Minimum Irradiance Research Articles

Mapping the light field of the Waldmann PDT 1200 lamp: potential for wide-field low light irradiance aminolevulinic acid photodynamic therapy.

A quantitative assessment of the light field produced by a Waldmann PDT 1200 lamp is presented. A photodiode detector array capable of measuring a beam diameter of 30 cm was used to map the light field. The irradiance was measured as a function of voltage. For lamp-detector distances of 10 cm (central axis irradiance = 250 mW/cm2), the spatial profile of irradiance was typically Gaussian. For lamp-detector distances of 30 cm (central axis irradiance = 79 mW/cm2), the spatial profile appeared more hemispherical in shape but with some asymmetry. The relative percentage variation between the maximum and minimum irradiance with respect to the central axis irradiance was approximately 13% and 3%, respectively, for a beam width of 12 cm. Beyond a lamp-detector distance of 50 cm (central axis irradiance = 32 mW/cm2), the spatial profile of irradiance was observed to become more crater-like in structure, with a minimum on the central axis and an approximately symmetric peak at a radial distance of 9 cm from the center. The relative percentage variation of this peak irradiance with respect to the central axis irradiance was approximately 17%. At lamp-detector distances of 70 and 90 cm (central axis irradiance = 19 and 13 mW/cm2, respectively), the beam's profile was asymmetric, and the irradiance was observed to increase from the center to a radial distance of 15 cm (beam width 30 cm). For a lamp-detector distance of 70 and 90 cm, the relative percentage variation between the maximum irradiance and the central axis irradiance was approximately 25% and 35%, respectively.

PHOTOSYNTHESIS IN THE DEEP BLUE SEA: PHOTOPHYSIOLOGICAL RESPONSES OF PHAEODACTYLUM TRICORNUTUM TO DYNAMIC AND STATIC IRRADIANCES

The kinetics of xanthophyll‐cycle pigment switching and fluorescence quenching dynamics in the marine diatom, Phaeodactylum tricornutum were determined in the context of dynamic and static growth light. Cultures were grown in a modified photobioreactor capable of producing dynamic light fields which exhibited attenuation characteristics similar to that of water; these cultures were pre‐acclimated to high and low, static and dynamic, growth‐light regimes for at least three days, and then examined under high, static and dynamic light. Pigment pools varied markedly. The two static light cultures had pigment complements that were very similar to “traditional” high and low‐light static cultures. The dynamic‐light grown cultures had pigment complements, which were very similar to each other but different from the static‐grown cultures. The maximum xanthophyll‐cycle pigment de‐epoxidation state attainable under saturating light was equal for all four treatments. Induction of fluorescence quenching was significantly faster in the static‐grown cultures, while xanthophyll‐cycle de‐epoxidation rates did not show as much variation. Minimum irradiances for xanthophyll‐cycle induction were correlated to average growth irradiance. Taken as a whole, the results from this work suggest that dynamic light‐grown phytoplankton have a unique photosynthetic functionality that is different from static light‐grown phytoplankton. The significance of these observations in the context of realistic light fields, and the photosynthetic response capabilities of algae grown under them will be discussed.

Impacts of Root Competition in Forests and Woodlands: A Theoretical Framework and Review of Experiments

