High Photon Irradiance Research Articles

Degradation of azo dye by an UV/H2O2 advanced oxidation process using an amalgam lamp

AbstractThe discharge of dyes into water is an ecological problem that can be alleviated by advanced oxidation processes (AOPs), such as UV/H2O2 treatments. Searching for more efficient light sources is a way to improve AOPs’ efficiency. This work tested the efficiency of an amalgam lamp on the degradation of an azo dye, studying the effect of dye and H2O2 concentrations and pH, and the influence of some salts on the decolouration rate of methyl orange. Actinometry showed that the amalgam lamp system was able to provide a high incident photon irradiance (6.30·10−5 mol/cm2 s). The amalgam lamp‐driven AOP was able to decolourize the dye at pseudo‐first‐order rates of 0.654–4.008 1/min, with increasing rates at low dye concentration and low pH and at high H2O2 concentrations until a maximum value is reached. The results show that the amalgam lamp can be an alternative light source for fast dye degradation by AOPs.

Diversity and Species Composition of Subaerial Algal Communities in Forested Areas of Meghalaya, India

The study deals with a comparative study on diversity and species composition of subaerial algal communities from tree barks of closed undisturbed sacred grove, mixed plantation, and open disturbed forest. A total of 85 taxa had been recorded, 30 cyanobacteria and 55 algal species belonging to six classes of algae. Sacred grove harboured the highest subaerial algal diversity compared to those of plantation and open disturbed forest. There was a strong significant difference in species composition among the three different sampling areas. High number of diatoms with 14 species was recorded in sacred grove. Cyanobacteria with 22 species were the frequent group in disturbed forest whereas Trentepohliales dominated in plantation. Canonical correspondence analysis confirmed that high photon irradiance favored the growth of cyanobacteria in disturbed forest. The abundance of Trentepohliales members correlated to high rainfall and photon irradiance. High diversity and presence of many diatom species in undisturbed Mawphlang sacred grove were associated with low photon irradiance and high relative humidity and could also be due to a presence of suitable substrata formed by the growth of mosses. Sunlight, relative humidity, and rainfall were the important factors which played a major role in determining the diversity and distribution of subaerial algal communities.

Brown Spot of Rice is Affected by Photon Irradiance and Temperature

Brown spot, caused by Bipolaris oryzae, is one of the most destructive diseases in rice. In this study, the effect of photon irradiance and temperature on brown spot development was evaluated. The concentration of total soluble sugars (fructose, glucose and sucrose) in rice leaves was also evaluated. Rice plants of cv. ‘Oochikara’ were inoculated with B. oryzae and kept in a greenhouse [20 ± 2°C (night time) and 35 ± 2°C (day time), ≈ 1000 μmol photons/m2/s] or two different mist chambers (25 or 32 ± 2°C, ≈ 15 μmol photons/m2/s at the top canopy). Plants kept in a mist chamber at 32 ± 2°C, under low photon irradiance, showed reduced incubation period (IP) and increase in the rate of lesion expansion. Brown spot severity in rice leaves was 67.8% at 32 ± 2°C, 27.8% at 25 ± 2°C and 11.4% under greenhouse conditions. The highest brown spot severity was found on plants grown under low photon irradiance, in which soluble sugar concentrations were lowest, suggesting that disease development was boosted under these particular growing conditions. Based on the results of this study, a continuous high temperature and low photon irradiance, in the presence of high relative humidity, and low soluble sugars contribute to an increase in brown spot development.

