infected with brown rust (Puccinia hordei Otth.) and within localized regions of the infected leaf, within brown rust pustules and in regions between them, from flecking to green island formation. In addition, oxygen evolution was measured from localized regions of a rusted leaf after feeding with inorganic phosphate (Pi) to investigate the hypothesis that fungal sequestration of Pi from the host cytoplasm may, totally or partially, be responsible for the decline in photosynthesis. The rate of net photosynthesis and the quantum yield of oxygen evolution declined in whole infected leaves as the disease progressed. However, the rate of gross photosynthesis (per unit chlorophyll) was increased in rusted leaves. Autoradiographic and oxygen evolution studies showed that the decline in net photosynthesis could be attributed largely to a reduction in the rate of photosynthesis in the regions between pustules. Within areas of the leaf invaded by the fungus, gross photosynthesis was increased in comparison to control tissue. Feeding leaf tissue with Pi did not significantly increase the rate of photosynthesis in pustules or regions between pustules, implying that Pi was not previously limiting the rate of photosynthesis. Possible mechanisms underlying the increase and decrease in photosynthesis in different regions of an infected leaf are discussed.

Carbohydrate metabolism in biotrophic plant pathogens.

Sucrose is the major carbon source in vivo. It is hydrolysed by extracellular fungal invertases and the hexoses taken up by proton symport. They are rapidly metabolized to hexitols as the endproduct of the uptake process. In further metabolism, in which pentitols are accumulated, fructose-6-phosphate has a central role.

THE LOCALIZATION AND FREQUENCY OF HAUSTORIA IN COLONIES OF BROWN RUST ON BARLEY LEAVES.

Serial sections were taken through colonies of brown rust (Puccinia hordei Otth.) on barley leaf blades. Fungal haustoria occurred in cells of the mesophyll, parenchymatous bundle sheath and epidermis (excluding guard cells) but not within the mestome sheath or vascular bundles. Within a colony, each cell contained 1.2 to 2 haustoria; each colony had about 104 haustoria on a total hyphal length of about 1 m. Surface areas of haustoria and intracellular hyphae were calculated and used to show that all of the hexose needed to sustain fungal growth and respiration could be taken up through either cell type.

Optical properties of the leaves of some African crop plants.

The fate of radiation incident on the leaves of some African crop plants is examined. Transmission coefficients for photosynthetically active radiation are 0.05-0.10 for mature leaves and 0.09-0.39 for young leaves. Increasing the angle of incidence of radiation results in a rise in reflectivity and a fall in absorptivity. Transmissivity rises to angles of incidence of 60 degrees and then falls sharply. Absorptivity and reflectivity are cubic functions of the angle of incidence of radiation. Consequences of these findings for photosynthesis are discussed.