Light is widely considered to be the most important factor limiting the performance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to predict the types of forest in which root competition affects seedling performance, and the types of plants that respond most strongly to release from root competition. We then test our predictions by reviewing experiments in which tree seedlings and forest herbs are released from belowground competition, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine-root mass along gradients of soil fertility and seasonal drought, keeping in mind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major nutrients. The changes are far more complex than those considered by allocation models. Dry woodlands typically allow 20 times as much light to penetrate as do wet forests, but there is surprisingly little evidence that they have greater fine-root densities in the topsoil. Tropical rain forests on highly infertile soils have only slightly more open canopies than those on fertile soils, but much greater fine-root densities. Northern temperate forests on highly acidic peats and sandy soils are often dominated by early-successional, open-canopied conifers (generally pines), mostly as a result of recurrent fires, and transmit about five times as much light as surrounding deciduous forests. A review of trenching experiments shows that light alone limits seedling growth in forests on moist, nutrient-rich soils, but competition for belowground resources becomes important on infertile soils and in drier regions. Secondly, we consider how root competition alters species' shade tolerances. Shade-house experiments demonstrate that species differ markedly in the minimum irradiance at which they respond to nutrient addition, but there generally tends to be a sizable response at >5% daylight and little response in <2% daylight. There is some evidence that species that have high potential growth rates and that respond markedly to increased irradiance are also most responsive to nutrient addition in 2–3% daylight. T. Smith and M. Huston have hypothesized that species cannot tolerate both shade and drought; this appears to be the case for species that tolerate shade chiefly by maximizing leaf area. However, many shade-tolerant woody plants in tropical and mediterranean-climate forests have thick, tough, long-lived leaves and a relatively high allocation to roots, and these species are much more drought tolerant. A few studies indicate that root trenching allows species to persist in deeper shade than that in which they are normally found and allows species from mesic sites to invade more xeric sites. Usually, the impact of trenching on growth rate is much greater in gaps than in the understory. Finally, we discuss the ways in which life-form composition and population structure of plant communities are shaped by reduced water supply and reduced nutrient availability, emphasizing the inadequacy of models that consider the impact of “belowground resource availability” in a generic sense. Competition in a dry climate leads to widely spaced dominants, a lack of interstitial plants, high rates of seedling mortality in the understory, and a restriction of regeneration to patches where established matrix-forming plants have died. In contrast, vegetation on moist, infertile sites is characterized by closely packed, slender dominants, miniaturized interstitial plants, and slow rates of seedling growth in the understory, combined with relatively low rates of seedling mortality. Consequently, there is a continuum of sizes among the individuals of the dominant species, and a lack of reliance on gaps for establishment.

IMPACTS OF ROOT COMPETITION IN FORESTS AND WOODLANDS: A THEORETICAL FRAMEWORK AND REVIEW OF EXPERIMENTS

Light is widely considered to be the most important factor limiting the performance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to predict the types of forest in which root competition affects seedling performance, and the types of plants that respond most strongly to release from root competition. We then test our predictions by reviewing experiments in which tree seedlings and forest herbs are released from belowground competition, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine-root mass along gradients of soil fertility and seasonal drought, keeping in mind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major nutrients. The changes are far more complex than those considered by allocation models. Dry woodlands typically allow 20 times as much light to penetrate as do wet forests, but there is surprisingly little evidence that they have greater fine-root densities in the topsoil. Tropical rain forests on highly infertile soils have only slightly more open canopies than those on fertile soils, but much greater fine-root densities. Northern temperate forests on highly acidic peats and sandy soils are often dominated by early-successional, open-canopied conifers (generally pines), mostly as a result of recurrent fires, and transmit about five times as much light as surrounding deciduous forests. A review of trenching experiments shows that light alone limits seedling growth in forests on moist, nutrient-rich soils, but competition for belowground resources becomes important on infertile soils and in drier regions. Secondly, we consider how root competition alters species' shade tolerances. Shade-house experiments demonstrate that species differ markedly in the minimum irradiance at which they respond to nutrient addition, but there generally tends to be a sizable response at >5% daylight and little response in <2% daylight. There is some evidence that species that have high potential growth rates and that respond markedly to increased irradiance are also most responsive to nutrient addition in 2–3% daylight. T. Smith and M. Huston have hypothesized that species cannot tolerate both shade and drought; this appears to be the case for species that tolerate shade chiefly by maximizing leaf area. However, many shade-tolerant woody plants in tropical and mediterranean-climate forests have thick, tough, long-lived leaves and a relatively high allocation to roots, and these species are much more drought tolerant. A few studies indicate that root trenching allows species to persist in deeper shade than that in which they are normally found and allows species from mesic sites to invade more xeric sites. Usually, the impact of trenching on growth rate is much greater in gaps than in the understory. Finally, we discuss the ways in which life-form composition and population structure of plant communities are shaped by reduced water supply and reduced nutrient availability, emphasizing the inadequacy of models that consider the impact of “belowground resource availability” in a generic sense. Competition in a dry climate leads to widely spaced dominants, a lack of interstitial plants, high rates of seedling mortality in the understory, and a restriction of regeneration to patches where established matrix-forming plants have died. In contrast, vegetation on moist, infertile sites is characterized by closely packed, slender dominants, miniaturized interstitial plants, and slow rates of seedling growth in the understory, combined with relatively low rates of seedling mortality. Consequently, there is a continuum of sizes among the individuals of the dominant species, and a lack of reliance on gaps for establishment.