Advances in the molecular breeding of forage crops for abiotic stress tolerance

Forages are the backbone of sustainable agriculture. They includes a wide variety of plant species ranging from grasses, such as tall fescue and bermudagrass, to herbaceous legumes, such as alfalfa and white clover. Abiotic stresses, especially salinity, drought, temperature extremes, high photon irradiance, and levels of inorganic solutes, are the limiting factors in the growth and productivity of major cultivated forage crops. Given the great complexity of forage species and the associated difficulties encountered in traditional breeding methods, the potential from molecular breeding in improving forage crops has been recognized. Plant engineering strategies for abiotic stress tolerance largely rely on the gene expression for enzymes involved in pathways leading to the synthesis of functional and structural metabolites, proteins that confer stress tolerance, or proteins in signaling and regulatory pathways. Genetic engineering allows researchers to control timing, tissue-specificity, and expression level for optimal function of the introduced genes. Thus, the use of either a constitutive or stress-inducible promoter may be useful in certain cases. In this review, we summarize the recent progress made towards the development of transgenic forage plants with improved tolerance to abiotic stresses.

Polychromatic UV Photon Irradiance Measurements Using Chemical Actinometers Based on NO3− and H2O2 Excitation: Applications for Industrial Photoreactors

Ultraviolet light (UV) is a proven technology for disinfecting and decontaminating drinking water. For this purpose, both low-pressure monochromatic and medium-pressure polychromatic mercury arc lamps are commonly being used. Methods for photon irradiance measurements of monochromatic light are not necessarily appropriate for polychromatic light, and there is a need for the latter especially when large volumes of water are involved. The present manuscript reports a comparative study of polychromatic UV photolysis of aqueous NO3- containing RH (RH = HCO2-, CH3OH, or C2H5OH) and H2O2 containing CH3OH, all in aerated buffered aqueous solutions. The UV photolysis of NO3- generates ONOO-, *OH, and *NO2 intermediates and the stable NO2- ion. The maximum yield of NO2- is obtained in the presence of RH. The UV photolysis of H2O2 generates *OH radicals, which in the presence of CH3OH form formaldehyde. The H2O2/CH3OH actinometer is limited to low and moderate light intensities because the reaction mechanism involves competition between second- and first-order processes. Therefore, the NO3- actinometer is preferable at high photon irradiance despite the relatively low quantum yield of NO2- and its dependence on the excitation wavelength. The two actinometers are compared to radiometry and to iodide/iodate actinometer. The latter is limited to the 200-280 nm range due to the absorption of the photoproduct I3-. The NO3-/C2H5OH actinometer is particularly useful for large volumes of water in industrial high-intensity UV photoreactors as the actinometer solution can be safely disposed.

Phytochrome A Function in Red Light Sensing

Light signals perceived by the phytochrome family of red (R) and far-red (FR) light-absorbing photoreceptors direct plant growth and development throughout their lifecycle. In contrast to other family members, phyA displays rapid light-induced proteolytic degradation upon conversion to the biologically active Pfr form and mediates high irradiance responses to continuous FR. These unique properties together with limited examples of phyA function in R have resulted in an over-simplified portrayal of phyA as a FR sensor which acts predominantly in seed germination and early stages of seedling de-etiolation. In a recent work, published in The Plant Journal, we report significant phyA activity in Arabidopsis thaliana at high (> 100 µmolm-2s-1) photon irradiances of R. Under these conditions, we observed retarded degradation of a pool of nuclear-localised phyA, consistent with the phenomenon of photoprotection, and showed phyBphyCphyDphyE quadruple null mutants, containing only functional phyA, to de-etiolate and survive to flowering. The photon irradiances used in this study were greater than those routinely used for photomorphogenic analysis in the laboratory but considerably lower than those commonly observed in daylight. In this addendum we present additional analyses of the phyBphyCphyDphyE mutant and discuss the possibility that phyA may perform a significant role in the growth and development of daylight-grown plants.