Re: Management of hyperbilirubinemia in term newborn infants

I was disappointed to see that the statement by the Canadian Paediatric Society (CPS) on hyperbilirubinemia in term newborn infants did not make more specific recommendations regarding the effective use of phototherapy (Paediatr Child Health 1999;4[2]:161–4). The widely used terms ‘single’, ‘double’ or ‘intensive’ phototherapy are quite meaningless unless there is an objective measurement of irradiance. The irradiance is dependent on the type of light source used and the distance of lights from the infant. In addition, the area of skin exposed to the light source will affect clinical effectiveness. Because infants admitted to hospital for the treatment of jaundice are now likely to have higher serum bilirubin levels than those previously seen when longer postnatal hospitalization was the norm, we need to ensure that the baby is being effectively treated and not just illuminated. Dosages are specified for any other measurable therapeutic intervention, why should phototherapy be different? I would suggest that the Fetus and Newborn Committee review this issue with a view towards specifying a minimum acceptable irradiance level for phototherapy of hyperbilirubinemia. There are good data on effective irradiance levels (1–3), and it should not be too difficult to reach agreement on a minimum irradiance level – perhaps greater than 15 μW/cm2/nm. It follows that any service providing phototherapy should be equipped with an irradiance meter. The expense involved would be tiny in relation to the substantial savings accruing from the reduction in the duration of hospitalization for jaundice (4), the incidence, costs and complications of exchange transfusion due to the ‘failure of phototherapy’, and the risk and incidence of bilirubin encephalopathy (5).

Patterns of tetraspore discharge in Caloglossa and Murrayella (Ceramiales, Rhodophyta)

SUMMARY Caloglossa released tetraspores only during dark periods in contrast to a marked light-period release exhibited by Murrayella. Both could be readily converted to a reversed light-dark cycle after a lag period of 1 day. Maximum tetraspore discharge by Caloglossa occurred approximately 4–5 h after the onset of darkness and by Murrayella 4–5 h after the onset of light. Neither 8- nor 16–h light periods affected spore release patterns in Caloglossa, while in Murrayella the spore release pattern was briefly disrupted by short day lengths, but recovered after several days. In both genera, the rhythmicity was disrupted in periods of continual darkness or continual light and appeared to be independent of light quality; however, minimum irradiance levels were critical to maintenance of release patterns. Possible mechanisms of tetraspore discharge periodicity are discussed but since both plants typically have a limited habitat as epiphytes on mangrove trunks and pneumatophores, any speculations as to the adaptive advantages of each pattern tend toward mutual contradiction.

Photosynthetic parameters and empirical modelling of primary production: a case study on the Antarctic Peninsula shelf

Eight hundred photosynthesis-irradiance relationships were determined in the shelf waters adjacent to Palmer Station, Antarctica during the spring/summer periods of 1991–94. Biomass specific maximum photosynthetic rate, PBmax, and the light limited photosynthetic efficiency, αB, were poorly correlated to the physical forcing and nutrient regimes at the sampling stations. The two photosynthetic parameters, however, did strongly covary indicating the minimum irradiance required to saturate photosynthesis, Ik, was relatively constant in this highly variable environment. The variability in Ik could partially be attributed to both depth in the water column and time of the year, with the highest values occurring for surface samples during the summer period of peak incident irradiance. Given this and the significant dependence of Pmax on phytoplankton biomass, a simple empirical model for primary productivity was developed. An independent test of the model was performed on data collected in a mesoscale offshore grid and predicted primary production was found to be within 13% of measured values. Although there are limitations to this approach (i.e. exclusion of diel periodicity in photosynthetic response), these results provide relatively robust estimates of daily primary production for the Southern Ocean.