Phytochrome A is an irradiance‐dependent red light sensor

Plants perceive red (R) and far-red (FR) light signals using the phytochrome family of photoreceptors. In Arabidopsis thaliana, five phytochromes (phyA-phyE) have been identified and characterized. Unlike other family members, phyA is subject to rapid light-induced proteolytic degradation and so accumulates to relatively high levels in dark-grown seedlings. The insensitivity of phyA mutant seedlings to prolonged FR and wild-type appearance in R has led to suggestions that phyA functions predominantly as an FR sensor during the early stages of seedling establishment. The majority of published photomorphogenesis experiments have, however, used <50 micromol m(-2) sec(-1) of R when characterizing phytochrome functions. Here we reveal considerable phyA activity in R at higher (>160 micromol m(-2) sec(-1)) photon irradiances. Under these conditions, plant architecture was observed to be largely regulated by the redundant actions of phytochromes A, B and D. Moreover, quadruple phyBphyCphyDphyE mutants containing only functional phyA displayed R-mediated de-etiolation and survived to flowering. The enhanced activity of phyA in continuous R (Rc) of high photon irradiance correlates with retarded degradation of the endogenous protein in wild-type plants and prolonged epifluorescence of nuclear-localized phyA:YFP in transgenic lines. Such observations suggest irradiance-dependent 'photoprotection' of nuclear phyA in R, providing a possible explanation for the increased activity observed. The discovery that phyA can function as an effective irradiance sensor, even in light environments that establish a high Pfr concentration, raises the possibility that phyA may contribute significantly to the regulation of growth and development in daylight-grown plants.

Effect of Irradiance and Temperature on Photosynthesis and Growth of Two Antarctic Benthic Diatoms, Gyrosigma subsalinum and Odontella litigiosa

Light and temperature demands for photosynthesis and growth of two benthic diatoms from Potter Cove, a shallow bay in King George Island, South Shetland Islands (Antarctica), were determined in laboratory cultures. The species investigated were Gyrosigma subsalinum var. antarctica and the Antarctic endemic Odontella litigiosa . The algae exhibited different light requirements for growth and photosynthesis. G. subsalinum was adapted to low light conditions, with a high light utilisation coefficient (α) and low saturating photon irradiances for photosynthesis and growth (~25 μmol m −2 s −1 and 11 μmol m −2 s −1 , respectively). Photosynthesis and growth in O. litigiosa were saturated at a photon irradiance of ~100 μmol m −2 s −1 . Both benthic diatoms exhibited extremely low upper survival temperature (UST, 5–7 °C) and growth only occurred over a narrow range from 0 and 7 °C. Optimal temperature for growth (0 °C) was slightly lower than optimum temperature for photosynthesis (5 °C). The results suggest that G. subsalinum is well-adapted to the low light and constant low temperature conditions present in water depths below 10 m where it occurs. In contrast, O. litigiosa requires more light and is able to tolerate the higher temperatures typical of shallow waters and tide pools. The northern distribution limit of G. subsalinum is the 4 °C summer isotherm, i.e. at the northern boundary of the Antarctic region. In contrast, O. litigiosa could potentially also occur in the sub-Antarctic region up to the 7 °C summer isotherm.

Laser-induced internal cooling of Yb 3+-doped fluoride-based glasses

Laser-induced internal cooling between room temperature and 77 K in a new fluorochloride glass (CNBZn) and a fluoride glass (BIG) doped with 1 mol% of YbF 3 has been demonstrated by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency for CNBZn glass which is ∼2.0% relative to the absorbed laser power at 1010 nm and 300 K falls about 20% at 77 K. The cooling efficiency for BIG glass was only ∼0.6% at room temperature. The experimental results are in good agreement with the predictions of a model based on the hypothesis of a second order process for the cooling mechanism.

On the origin of anti-Stokes laser-induced cooling of Yb3+-doped glass

Abstract Anti-Stokes cooling between room temperature and 77 K in a new fluorochloride glass (CNBZn) doped with 1 mol% of YbF 3 has been demonstrated by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency of CNBZn glass which is ∼2.0% relative to the absorbed laser power at 1010 nm and 300 K falls by about 20% at 77 K. A model accounting for the photon–ion–phonon interaction is in good agreement with the observed temperature dependence of the cooling process and shows its relation with the vibrational properties of the material.