Effect of photon irradiance and photoperiod on young sporophytes of four species of the Laminariales

Four kelp species (Alaria esculenta, Laminaria digitata, L. hyperborea and L. saccharina) were cultured in the laboratory and pretreated so that gametophytes became mature and sporophytes were produced. Young sporophytes less than 3 weeks old were exposed to various light conditions and their survival or relative growth rate in length (R L), and width (R W) in one experiment, determined. Sporophytes of L. digitata survived poorly in the dark compared with L. hyperborea. The minimum photon irradiance allowing growth in L. hyperborea was about 1 μmol m-2 s-1; 1–2 μmol m-2 s-2 sustained growth in all the species. The growth of all four species was saturated at 20–30 μmol m-2 s-1 of continuous photon irradiance. In shorter daylengths the growth rate was lower both below and above light saturation. Above saturation the growth was considerably faster than would be expected if no photosynthetic products were carried over from the light to the dark period. There was evidence that 2–3 times as much light was neces...

Effects of Temperature, Photoperiod, and Light Quality on Flowering in Several Herbaceous Perennial Species

The influence of low temperatures on Campanula carpatica `Blue Clips' and Lavandula angustifolia `Munstead' flowering was determined; plants were stored at 5C for several weeks and forced under 9-h photoperiods with a 4-h night interruption (NI). C. carpatica, L. angustifolia, and Asclepias tuberosa were forced under NI at five temperatures (15–27C) and time to flower under each treatment was calculated. Flower number and size were reduced at highest temperatures. The effectiveness of cool-white fluorescent (CWF), high-pressure sodium (HPS), incandescent (I), and metal halide (MH) lights in inducing flowering in C. carpatica and Coreopsis lanceolata `Early Sunrise' was compared. Lighting was delivered as a 7-h daylength extension with PPF ranging from 0.05–2.0 μmol·m–2·s–1. Minimum irradiances above which all C. carpatica flowered were approximately 0.14, 0.12, 0.1, and 0.17 μmol·m–2·s–1, respectively. C. lanceolata under CWF displayed irregular flowering throughout the range of intensities used. Under HPS and MH, minimum irradiances for 100% flowering were 0.37 and 1.0 μmol·m–2·s–1, respectively, with sporadic flowering at lower intensities. Under I light, all C. lanceolata exposed to 0.12 μmol·m–2·s–1 or more flowered.

Design of a Multiple-Lamp Large-Scale Solar Simulator

A simple solution to the conflicting constraints of providing uniformity and collimation of irradiance in multiple-lamp solar simulators is proposed. As proof of concept, irradiance measurements obtained in a simulator comprised of 28 1-kW mercury-iodide gas discharge lamps and capable of irradiating a 1.22 m-by-2.44 m collector plane are given. The design is based on preventing a portion of the light from each bulb from reaching the collector plane. Light blockage is achieved by placing a “shadow board” 1.02 m from and parallel to the plane of the lamps. Lamps are arranged in an hexagonal pattern with 4 columns of 7 lamps at a lamp-to-lamp spacing and column-to-column spacing of 0.45 m. Lamp-to-collector plane distance is 3.05 m. The design is determined from measurements of the spatial distribution of radiant energy from a single lamp. Irradiance from an array of lamps is then simulated. Measurements of irradiance in the full-scale simulator confirm that uniformity and collimation conform to the American Society of Heating, Refrigerating and Air Conditioning Engineers’ standard. Average irradiance is 1080 W/m2. Maximum irradiance is 1190 W/m2 and minimum irradiance is 980 W/m2. Every point on the plane of the collector receives 100 percent of radiant energy from an area on the lamp array contained within a subtended angle of 20 deg.

Light requirements for growth of six shade-acclimated Mediterranean macroalgae

Six mediterranean macroalgae were cultivated for more than 2 yr under shade culture conditions, after which light requirements for growth were investigated at 16±2°C. The saturation light levels for growth in the logarithmic phase were related to the bathymetric distribution of the algae on the shore. The eulittoral to supralittoral red alga Bangia atropurpurea was saturated at a photon fluence rate of 71 μmol photons m-2 s-1, the upper sublittoral to eulittoral brown algae Scytosiphon lomentaria, Colpomenia peregrina and Kuckuckia spinosa and the sublittoral brown alga Stictyosiphon soriferus at 39 to 71 μmol photons m-2 s-1, and the deep-water alga Choristocarpus tenellus at 19 μmol photons m-2 s-1. The minimum light requirements for growth of B. atropurpurea and C. tenellus were determined by observing length increase for 56 d under limiting light conditions. The compensation and minimum irradiances required for growth of B. atropurpurea were 0.5 and 1 μmol photons m-2 s-1 respectively. The corresponding values for C. tenellus were 0.15 to 0.28 and 0.5 μmol photons m-2 s-1 respectively. C. tenellus was the siowest-growing species tested at saturating light conditions, but it grew faster than B. atropurpurea at 1 μmol photons m-2 s-1. Both B. atropurpurea and C. tenellus were able to survive 56 d in darkness, but only the latter grew under darkness in the first 14 d.

Sensitivity and integration in a visual pathway for circadian entrainment in the hamster (Mesocricetus auratus).

1. Light-induced phase shifts of the circadian rhythm of wheel-running activity were used to measure the photic sensitivity of a circadian pacemaker and the visual pathway that conveys light information to it in the golden hamster (Mesocricetus auratus). The sensitivity to stimulus irradiance and duration was assessed by measuring the magnitude of phase-shift responses to photic stimuli of different irradiance and duration. The visual sensitivity was also measured at three different phases of the circadian rhythm. 2. The stimulus-response curves measured at different circadian phases suggest that the maximum phase-shift is the only aspect of visual responsivity to change as a function of the circadian day. The half-saturation constants (sigma) for the stimulus-response curves are not significantly different over the three circadian phases tested. The photic sensitivity to irradiance (1/sigma) appears to remain constant over the circadian day. 3. The hamster circadian pacemaker and the photoreceptive system that subserves it are more sensitive to the irradiance of longer-duration stimuli than to irradiance of briefer stimuli. The system is maximally sensitive to the irradiance of stimuli of 300 s and longer in duration. A quantitative model is presented to explain the changes that occur in the stimulus-response curves as a function of photic stimulus duration. 4. The threshold for photic stimulation of the hamster circadian pacemaker is also quite high. The threshold irradiance (the minimum irradiance necessary to induce statistically significant responses) is approximately 10(11) photons cm-2 s-1 for optimal stimulus durations. This threshold is equivalent to a luminance at the cornea of 0.1 cd m-2. 5. We also measured the sensitivity of this visual pathway to the total number of photons in a stimulus. This system is maximally sensitive to photons in stimuli between 30 and 3600 s in duration. The maximum quantum efficiency of photic integration occurs in 300 s stimuli. 6. These results suggest that the visual pathways that convey light information to the mammalian circadian pacemaker possess several unique characteristics. These pathways are relatively insensitive to light irradiance and also integrate light inputs over relatively long durations. This visual system, therefore, possesses an optimal sensitivity of 'tuning' to total photons delivered in stimuli of several minutes in duration. Together these characteristics may make this visual system unresponsive to environmental 'noise' that would interfere with the entrainment of circadian rhythms to light-dark cycles.

Ignition and maintenance of laser-supported detonation waves

Laser-driven detonation waves are being considered for use in ground to low-Earth-orbit launch applications. This paper addresses the problem of determining minimum laser irradiance for ignition and maintenace of laser-supported detonations waves. A means of igniting a detonation wave at low irradiance is to embed in the propellant thin metallic flakes that break down upon vaporization. A computational scheme has been developed for determining the structure and thickness of the waves. Results are presented for seeded H 2 O, LiH, and glass propellants in the density range 10 -4 to 10 -2 g/cm 3

Photic and circadian regulation of c- fos gene expression in the hamster suprachiasmatic nucleus

Photic information entrains a circadian pacemaker located in the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus to environmental light/dark cycles. To determine whether light regulates c- fos gene expression in the SCN, we have measured c- fos mRNA levels in the SCN of the golden hamster. We report that, during the subjective night, light causes a rapid increase in levels of c- fos mRNA in the SCN. Light pulses of 5 min duration are sufficient to induce c- fos mRNA, and the highest mRNA levels occur 30 min following the onset of light. The minimum level of illumination required to induce an increase in c- fos mRNA is indistinguishable from the minimum irradiance that produces a phase shift in the hamster's circadian rhythm of activity. In addition, the induction of c- fos mRNA in the SCN by light is itself under circadian regulation. Light induction of c- fos mRNA occurs only during the subjective night, at circadian times when photic phase shifting of activity occurs. Taken together, these data suggest that c- fos may be a molecular component of the photic pathway for entrainment of mammalian circadian rhythms.