Anti-Stokes laser-induced internal cooling ofYb3+-doped glasses

Anti-Stokes cooling between room temperature and 77 K in a fluorochloride glass (CNBZn) and a fluoride glass (BIG) doped with 1 mol % of ${\mathrm{YbF}}_{3}$ has been demonstrated by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency for CNBZn glass which is \ensuremath{\sim}2.0% relative to the absorbed laser power at 1010 nm and 300 K falls about 20% at 77 K. The cooling efficiency for BIG glass was only \ensuremath{\sim}0.6% at room temperature. A model accounting for the photon-ion-phonon interaction is in good agreement with the observed temperature dependence of the cooling process and shows its relation with the vibrational properties of the material.

Photoinhibition and the availability of inorganic carbon restrict photosynthesis by surface blooms of cyanobacteria

When buoyant cyanobacteria form a surface bloom, they may obtain inorganic carbon from the water and the atmosphere. In this study, artificial blooms of cyanobacteria were exposed to different concentrations of CO2 in the atmosphere and HCO2-1, in the water. The conditions and photosynthetic activity within the bloom were measured with microelectrodes sensing O2, pH, and light. Net rates of photosynthesis increased with the concentration of CO2 in the atmosphere (air = 350 ppm). However, even under 3,500 ppm CO2 , reducing the concentration of inorganic carbon in the water reduced rates of photosynthesis. Thus, both air and water acted as sources of inorganic carbon. Bloom formation may give cyanobacteria an advantage in competition for light and inorganic carbon with other groups of phytoplankton; by placing a dense biomass close to the water surface they are able to intercept a large amount of the flux of light and inorganic carbon. The high surface pH produced in the bloom will promote influx of CO2 from the atmosphere by “chemical enhancement.” The obvious advantage of being close to the water surface may be offset by the risk of photoinhibition at high photon irradiance. The dense packing of colonies in the bloom furthermore caused a high local demand for inorganic carbon and consequently extreme carbon depletion. This combination of high photon irradiance and carbon limitation proved to be especially detrimental in blooms. When the availability of inorganic carbon was restricted, nonphotochemical quenching of fluorescence increased, showing some removal of excess excitation energy, but not to the extent that photodamage was prevented. Depending on the outcome of several interacting processes, surface bloom formation may be advantageous or deleterious for the cyanobacteria involved. This also depends on the environmental conditions to which cells were acclimated prior to bloom formation. Cyanobacteria have a better chance of withstanding the conditions in surface blooms if the cells were already acclimated to high average irradiance prior to floating to the surface.

Fluxes of carbon dioxide and water vapour over a C4 pasture in southwestern Amazonia (Brazil)

In Brazil, pastures for cattle ranching are being established in areas that were previously forested. To investigate some consequences of this change in land use we measured fluxes of CO2 and water vapour over a typical pasture, dominated by the introduced C4 grass Brachiaria brizantha. In addition, we compared the CO2, water vapour fluxes and canopy stomatal conductances observed with those obtained simultaneously over a nearby undisturbed rain forest. Measurements were made near the end of the wet season under conditions of ample soil moisture. Leaf area index of the pasture was 3.9. The pasture had a lower canopy stomatal conductance than the forest (typically 0.2–0.3 mol m-2 s-1 versus 0.4–0.9 mol m-2 s-1 at high photon irradiance) and was less responsive to the canopy-to-air vapour pressure difference. As a consequence of these lower canopy stomatal conductances, the pasture used much less water than the forest with average values over the period examined being 153 mol H2O m-2 d-1 and 249 mol H2O m-2 d-1 for pasture and forest respectively (2.74 and 4.48 mm d-1 respectively). This was also reflected by differing fractions of the absorbed energy being dissipated as evaporation. This proportion was typically 0.56 for the pasture and 0.74 for the forest. After allowing for soil and plant respiration, average daily photosynthetic rates were 0.67 mol C m-2 d-1 for the pasture and 0.57 mol C m-2 d-1for the forest (8.0 and 6.8 g C m-2 d-1, respectively). Thus, despite an appreciably lower rate of water use the pasture assimilated more carbon on a daily basis. Nevertheless, Brachiaria displayed a somewhat lower rate of photosynthesis than expected for a C4 grass, perhaps because of a low nutrient status. Indeed, at low and medium photon irradiance the pasture and forest showed remarkably similar photosynthetic performance. There was, however, less tendency for CO2 assimilation rates of the pasture canopy to saturate at high photon irradiance. The respiratory fluxes from the two ecosystems at night were quite similar, 6–8 µmol m-2 s-1. The ratio of intercellular CO2 concentration to ambient CO2 concentration was usually 0.4 to 0.6 for the pasture, a range which is higher than that often reported for C4 plants but possibly not unusual for tropical grasses in their natural environment.

Determination of microphytobenthos PSII quantum efficiency and photosynthetic activity by means of variable chlorophyll fluorescence

A pulse amplitude modulated fluorometer (PAM) was used to investigate photosynthetic activity of microphytobenthos on an intertidal mudflat. Spectral irradiance measurements indicate that 75% of the signal detectable by the PAM originates in the upper 150 mu m of the sediment. From the photosynthetic electron transport rate (ETR) measurements, it was concluded that the PAM could be used to observe changes in photosynthetic parameters during the day or the season. Photoacclimation to lower irradiance was indicated by changes in the maximum ETR and the saturating photon irradiance parameter I-k. When cores were exposed to a high photon irradiance for several hours, vertical migration could be followed using reflectance spectra. The data also showed that the benthic algae did not seem to experience photoinhibition or CO2 limitation. To explain this, it is hypothesised that there is a continuous vertical migration in the top layer of the sediment, where algae can avoid photoinhibition due to prolonged periods of high irradiance and lack of CO2 by migrating downwards while others migrate upwards. [KEYWORDS: microphytobenthos; chlorophyll fluorescence photosynthesis; vertical migration; C-limitation Estuary sw netherlands; photosystem-ii; marine-phytoplankton; light; photoinhibition; sediments; stabilization; respiration;communities; limitations]

Modelling vertical migration of the cyanobacterium Microcystis

Computer models can be helpful tools to provide a better understanding of the mechanisms responsible for the complex movements of cyanobacteria resulting from changes in buoyancy and mixing of the water column in a lake. Kromkamp & Walsby (1990) developed a vertical migration model for Oscillatoria, that was based on the experimentally determined relationship between the rates of density change and photon irradiance in this cyanobacterium. To adapt this model to Microcystis, we determined related changes in carbohydrate content in cultures of Microcystis. Samples were incubated at various constant values of photon irradiance and then placed in the dark. The changes in carbohydrate content of the cells during these incubations were investigated. The relationship between the ratio of carbohydrate to protein and cell density in Microcystis was established to permit conversion of the rates of carbohydrate change to rates of density change. By plotting the calculated rates of density change against the values of photon irradiance experienced during the incubations, an irradiance-response curve of density change was established. The curve showed a distinct maximum at 278 mol photons m 2 s 1 . At higher values of photon irradiance, the rate of density change was strongly inhibited. A positive linear correlation was found between cell density and the rates of density decrease in the dark. The validity of the use of rate equations of density change, which are based on short-term incubations at constant values of photon irradiance, to predict density changes in Microcystis in fluctuating light regimes was tested. This was accomplished by measuring the time course of change in carbohydrate content of two continuous cultures of Microcystis, which were submitted to fluctuating light regimes, and comparing the results with the changes in the carbohydrate contents of these cultures predicted by the rate equations of carbohydrate change. The results showed good agreement: the rate equations of density change were therefore introduced into the model to simulate vertical migration of Microcystis .T he model predicts that the maximum migration depth of Microcystis will increase with colony size up to a maximum of 200 m radius. The effect of colony size on the net increase in cell density during the light period was also investigated with the model. It predicts that small colonies have a higher net increase in cell density than large colonies, but are inhibited at high photon irradiances at the surface.