Carbohydrate Changes of Easter Lilies during Growth in Normal and Reduced Irradiance Environments

Abstract Changes in carbohydrate types and quantities in the bulbs, stems, leaves, and buds of Easter lilies (Lilium longiflorum Thunb. ‘Nellie White’) forced under ambient or reduced irradiance conditions were investigated. Analysis by high-performance liquid chromatography (HPLC) revealed that sucrose is the dominant soluble carbohydrate in bulb tissues, with glucose, fructose, and mannose present at significantly lower concentrations. During growth of the flowering shoot, mother bulb reserves are preferentially used regardless of greenhouse irradiance. Beginning 40 days after planting, there was a steady decrease in mother bulb starch concentration until anthesis, 70 days later. Increased bulb sucrose, glucose, mannose, and fructose concentrations were correlated with the induction of starch breakdown and carbohydrate export. Under natural greenhouse irradiance conditions, daughter bulb carbohydrate reserves were not used, as starch and soluble carbohydrate concentrations remained constant. Irradiance reductions of 50% to 85% significantly reduced total carbohydrate concentration in leaves and floral buds and induced export from the daughter bulb, as evidenced by elevated levels of daughter bulb soluble carbohydrates and reductions in starch concentration. Dark-grown plants exhibited similar daughter bulb carbohydrate metabolism patterns as plants grown in full sun: starch was not hydrolyzed and soluble carbohydrates did not increase in concentration. Collectively, these results suggest 1) bulb export metabolism is characterized by elevated soluble carbohydrate concentrations, 2) there is a minimum irradiance requirement for carbohydrate export processes from the daughter bulb, and 3) reductions in greenhouse irradiance result in reduced carbohydrate levels in Easter lily leaves and flower buds.

Model of the Ignition and Flame Development on a Vaporizing Combustible Surface in a Stagnation Point Flow: Ignition by Vapor Fuel Radiation Absorption

Abstract A numerical analysis is conducted of the initiation and evolution of a combustion reaction generated by fuel vapor absorption of radiation over an evaporating combustible surface in an oxidizer stagnation point flow. The combustible is initially evaporating due to an externally applied irradiance, and is assumed to be in equilibrium vaporization. The transient, stagnation point, gas conservation equations, including one-step Arrhenius type kinetics and fuel vapor absorption of radiation, are solved numerically for the case of PMMA as combustible evaporating in air. Detailed calculations are presented, for a specific case of irradiance and flow velocity, of the evolution of the velocity field, and temperature and species concentration distributions during the ignition of the mixture, and subsequent establishment of a diffusion flame over the combustible surface. Ignition is characterized by thermal run-away of the gas and it is considered to have occurred if, after discontinuing the external irradiance, a self-sustained reaction is present. It is predicted that ignition occurs in the fuel-rich side of the mixing layer formed around the dividing streamlines of the fuel and oxidizer opposing stagnation flows. After ignition a premixed reaction front moves toward the lean side separating the fuel from the oxidizer and leaving behind a diffusion flame which eventually reaches steady conditions. A parametric study is also conducted on the effect of the flow velocity on the minimum irradiance for ignition and on the ignition time. It is predicted that the minimum irradiance for ignition increases approximately linearly with the velocity and that for a given velocity the ignition time decreases approximately inversely with the irradiance.

The Stoichiometries of Supramolecular Complexes in Thylakoid Membranes From Spinach Chloroplasts