CHANGES IN PHOTOSYNTHESIS IN RESPONSE TO COMBINED IRRADIANCE AND TEMPERATURE STRESS IN CYANOBACTERIAL SURFACE WATERBLOOMS1

ABSTRACTBuoyant cyanobacteria, previously mixed throughout the water column, float to the lake surface and form a surface waterbloom when mixing subsides. At the surface, the cells are exposed to full sunlight, and this abrupt change in photon irradiance may induce photoinhibition; at the same time, temperature rises as well. This study investigated the damaging effects of this increase in temperature as well as the ecologically more relevant combination of both an increased temperature and a high photon irradiance. Analysis of surface blooms with oxygen microelectrodes showed that integrated oxygen contents that are dependent on the balance of photosynthetic oxygen evolution and respiratory oxygen uptake decreased when temperature was raised above the lake temperature. Gross rates of photosynthesis were unaffected by temperatures up to of 35°C; hence, a moderate increase in temperature mainly stimulated oxygen uptake. Preincubation of cells of the cyanobacterium Anabaena flos‐aquae (Lyngb.) de Brébisson at temperatures up to 35°C did not affect the subsequent measurement of rates of net photosynthesis. Another 5°C rise in temperature severely damaged the photosynthetic apparatus. Failure to restore net rates of photosynthesis was coupled to a strong quenching of the ratio of variable to maximum fluorescence, Fv/Fm, that was the result of a rise in Fo. A combination of high temperature and high photon irradiance was more damaging than high temperature alone. In contrast, low photon irradiances offered substantial protection against heat injury of the photosynthetic apparatus. I conclude from this study that because cyanobacteria usually are acclimated to low average irradiance prior to bloom formation, there is a reasonable risk of chronic photoinhibition. The increase in temperature will enhance the photodamage of cells in the top layer of the bloom. Low photon irradiances in subsurface layers will offer protection against heat injury. If the high temperatures extend to the deepest, dark layers of the bloom, damage in those layers is likely to occur.

Lacunar pressures in Myriophyllum heterophyllum: manometric measurement and diurnal gas dynamics in field populations

A simple, inexpensive manometer was developed to measure lacunar pressures in submersed vascular plants. Manometers consisted of a sealed micropipet connected to a hypodermic needle and partially filled with manometer fluid. The accuracy and sensitivity of the manometer was determined by the amount of fluid and manometers were reliable over the range of 80 to 120 kPa. Diurnal changes in lacunar gas pressure and composition were examined in intact plants of Myriophyllum heterophyllum Michx. exposed to different current velocities in the field. Lacunar pressures were about 1 kPa below atmospheric in the morning, and by early afternoon reached maximum levels of 3 kPa and 5 kPa above atmospheric at a high and low velocity site, respectively. Lacunar O 2 concentrations increased as pressures increased and ranged from 17 to 23% and 15 to 34% at these sites. Lacunar O 2 concentrations at the low velocity site were higher at the shoot apex than toward the base and cutting of these stems lowered the O 2 concentration in basal segments, indicating that O 2 distribution may be influenced by photosynthetic differences and O 2 transport. The behavior of lacunar gases was examined by comparing measured gas pressures with pressures predicted based on measured lacunar O 2 concentrations. Gas pressures measured at the high velocity site were slightly higher than predicted, but in general followed the ideal gas law. Measured gas pressures at the low velocity site, however, became progressively lower than predicted throughout the afternoon. High photon irradiance and thick boundary layers at this site apparently enhanced lacunar O 2 accumulation, promoted the release of gas bubbles from the lacunar atmosphere, and led to non-equilibrium lacunar N 2 content and to lacunar O 2 enrichment without increased lacunar pressure. Lacunar pressures measured in this field study were lower than those reported for laboratory studies. Factors which can influence lacunar pressurization and lacunar function are discussed.

Modelling Stomatal Responses to Environment in Macadamia integrifolia

Gas exchange measurements were made of photosynthetic and stomatal responses of Macadamia integrifolia under controlled conditions. Test leaves were subjected to a range of temperatures, humidities and photon irradiances. When stomatal responses to humidity were plotted as a function of vapour mol fraction difference (D) a similar curvilinear response was observed at all temperatures and at photon irradiances of 200 and 1500 μmol quanta m-2 s-1. By contrast, when expressed as a function of relative humidity, different slopes in the humidity response were observed, and at high photon irradiances, stomatal conductances (gs) appeared to have an optimum temperature below 15�C. Simple equations to quantify responses to leaf temperature (TI) and D were developed, the best of which was gs = [1-k1(1-[Tl/Topt)]/k2√D, where Topt is the leaf temperature at which maximal stomatal opening is observed and k1 and k2 are constants fitted by non-linear least squares regression analysis. Calculation of the gain ratio of CO2 assimilation (A) to transpiration (E) (δA/δE) was complicated by effects of D on the relationship between A and leaf intercellular mol fraction of CO2 (CI). Calculation of δA/δE using A/CI relationships derived by varying external CO2 mol fraction at constant D showed the gain ratio to be virtually constant (1.5 mmol mol-1) across a range of leaf temperatures and vapour mol fraction differences but, when calculated directly from the relationship between A and gs, a decrease in δA/δE with D was observed. Macadamia leaves have heavily sclerified bundle sheath extensions and it is considered that this dependence was an artefact due to non-uniform stomatal closure in response to increasing D. It is shown that, at any given temperature, a stomatal response of the form gsD-1/2 gives rise to an approximately constant δA/δE.

Influence of tissue source and growth rates on dry weight and carrageenan composition of Chondrus crispus (Gigartinales, Rhodophyta)

Carrageenan analyses were conducted on vegetative female clones of Chondrus crispus that were cultured to provide tissues with differing growth rates. Tissue dry weights increased from apex to base of fronds. Total carrageenan contents were lower in apical 1 to 2 cm segments than elsewhere in the frond, except when the alga was grown at high photon irradiances. Clone 373A contained more carrageenan than clone G8. The proportion of 0.3 M KCl-soluble polymers in the total native carrageenans varied from 44 to 92%, being highest in older tissues of fronds cultured at high photon irradiances. The apical 1 cm segments contained less KCl-soluble carrageenans than other tissues from the corresponding fronds. The KCl-soluble carrageenans, when alkali-modified and refractionated, afforded the expected kappa-iota carrageenan in > 79% yields. The remainder consisted of a polymer containing 23.1% SO3Na and 8.4% 3,6-anhydrogalactose. Lambda carrageenan was not detected. Variations in carrageenan distribution between the apical region and other parts of the frond may reflect the increasing influence of medullary tissue developed as the immature cells differentiate.

Photosynthetic Characterisation of Rice Varieties in Relation to Growth Irradiance

The growth irradiances in the primary habitats of newly popularised Chinese rice 'Gui-Chao 2' and modern U.S. rice varieties 'Bellemont' and 'Lemont' are quite different, with only 2-8 h sunshine per day in Guangzhou, China, and 8-12 h sunshine per day in Texas, U.S.A. The U.S. varieties Bellemont and Lemont adapted to higher photon irradiance, showing increase of photosynthesis and increase of leaf thickness under high photon irradiance (1000 �mol photons m-2 s-1). Bright light reduced leaf chlorophyll content less in Bellemont than in Gui-Chao 2. The Chinese variety Gui-Chao 2 seemed adapted to low photon irradiance, showed delayed leaf senescence and retained higher photosynthetic rate after 20 days shading treatment. Thus, the yield reduction under shading of Gui-Chao 2 was less than that of Bellemont. After treatment for 9 days at 1000 �mol photons m-2 s-1, the quantum yield and light- and CO2-saturated photosynthesis of Gui-Chao 2 was lower than that of Lemont, suggesting possible photoinhibition in Gui-Chao 2 under high irradiance. It was proposed that these varieties represent two light-intensity ecotypes of rice. The possible application of this research to rice breeding and agricultural cultivation is discussed.