The concentrations of photosystem II (PS II), photosystem I (PS I) and cytochrome b/f complexes on a chlorophyll basis have been determined for chloroplasts of spinach grown under three irradiances in a glasshouse. Assaying PS II by atrazine binding gave concentrations that exceeded the estimates from flash-induced O2 or H+ yield by a factor of 1.21 � 0.04 (23). Since part of this factor (1.14) is expected to arise from missed turnovers in excited reaction centres, it is concluded that both methods are valid to determine [PS II] in gently isolated chloroplasts. The agreement between the methods also suggests that atrazine does not bind to additional sites of quenchers such as 'Q400', contrary to the suggestion of R. J. Dennenberg and P. A. Jursinic [Biochim. Biophys. Acta 808 (1985), 192-200]. Whilst [PS I] was independent of growth irradiances, [PS II] and [cyt f ] increased with irradiance, as did the latent ATPase activity, a measure of the concentration of coupling factor 1. If [PS I] is taken as constant at 1.65 mmol (mol Chl)-1, the mean stoichiometries of PS II : cyt b/f complex : PS I obtained at the minimum and maximum irradiances were 1.2 : 0.7 : 1 and 1.6 : 1.0 : 1 respectively, PS II being determined by atrazine binding.

Adaptation of Chrysanthemum morifolium to low light levels

Fourteen genotypes of Chrysanthemum morifolium were induced to flower under a gradient that ran from 3 to 35 W m −2 visible light at a daylength of 9 h. The first sign of light stress was a reduction in number of flowers, followed by delayed flowering and finally, with increasing stress, no flowering at all. The minimum irradiance required for flowering may be as low as 5 W m −2 for some cultivars, which is considerably lower than the irradiance of 16 W m −2 falling on the plants in the greenhouse in winter. Large cultivar-differences in minimum light requirement for flowering were detected. At lower temperatures, the minimum level of light required for flowering increased.

Physical control of the horizontal patchiness of sea-ice microalgae

Factors controlling the horizontal distribution of sea-ice microalgae were studied in Southeastern Hudson Bay and adjacent Manitounuk Sound (Canadian Arctic). Both large (-30 km) and small (0.3 to 500m) scales of variability were investigated. Results showed that salinity was the most important factor controlling large scale distribution of the ice-microalgal biomass, through its effect on the structure of the ice (surface available for colonization). Variation in the thickness of the snow-ice cover, which determines irradiance at the bottom of the ice, was the factor controlling distribution of the algal biomass at smaller scale (diameter of patches of microalgal biomass ranging between 20 and 90 m). The relation between ice-algal abundance and snow-ice thickness changed however over the season. At the beginning of the growing season (in April when the bottom-ice irradiance was higher than a minimum critical irradiance), maximum algal biomass was observed under areas covered by the smallest snow depths. Towards the end of the season, when light transmitted through the snow-lce cover increased, maximum algal biomass was observed under areas covered by the deepest snow. This suggests that ice algae have both minimum and maximum critical light levels. The minimum level is the irradiance below which there is no photosynthetic activity (Imi, 7.6 CL Einst m-2 S-') and the maximum level corresponds to the inhibiting light intensity, which may vary during the growth season. The horizontal heterogeneity of the snow-ice cover, whlch is influenced by wind at the air-ice interface, thus provides diversified bottom-ice habitats where irradiance is compatible or not with the physiological limits of the ice microalgals cells. This results in a strong patchiness in distribution of ice-bottom microalgae.

Influence of light and temperature on the growth rate of estuarine benthic diatoms in culture

Four species of estuarine benthic diatoms: Amphiprora c. f. paludosa W. Smith, Nitzschia c. f. dissipata (Kutzing) Grunow, Navicula arenaria Donkin, and Nitzschia sigma (Kutzing) W. Smith were grown in unialgal cultures. The growth rates of the diatoms were determined as the rate of increase of the chlorophyll a content of the cultures. The diatoms were cultured at different combinations of temperture, daylength, and quantum irradiance. The highest growth rates of Navicula arenaria occurred at 16° to 20°C; the other 3 species had their optimum at 25°C or higher. The small-celled species had higher growth rates at their optimum temperature, but at lower temperatures the growth rates of all 4 species became very similar. The minimum daily quantum irradiance that could effect light-saturated growth at 12° and 20°C ranged from 2.5 to 5.0 E.m-2.day-1. At 12°C, two species had their highest growth rates under an 8 h daily photoperiod. At 20°C, the three species tested all had highest growth rates under 16 h daily photoperiod. The growth response of the benthic diatoms is comparable to that of several cultures of planktonic diatoms, as described in the literature. The influence of temperature and quantum irradiance on the diatoms in the present investigation was comparable to the influence of temperature and light intensity on the 14C-fixation of marine benthic diatoms (Colijn and van Buurt, 1975).